Add Slice 5 ADAU1701 runtime audio control (firmware 0.5.0).

Safeload mixer/EQ/master on the ADAU1701, persist AudioProfile in NVS,
expose /api/audio routes and web UI controls, with host tests and manual sync.

Co-authored-by: Cursor <cursoragent@cursor.com>

Software/Firmware/ADAU1701-Firmware/README.md

@@ -5,9 +5,13 @@ SigmaStudio project **DigiRadio**, IC1 = ADAU1701.
 Key files:
 - `DigiRadio_IC_1.h` — program/param RAM data + `default_download_IC_1()`
 - `DigiRadio_IC_1_REG.h` — register map
-- `DigiRadio_IC_1_PARAM.h` — parameter handles (safeload, Slice 5+)
+- `DigiRadio_IC_1_PARAM.h` — parameter handles (safeload addresses)
 
 The ESP32 replays `default_download_IC_1()` over I2C on every boot via
-`adau1701::Adau1701Driver` (see `components/drivers/adau1701/`).
+`adau1701::Adau1701Driver`. Runtime mixer/EQ/master changes use the
+ADAU1701 safeload mechanism (`sigma_safeload_param` /
+`sigma_safeload_block` in `SigmaStudioFW.c`).
 
 I2C address: 7-bit `0x34` (ADDR0=ADDR1=GND), matching `board_pins.hpp`.
+
+Sample rate: **48 kHz** (see `DigiRadio_NetList.xml`).

Software/Firmware/ADAU1701-Firmware/SigmaStudioFW.h

@@ -49,6 +49,35 @@ void SIGMA_WRITE_REGISTER_BLOCK(unsigned char devAddress,
  */
 void adau1701_run_default_download(void);
 
+/** Safeload data register base (0x0810..0x0814). */
+#define ADAU1701_SAFELOAD_DATA_BASE 0x0810U
+/** Safeload address register base (0x0815..0x0819). */
+#define ADAU1701_SAFELOAD_ADDR_BASE 0x0815U
+/** DSP core control register (IST bit triggers safeload). */
+#define ADAU1701_CORE_CONTROL_REG   0x081CU
+/** IST bit value OR-ed into core control to commit safeload. */
+#define ADAU1701_CORE_IST_TRIGGER   0x003CU
+
+/**
+ * @brief    Safeload one 8.23 fixpoint parameter (click-free update).
+ *
+ * @param    paramAddr  Parameter RAM address from DigiRadio_IC_1_PARAM.h.
+ * @param    fixpoint   32-bit SigmaStudio fixpoint value.
+ * @return   0 on success, non-zero on I2C failure.
+ */
+int sigma_safeload_param(unsigned int paramAddr, int fixpoint);
+
+/**
+ * @brief    Safeload up to five parameters in one IST transfer (e.g. biquad).
+ *
+ * @param    count      Number of pairs (1..5).
+ * @param    paramAddrs Parameter RAM addresses.
+ * @param    fixpoints  32-bit fixpoint values.
+ * @return   0 on success, non-zero on I2C failure.
+ */
+int sigma_safeload_block(unsigned char count, const unsigned int* paramAddrs,
+                         const int* fixpoints);
+
 #ifdef __cplusplus
 }
 #endif

Software/components/core/CMakeLists.txt

@@ -10,6 +10,15 @@ idf_component_register(
         "src/EmbeddedBlobReader.cpp"
         "src/TunerJson.cpp"
         "src/FrequencyKHz.cpp"
+        "src/GainDb.cpp"
+        "src/FrequencyHz.cpp"
+        "src/EqBandIndex.cpp"
+        "src/BiquadCoefficients.cpp"
+        "src/BiquadDesign.cpp"
+        "src/MixerState.cpp"
+        "src/EqProfile.cpp"
+        "src/AudioProfile.cpp"
+        "src/AudioProfileJson.cpp"
     INCLUDE_DIRS "include"
 )
 

Software/components/core/include/core/AudioProfile.hpp

@@ -0,0 +1,51 @@
+/**
+ * @file    AudioProfile.hpp
+ * @brief   Combined mixer, EQ, and master volume snapshot.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include "core/EqProfile.hpp"
+#include "core/GainDb.hpp"
+#include "core/MixerState.hpp"
+
+namespace core {
+
+/**
+ * @brief    AudioProfile — full ADAU1701 user configuration snapshot.
+ *
+ * @dname    AudioProfile
+ * @return   n/a (type)
+ * @pubstate Plain aggregate persisted by IAudioProfileStore and applied by
+ *           AudioService after boot.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+struct AudioProfile {
+    MixerState mixer;     ///< Input and stereo-mixer gains.
+    EqProfile eq;         ///< Six-band parametric EQ.
+    GainDb masterLeft;    ///< Multiple 1 master volume, left.
+    GainDb masterRight;   ///< Multiple 1 master volume, right.
+
+    /**
+     * @brief    factoryDefault — factory-flat audio path (0 dB everywhere).
+     *
+     * @dname    factoryDefault
+     * @return   AudioProfile suitable for first boot and reset.
+     * @pubstate none
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] static AudioProfile factoryDefault() noexcept;
+};
+
+} // namespace core

Software/components/core/include/core/AudioProfileJson.hpp

@@ -0,0 +1,77 @@
+/**
+ * @file    AudioProfileJson.hpp
+ * @brief   JSON parse/serialise for audio profile API (pure core).
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include "core/AudioProfile.hpp"
+#include "core/ParseError.hpp"
+
+#include <expected>
+#include <string>
+#include <string_view>
+
+namespace core {
+
+/**
+ * @brief    serializeAudioProfileJson — serialise profile for GET /api/audio.
+ *
+ * @dname    serializeAudioProfileJson
+ * @param    profile  Domain snapshot from AudioService.
+ * @return   JSON object string for the HTTP response body.
+ * @pubstate none
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+[[nodiscard]] std::string serializeAudioProfileJson(
+    const AudioProfile& profile);
+
+/**
+ * @brief    parseAudioProfileJson — parse PUT /api/audio/profile body.
+ *
+ * @dname    parseAudioProfileJson
+ * @param    json  Untrusted request body.
+ * @return   AudioProfile on success, or ParseError.
+ * @pubstate none
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+[[nodiscard]] std::expected<AudioProfile, ParseError> parseAudioProfileJson(
+    std::string_view json);
+
+/**
+ * @brief    serializeAudioSavedJson — success response after profile apply.
+ *
+ * @dname    serializeAudioSavedJson
+ * @return   JSON object \texttt{\{"status":"saved"\}}.
+ * @pubstate none
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+[[nodiscard]] std::string serializeAudioSavedJson();
+
+/**
+ * @brief    serializeAudioErrorJson — error response for audio routes.
+ *
+ * @dname    serializeAudioErrorJson
+ * @param    reason  Safe token (never includes secrets).
+ * @return   JSON object with status and reason fields.
+ * @pubstate none
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+[[nodiscard]] std::string serializeAudioErrorJson(std::string_view reason);
+
+} // namespace core

Software/components/core/include/core/BiquadCoefficients.hpp

@@ -0,0 +1,50 @@
+/**
+ * @file    BiquadCoefficients.hpp
+ * @brief   Float and fixpoint biquad coefficients for ADAU1701 PEQ.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include <array>
+#include <cstdint>
+
+namespace core {
+
+/**
+ * @brief    BiquadCoefficients — five PEQ coefficients (b0..a1).
+ *
+ * @dname    BiquadCoefficients
+ * @return   n/a (type)
+ * @pubstate Plain value type matching SigmaStudio Param EQ cell order.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+struct BiquadCoefficients {
+    float b0; ///< Numerator b0.
+    float b1; ///< Numerator b1.
+    float b2; ///< Numerator b2.
+    float a0; ///< Denominator a0 (ADI stores feedback terms).
+    float a1; ///< Denominator a1.
+
+    /**
+     * @brief    toFixpoint823 — convert each coefficient to 8.23 fixpoint.
+     *
+     * @dname    toFixpoint823
+     * @return   Array of five 32-bit fixpoint words for safeload.
+     * @pubstate none
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] std::array<std::int32_t, 5> toFixpoint823() const noexcept;
+};
+
+} // namespace core

Software/components/core/include/core/BiquadDesign.hpp

@@ -0,0 +1,80 @@
+/**
+ * @file    BiquadDesign.hpp
+ * @brief   Host-testable biquad coefficient design for ADAU1701 PEQ.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include "core/BiquadCoefficients.hpp"
+#include "core/FrequencyHz.hpp"
+#include "core/GainDb.hpp"
+
+namespace core {
+
+/** Sample rate of the DigiRadio SigmaStudio project (Hz). */
+inline constexpr std::uint32_t kAdauSampleRateHz = 48000U;
+
+/**
+ * @brief    designPeakingEq — compute PEQ coefficients for one band.
+ *
+ * @dname    designPeakingEq
+ * @param    center     Validated centre frequency.
+ * @param    gain       Band gain in dB (0 dB yields flat response).
+ * @param    q          Quality factor (typically 0.5..10).
+ * @return   BiquadCoefficients in SigmaStudio Param EQ order.
+ * @pubstate none
+ *
+ * Uses the Robert Bristow-Johnson peaking EQ formulae at kAdauSampleRateHz.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+[[nodiscard]] BiquadCoefficients designPeakingEq(FrequencyHz center, GainDb gain,
+                                                 float q) noexcept;
+
+/**
+ * @brief    designFlatEq — unity-gain bypass coefficients for a PEQ band.
+ *
+ * @dname    designFlatEq
+ * @return   Identity biquad (b0=1, others 0) as in SigmaStudio defaults.
+ * @pubstate none
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+[[nodiscard]] BiquadCoefficients designFlatEq() noexcept;
+
+/**
+ * @brief    gainDbToLinearFixpoint — map dB to ADAU 8.23 linear gain word.
+ *
+ * @dname    gainDbToLinearFixpoint
+ * @param    gain  Validated gain in dB.
+ * @return   32-bit fixpoint suitable for safeload (0 dB = 0x00800000).
+ * @pubstate none
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+[[nodiscard]] std::int32_t gainDbToLinearFixpoint(GainDb gain) noexcept;
+
+/**
+ * @brief    floatToFixpoint823 — convert a float to ADAU 8.23 fixpoint.
+ *
+ * @dname    floatToFixpoint823
+ * @param    value  Floating value (typically -16..+16 for filter coeffs).
+ * @return   Rounded 32-bit fixpoint word.
+ * @pubstate none
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+[[nodiscard]] std::int32_t floatToFixpoint823(float value) noexcept;
+
+} // namespace core

Software/components/core/include/core/DspError.hpp

@@ -0,0 +1,33 @@
+/**
+ * @file    DspError.hpp
+ * @brief   Typed errors for DSP control operations.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+namespace core {
+
+/**
+ * @brief    DspError — failure causes for IDsp operations.
+ *
+ * @dname    DspError
+ * @return   n/a (type)
+ * @pubstate n/a
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+enum class DspError {
+    NotBooted,
+    SafeloadFailed,
+    InvalidParameter,
+};
+
+} // namespace core

Software/components/core/include/core/EqBandIndex.hpp

@@ -0,0 +1,69 @@
+/**
+ * @file    EqBandIndex.hpp
+ * @brief   Strong index for ADAU1701 Param EQ1 bands (0–5).
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include "core/ParseError.hpp"
+
+#include <cstdint>
+#include <expected>
+
+namespace core {
+
+/**
+ * @brief    EqBandIndex — validated index into the 6-band Param EQ1 module.
+ *
+ * @dname    EqBandIndex
+ * @return   n/a (type)
+ * @pubstate Owns index_ in 0..kBandCount-1 after construction.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+class EqBandIndex {
+public:
+    /** Number of parametric EQ bands in the SigmaStudio export. */
+    static constexpr std::uint8_t kBandCount = 6U;
+
+    /**
+     * @brief    tryFromIndex — validate a band index at the boundary.
+     *
+     * @dname    tryFromIndex
+     * @param    index  Untrusted band index from JSON input.
+     * @return   EqBandIndex on success, or ParseError::MissingField.
+     * @pubstate none
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] static std::expected<EqBandIndex, ParseError> tryFromIndex(
+        std::uint32_t index) noexcept;
+
+    /**
+     * @brief    value — read the stored band index.
+     *
+     * @dname    value
+     * @return   Band index 0..5.
+     * @pubstate reads index_.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] std::uint8_t value() const noexcept;
+
+private:
+    explicit EqBandIndex(std::uint8_t index) noexcept;
+
+    std::uint8_t index_;
+};
+
+} // namespace core

Software/components/core/include/core/EqBandSettings.hpp

@@ -0,0 +1,36 @@
+/**
+ * @file    EqBandSettings.hpp
+ * @brief   Per-band EQ settings for EqProfile.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include "core/FrequencyHz.hpp"
+#include "core/GainDb.hpp"
+
+namespace core {
+
+/**
+ * @brief    EqBandSettings — gain, centre frequency, and Q for one PEQ band.
+ *
+ * @dname    EqBandSettings
+ * @return   n/a (type)
+ * @pubstate Plain value type; q in 0.2..10.0 when validated via EqProfile.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+struct EqBandSettings {
+    GainDb gain;           ///< Band gain in dB.
+    FrequencyHz center;    ///< Centre frequency in Hz.
+    float q;               ///< Quality factor.
+};
+
+} // namespace core

Software/components/core/include/core/EqProfile.hpp

@@ -0,0 +1,94 @@
+/**
+ * @file    EqProfile.hpp
+ * @brief   Six-band parametric EQ profile for the ADAU1701.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include "core/EqBandIndex.hpp"
+#include "core/EqBandSettings.hpp"
+#include "core/FrequencyHz.hpp"
+#include "core/GainDb.hpp"
+
+#include <array>
+
+namespace core {
+
+/**
+ * @brief    EqProfile — six PEQ bands persisted and applied via AudioService.
+ *
+ * @dname    EqProfile
+ * @return   n/a (type)
+ * @pubstate Owns bands_ array; default centres match typical listening curve.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+class EqProfile {
+public:
+    /**
+     * @brief    factoryDefault — flat EQ at standard centre frequencies.
+     *
+     * @dname    factoryDefault
+     * @return   EqProfile with 0 dB on all bands.
+     * @pubstate none
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] static EqProfile factoryDefault() noexcept;
+
+    /**
+     * @brief    band — read settings for one PEQ band.
+     *
+     * @dname    band
+     * @param    index  Band index 0..5.
+     * @return   Const reference to band settings.
+     * @pubstate reads bands_.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] const EqBandSettings& band(EqBandIndex index) const noexcept;
+
+    /**
+     * @brief    setBand — replace settings for one PEQ band.
+     *
+     * @dname    setBand
+     * @param    index     Band index 0..5.
+     * @param    settings  New gain, centre, and Q.
+     * @pubstate writes bands_.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    void setBand(EqBandIndex index, EqBandSettings settings) noexcept;
+
+    /**
+     * @brief    bands — access the full band array.
+     *
+     * @dname    bands
+     * @return   Const reference to all six band settings.
+     * @pubstate reads bands_.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] const std::array<EqBandSettings, EqBandIndex::kBandCount>&
+    bands() const noexcept;
+
+private:
+    explicit EqProfile(std::array<EqBandSettings, EqBandIndex::kBandCount> bands)
+        noexcept;
+
+    std::array<EqBandSettings, EqBandIndex::kBandCount> bands_;
+};
+
+} // namespace core

Software/components/core/include/core/FrequencyHz.hpp

@@ -0,0 +1,71 @@
+/**
+ * @file    FrequencyHz.hpp
+ * @brief   Strong type for audio centre frequencies (EQ bands).
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include "core/ParseError.hpp"
+
+#include <cstdint>
+#include <expected>
+
+namespace core {
+
+/**
+ * @brief    FrequencyHz — validated centre frequency for parametric EQ.
+ *
+ * @dname    FrequencyHz
+ * @return   n/a (type)
+ * @pubstate Owns hz_ within 20..20\,000 Hz (48 kHz SigmaStudio project).
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+class FrequencyHz {
+public:
+    /** Minimum EQ centre frequency (Hz). */
+    static constexpr std::uint32_t kMinHz = 20U;
+    /** Maximum EQ centre frequency (Hz). */
+    static constexpr std::uint32_t kMaxHz = 20000U;
+
+    /**
+     * @brief    tryFromHz — validate a centre frequency at the boundary.
+     *
+     * @dname    tryFromHz
+     * @param    hz  Untrusted frequency in hertz.
+     * @return   FrequencyHz on success, or ParseError::MissingField.
+     * @pubstate none
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] static std::expected<FrequencyHz, ParseError> tryFromHz(
+        std::uint32_t hz) noexcept;
+
+    /**
+     * @brief    value — read the stored frequency in hertz.
+     *
+     * @dname    value
+     * @return   Validated centre frequency in Hz.
+     * @pubstate reads hz_.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] std::uint32_t value() const noexcept;
+
+private:
+    explicit FrequencyHz(std::uint32_t hz) noexcept;
+
+    std::uint32_t hz_;
+};
+
+} // namespace core

Software/components/core/include/core/GainDb.hpp

@@ -0,0 +1,95 @@
+/**
+ * @file    GainDb.hpp
+ * @brief   Strong type for decibel gain/attenuation values.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include "core/ParseError.hpp"
+
+#include <cmath>
+#include <expected>
+
+namespace core {
+
+/**
+ * @brief    GainDb — validated gain in decibels for DSP volume and EQ.
+ *
+ * @dname    GainDb
+ * @return   n/a (type)
+ * @pubstate Owns db_ within kMinDb..kMaxDb after construction.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+class GainDb {
+public:
+    /** Minimum attenuation (effectively muted on the linear path). */
+    static constexpr float kMinDb = -96.0F;
+    /** Maximum boost allowed on the DSP path. */
+    static constexpr float kMaxDb = 12.0F;
+
+    /**
+     * @brief    tryFromDb — validate a gain value at the HTTP boundary.
+     *
+     * @dname    tryFromDb
+     * @param    db  Untrusted gain in decibels.
+     * @return   GainDb on success, or ParseError::MissingField.
+     * @pubstate none
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] static std::expected<GainDb, ParseError> tryFromDb(
+        float db) noexcept;
+
+    /**
+     * @brief    zero — unity gain (0 dB).
+     *
+     * @dname    zero
+     * @return   GainDb at 0 dB.
+     * @pubstate none
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] static GainDb zero() noexcept;
+
+    /**
+     * @brief    value — read the stored gain in decibels.
+     *
+     * @dname    value
+     * @return   Validated gain in dB.
+     * @pubstate reads db_.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] float value() const noexcept;
+
+    /**
+     * @brief    linear — convert to linear amplitude (not dB).
+     *
+     * @dname    linear
+     * @return   Linear gain factor (1.0 at 0 dB).
+     * @pubstate none
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] float linear() const noexcept;
+
+private:
+    explicit GainDb(float db) noexcept;
+
+    float db_;
+};
+
+} // namespace core

Software/components/core/include/core/IAudioProfileStore.hpp

@@ -0,0 +1,88 @@
+/**
+ * @file    IAudioProfileStore.hpp
+ * @brief   Persistence boundary for user audio configuration.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include "core/AudioProfile.hpp"
+#include "core/StoreError.hpp"
+
+#include <expected>
+
+namespace core {
+
+/**
+ * @brief    IAudioProfileStore — load/save AudioProfile in NVS (non-secret).
+ *
+ * @dname    IAudioProfileStore
+ * @return   n/a (type)
+ * @pubstate Shell implementations own NVS handles; core uses fakes in tests.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+class IAudioProfileStore {
+public:
+    virtual ~IAudioProfileStore() = default;
+
+    /**
+     * @brief    hasProfile — check whether a saved profile exists.
+     *
+     * @dname    hasProfile
+     * @return   true when loadProfile would succeed.
+     * @pubstate reads backing storage via implementation.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] virtual bool hasProfile() const = 0;
+
+    /**
+     * @brief    saveProfile — persist a validated audio profile.
+     *
+     * @dname    saveProfile
+     * @param    profile  User mixer/EQ/master snapshot.
+     * @return   Ok on success, or StoreError::IoFailed.
+     * @pubstate writes backing storage via implementation.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] virtual std::expected<void, StoreError> saveProfile(
+        const AudioProfile& profile) = 0;
+
+    /**
+     * @brief    loadProfile — read the stored audio profile.
+     *
+     * @dname    loadProfile
+     * @return   AudioProfile on success, or StoreError::NotFound / IoFailed.
+     * @pubstate reads backing storage via implementation.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] virtual std::expected<AudioProfile, StoreError> loadProfile()
+        const = 0;
+
+    /**
+     * @brief    clearProfile — erase the stored audio profile.
+     *
+     * @dname    clearProfile
+     * @return   Ok on success, or StoreError::IoFailed.
+     * @pubstate clears backing storage via implementation.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] virtual std::expected<void, StoreError> clearProfile() = 0;
+};
+
+} // namespace core

Software/components/core/include/core/IDsp.hpp

@@ -0,0 +1,135 @@
+/**
+ * @file    IDsp.hpp
+ * @brief   Abstract ADAU1701 DSP control boundary (host-testable).
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include "core/AudioProfile.hpp"
+#include "core/DspError.hpp"
+#include "core/EqBandIndex.hpp"
+#include "core/FrequencyHz.hpp"
+#include "core/GainDb.hpp"
+#include "core/MixSource.hpp"
+#include "core/MixerState.hpp"
+#include "core/EqProfile.hpp"
+
+#include <expected>
+
+namespace core {
+
+/**
+ * @brief    IDsp — hardware abstraction for ADAU1701 runtime control.
+ *
+ * @dname    IDsp
+ * @return   n/a (type)
+ * @pubstate Implemented by adau1701::Adau1701Dsp on device; fakes in tests.
+ *
+ * All parameter updates use safeload on the implementation side.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+class IDsp {
+public:
+    virtual ~IDsp() = default;
+
+    /**
+     * @brief    applyProfile — safeload mixer, EQ, and master from a snapshot.
+     *
+     * @dname    applyProfile
+     * @param    profile  Validated user configuration.
+     * @return   Ok on success, or DspError.
+     * @pubstate writes ADAU1701 parameter RAM via safeload.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] virtual std::expected<void, DspError> applyProfile(
+        const AudioProfile& profile) = 0;
+
+    /**
+     * @brief    applyMixer — safeload input and stereo-mixer gains.
+     *
+     * @dname    applyMixer
+     * @param    mixer  Per-source and St Mixer1 levels.
+     * @return   Ok on success, or DspError.
+     * @pubstate writes ADAU1701 parameter RAM via safeload.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] virtual std::expected<void, DspError> applyMixer(
+        const MixerState& mixer) = 0;
+
+    /**
+     * @brief    applyEq — safeload all six PEQ bands.
+     *
+     * @dname    applyEq
+     * @param    eq  Six-band parametric EQ settings.
+     * @return   Ok on success, or DspError.
+     * @pubstate writes ADAU1701 parameter RAM via safeload.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] virtual std::expected<void, DspError> applyEq(
+        const EqProfile& eq) = 0;
+
+    /**
+     * @brief    setInputVolume — safeload one input path (Si4684 or ESP32).
+     *
+     * @dname    setInputVolume
+     * @param    source  Tuner or ESP32 I2S path.
+     * @param    left    Left channel gain.
+     * @param    right   Right channel gain.
+     * @return   Ok on success, or DspError.
+     * @pubstate writes ADAU1701 parameter RAM via safeload.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] virtual std::expected<void, DspError> setInputVolume(
+        MixSource source, GainDb left, GainDb right) = 0;
+
+    /**
+     * @brief    setMasterVolume — safeload Multiple 1 master output gain.
+     *
+     * @dname    setMasterVolume
+     * @param    left   Left master gain.
+     * @param    right  Right master gain.
+     * @return   Ok on success, or DspError.
+     * @pubstate writes ADAU1701 parameter RAM via safeload.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] virtual std::expected<void, DspError> setMasterVolume(
+        GainDb left, GainDb right) = 0;
+
+    /**
+     * @brief    setEqBand — design and safeload one PEQ band.
+     *
+     * @dname    setEqBand
+     * @param    band   Band index 0..5.
+     * @param    gain   Band gain in dB.
+     * @param    center Centre frequency.
+     * @param    q      Quality factor.
+     * @return   Ok on success, or DspError.
+     * @pubstate writes five coefficients via a single safeload transfer.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] virtual std::expected<void, DspError> setEqBand(
+        EqBandIndex band, GainDb gain, FrequencyHz center, float q) = 0;
+};
+
+} // namespace core

Software/components/core/include/core/MixSource.hpp

@@ -0,0 +1,32 @@
+/**
+ * @file    MixSource.hpp
+ * @brief   ADAU1701 input path selector for per-source volume control.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+namespace core {
+
+/**
+ * @brief    MixSource — Si4684 tuner or ESP32 I2S input on the ADAU1701.
+ *
+ * @dname    MixSource
+ * @return   n/a (type)
+ * @pubstate n/a
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+enum class MixSource {
+    Si4684,
+    Esp32,
+};
+
+} // namespace core

Software/components/core/include/core/MixerState.hpp

@@ -0,0 +1,52 @@
+/**
+ * @file    MixerState.hpp
+ * @brief   ADAU1701 input and stereo-mixer gain snapshot.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include "core/GainDb.hpp"
+
+namespace core {
+
+/**
+ * @brief    MixerState — per-source and St Mixer1 levels for the ADAU1701.
+ *
+ * @dname    MixerState
+ * @return   n/a (type)
+ * @pubstate Immutable snapshot applied by AudioService via IDsp.
+ *
+ * Maps to SigmaStudio modules Si4674, ESP32, and St Mixer1.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+struct MixerState {
+    GainDb si4684Left;   ///< Si4674 volume control, left channel.
+    GainDb si4684Right;  ///< Si4674 volume control, right channel.
+    GainDb esp32Left;    ///< ESP32 volume control, left channel.
+    GainDb esp32Right;   ///< ESP32 volume control, right channel.
+    GainDb mixLeft;      ///< St Mixer1 left blend level.
+    GainDb mixRight;     ///< St Mixer1 right blend level.
+
+    /**
+     * @brief    factoryDefault — unity gains on all paths (radio-first mix).
+     *
+     * @dname    factoryDefault
+     * @return   MixerState with 0 dB on every control.
+     * @pubstate none
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] static MixerState factoryDefault() noexcept;
+};
+
+} // namespace core

Software/components/core/src/AudioProfile.cpp

@@ -0,0 +1,29 @@
+/**
+ * @file    AudioProfile.cpp
+ * @brief   AudioProfile factory defaults.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "core/AudioProfile.hpp"
+
+namespace core {
+
+AudioProfile AudioProfile::factoryDefault() noexcept
+{
+    const GainDb unity = GainDb::zero();
+    return AudioProfile{
+        .mixer = MixerState::factoryDefault(),
+        .eq = EqProfile::factoryDefault(),
+        .masterLeft = unity,
+        .masterRight = unity,
+    };
+}
+
+} // namespace core

Software/components/core/src/AudioProfileJson.cpp

@@ -0,0 +1,227 @@
+/**
+ * @file    AudioProfileJson.cpp
+ * @brief   AudioProfileJson implementation.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "core/AudioProfileJson.hpp"
+
+#include <cstdlib>
+#include <sstream>
+
+namespace core {
+
+namespace {
+
+[[nodiscard]] bool extractJsonFloat(std::string_view json,
+                                    std::string_view key,
+                                    float& out)
+{
+    const std::string needle =
+        std::string("\"") + std::string(key) + "\":";
+    const std::size_t start = json.find(needle);
+    if (start == std::string_view::npos) {
+        return false;
+    }
+    const std::size_t valueStart = start + needle.size();
+    char* end = nullptr;
+    out = std::strtof(json.data() + valueStart, &end);
+    return end != json.data() + valueStart;
+}
+
+[[nodiscard]] bool extractJsonUint(std::string_view json,
+                                   std::string_view key,
+                                   unsigned long& out)
+{
+    const std::string needle =
+        std::string("\"") + std::string(key) + "\":";
+    const std::size_t start = json.find(needle);
+    if (start == std::string_view::npos) {
+        return false;
+    }
+    const std::size_t valueStart = start + needle.size();
+    char* end = nullptr;
+    out = std::strtoul(json.data() + valueStart, &end, 10);
+    return end != json.data() + valueStart;
+}
+
+[[nodiscard]] std::expected<GainDb, ParseError> parseGainField(
+    std::string_view json, std::string_view key)
+{
+    float db = 0.0F;
+    if (!extractJsonFloat(json, key, db)) {
+        return std::unexpected(ParseError::MissingField);
+    }
+    return GainDb::tryFromDb(db);
+}
+
+[[nodiscard]] std::expected<MixerState, ParseError> parseMixerJson(
+    std::string_view json)
+{
+    const std::size_t mixerStart = json.find("\"mixer\"");
+    if (mixerStart == std::string_view::npos) {
+        return std::unexpected(ParseError::MissingField);
+    }
+    const std::string_view mixer = json.substr(mixerStart);
+
+    const auto si4684Left = parseGainField(mixer, "si4684_left_db");
+    const auto si4684Right = parseGainField(mixer, "si4684_right_db");
+    const auto esp32Left = parseGainField(mixer, "esp32_left_db");
+    const auto esp32Right = parseGainField(mixer, "esp32_right_db");
+    const auto mixLeft = parseGainField(mixer, "mix_left_db");
+    const auto mixRight = parseGainField(mixer, "mix_right_db");
+
+    if (!si4684Left || !si4684Right || !esp32Left || !esp32Right || !mixLeft
+        || !mixRight) {
+        return std::unexpected(ParseError::MissingField);
+    }
+
+    return MixerState{
+        .si4684Left = *si4684Left,
+        .si4684Right = *si4684Right,
+        .esp32Left = *esp32Left,
+        .esp32Right = *esp32Right,
+        .mixLeft = *mixLeft,
+        .mixRight = *mixRight,
+    };
+}
+
+[[nodiscard]] std::expected<EqProfile, ParseError> parseEqJson(
+    std::string_view json)
+{
+    EqProfile profile = EqProfile::factoryDefault();
+    const std::size_t eqStart = json.find("\"eq\"");
+    if (eqStart == std::string_view::npos) {
+        return std::unexpected(ParseError::MissingField);
+    }
+
+    std::string_view rest = json.substr(eqStart);
+    for (std::uint8_t band = 0; band < EqBandIndex::kBandCount; ++band) {
+        const std::size_t objStart = rest.find('{');
+        if (objStart == std::string_view::npos) {
+            return std::unexpected(ParseError::MissingField);
+        }
+        const std::size_t objEnd = rest.find('}', objStart);
+        if (objEnd == std::string_view::npos) {
+            return std::unexpected(ParseError::MissingField);
+        }
+        const std::string_view obj = rest.substr(objStart, objEnd - objStart + 1U);
+
+        float gainDb = 0.0F;
+        unsigned long centerHz = 0U;
+        float q = 0.0F;
+        if (!extractJsonFloat(obj, "gain_db", gainDb)
+            || !extractJsonUint(obj, "center_hz", centerHz)
+            || !extractJsonFloat(obj, "q", q)) {
+            return std::unexpected(ParseError::MissingField);
+        }
+        if (q < 0.2F || q > 10.0F) {
+            return std::unexpected(ParseError::MissingField);
+        }
+
+        const auto gain = GainDb::tryFromDb(gainDb);
+        const auto center = FrequencyHz::tryFromHz(
+            static_cast<std::uint32_t>(centerHz));
+        const auto index = EqBandIndex::tryFromIndex(band);
+        if (!gain || !center || !index) {
+            return std::unexpected(ParseError::MissingField);
+        }
+
+        profile.setBand(*index, EqBandSettings{
+                                   .gain = *gain,
+                                   .center = *center,
+                                   .q = q,
+                               });
+        rest = rest.substr(objEnd + 1U);
+    }
+    return profile;
+}
+
+} // namespace
+
+std::string serializeAudioProfileJson(const AudioProfile& profile)
+{
+    std::ostringstream out;
+    const auto& m = profile.mixer;
+    out << "{\"mixer\":{"
+        << "\"si4684_left_db\":" << m.si4684Left.value() << ','
+        << "\"si4684_right_db\":" << m.si4684Right.value() << ','
+        << "\"esp32_left_db\":" << m.esp32Left.value() << ','
+        << "\"esp32_right_db\":" << m.esp32Right.value() << ','
+        << "\"mix_left_db\":" << m.mixLeft.value() << ','
+        << "\"mix_right_db\":" << m.mixRight.value() << "},"
+        << "\"master\":{"
+        << "\"left_db\":" << profile.masterLeft.value() << ','
+        << "\"right_db\":" << profile.masterRight.value() << "},"
+        << "\"eq\":[";
+
+    const auto& bands = profile.eq.bands();
+    for (std::size_t i = 0; i < bands.size(); ++i) {
+        if (i > 0U) {
+            out << ',';
+        }
+        const auto& b = bands[i];
+        out << "{\"gain_db\":" << b.gain.value()
+            << ",\"center_hz\":" << b.center.value() << ",\"q\":" << b.q
+            << '}';
+    }
+    out << "]}";
+    return out.str();
+}
+
+std::expected<AudioProfile, ParseError> parseAudioProfileJson(
+    std::string_view json)
+{
+    if (json.find('{') == std::string_view::npos) {
+        return std::unexpected(ParseError::InvalidJson);
+    }
+
+    const auto mixer = parseMixerJson(json);
+    if (!mixer) {
+        return std::unexpected(mixer.error());
+    }
+
+    const auto eq = parseEqJson(json);
+    if (!eq) {
+        return std::unexpected(eq.error());
+    }
+
+    const std::size_t masterStart = json.find("\"master\"");
+    if (masterStart == std::string_view::npos) {
+        return std::unexpected(ParseError::MissingField);
+    }
+    const std::string_view master = json.substr(masterStart);
+    const auto masterLeft = parseGainField(master, "left_db");
+    const auto masterRight = parseGainField(master, "right_db");
+    if (!masterLeft || !masterRight) {
+        return std::unexpected(ParseError::MissingField);
+    }
+
+    return AudioProfile{
+        .mixer = *mixer,
+        .eq = *eq,
+        .masterLeft = *masterLeft,
+        .masterRight = *masterRight,
+    };
+}
+
+std::string serializeAudioSavedJson()
+{
+    return R"({"status":"saved"})";
+}
+
+std::string serializeAudioErrorJson(std::string_view reason)
+{
+    std::ostringstream out;
+    out << R"({"status":"error","reason":")" << reason << R"("})";
+    return out.str();
+}
+
+} // namespace core

Software/components/core/src/BiquadCoefficients.cpp

@@ -0,0 +1,30 @@
+/**
+ * @file    BiquadCoefficients.cpp
+ * @brief   BiquadCoefficients fixpoint conversion.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "core/BiquadCoefficients.hpp"
+#include "core/BiquadDesign.hpp"
+
+namespace core {
+
+std::array<std::int32_t, 5> BiquadCoefficients::toFixpoint823() const noexcept
+{
+    return {
+        floatToFixpoint823(b0),
+        floatToFixpoint823(b1),
+        floatToFixpoint823(b2),
+        floatToFixpoint823(a0),
+        floatToFixpoint823(a1),
+    };
+}
+
+} // namespace core

Software/components/core/src/BiquadDesign.cpp

@@ -0,0 +1,94 @@
+/**
+ * @file    BiquadDesign.cpp
+ * @brief   Biquad coefficient design and ADAU fixpoint helpers.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "core/BiquadDesign.hpp"
+
+#include <cmath>
+#include <numbers>
+
+namespace core {
+
+namespace {
+
+constexpr float kPi = std::numbers::pi_v<float>;
+
+[[nodiscard]] float clampQ(float q) noexcept
+{
+    if (q < 0.2F) {
+        return 0.2F;
+    }
+    if (q > 10.0F) {
+        return 10.0F;
+    }
+    return q;
+}
+
+} // namespace
+
+std::int32_t floatToFixpoint823(float value) noexcept
+{
+    const double scaled =
+        static_cast<double>(value) * static_cast<double>(1U << 23);
+    const auto rounded = static_cast<std::int64_t>(std::llround(scaled));
+    return static_cast<std::int32_t>(rounded);
+}
+
+std::int32_t gainDbToLinearFixpoint(GainDb gain) noexcept
+{
+    return floatToFixpoint823(gain.linear());
+}
+
+BiquadCoefficients designFlatEq() noexcept
+{
+    return BiquadCoefficients{
+        .b0 = 1.0F,
+        .b1 = 0.0F,
+        .b2 = 0.0F,
+        .a0 = 0.0F,
+        .a1 = 0.0F,
+    };
+}
+
+BiquadCoefficients designPeakingEq(FrequencyHz center, GainDb gain,
+                                   float q) noexcept
+{
+    if (std::fabs(gain.value()) < 0.001F) {
+        return designFlatEq();
+    }
+
+    const float qClamped = clampQ(q);
+    const float fs = static_cast<float>(kAdauSampleRateHz);
+    const float f0 = static_cast<float>(center.value());
+    const float a = std::pow(10.0F, gain.value() / 40.0F);
+    const float omega = 2.0F * kPi * f0 / fs;
+    const float sinOmega = std::sin(omega);
+    const float cosOmega = std::cos(omega);
+    const float alpha = sinOmega / (2.0F * qClamped);
+
+    const float b0 = 1.0F + alpha * a;
+    const float b1 = -2.0F * cosOmega;
+    const float b2 = 1.0F - alpha * a;
+    const float a0 = 1.0F + alpha / a;
+    const float a1 = -2.0F * cosOmega;
+    const float a2 = 1.0F - alpha / a;
+
+    return BiquadCoefficients{
+        .b0 = b0 / a0,
+        .b1 = b1 / a0,
+        .b2 = b2 / a0,
+        .a0 = a1 / a0,
+        .a1 = a2 / a0,
+    };
+}
+
+} // namespace core

Software/components/core/src/EqBandIndex.cpp

@@ -0,0 +1,37 @@
+/**
+ * @file    EqBandIndex.cpp
+ * @brief   EqBandIndex implementation.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "core/EqBandIndex.hpp"
+
+namespace core {
+
+EqBandIndex::EqBandIndex(std::uint8_t index) noexcept
+    : index_(index)
+{
+}
+
+std::expected<EqBandIndex, ParseError> EqBandIndex::tryFromIndex(
+    std::uint32_t index) noexcept
+{
+    if (index >= kBandCount) {
+        return std::unexpected(ParseError::MissingField);
+    }
+    return EqBandIndex(static_cast<std::uint8_t>(index));
+}
+
+std::uint8_t EqBandIndex::value() const noexcept
+{
+    return index_;
+}
+
+} // namespace core

Software/components/core/src/EqProfile.cpp

@@ -0,0 +1,71 @@
+/**
+ * @file    EqProfile.cpp
+ * @brief   EqProfile implementation.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "core/EqProfile.hpp"
+
+namespace core {
+
+namespace {
+
+constexpr std::array<std::uint32_t, EqBandIndex::kBandCount> kDefaultCenters{
+    40U, 100U, 250U, 1000U, 4000U, 12000U};
+
+[[nodiscard]] std::array<EqBandSettings, EqBandIndex::kBandCount>
+makeDefaultBands() noexcept
+{
+    return std::array<EqBandSettings, EqBandIndex::kBandCount>{
+        EqBandSettings{GainDb::zero(), *FrequencyHz::tryFromHz(kDefaultCenters[0]),
+                       1.414F},
+        EqBandSettings{GainDb::zero(), *FrequencyHz::tryFromHz(kDefaultCenters[1]),
+                       1.414F},
+        EqBandSettings{GainDb::zero(), *FrequencyHz::tryFromHz(kDefaultCenters[2]),
+                       1.414F},
+        EqBandSettings{GainDb::zero(), *FrequencyHz::tryFromHz(kDefaultCenters[3]),
+                       1.414F},
+        EqBandSettings{GainDb::zero(), *FrequencyHz::tryFromHz(kDefaultCenters[4]),
+                       1.414F},
+        EqBandSettings{GainDb::zero(), *FrequencyHz::tryFromHz(kDefaultCenters[5]),
+                       1.414F},
+    };
+}
+
+} // namespace
+
+EqProfile::EqProfile(
+    std::array<EqBandSettings, EqBandIndex::kBandCount> bands) noexcept
+    : bands_(bands)
+{
+}
+
+EqProfile EqProfile::factoryDefault() noexcept
+{
+    return EqProfile(makeDefaultBands());
+}
+
+const EqBandSettings& EqProfile::band(EqBandIndex index) const noexcept
+{
+    return bands_[index.value()];
+}
+
+void EqProfile::setBand(EqBandIndex index, EqBandSettings settings) noexcept
+{
+    bands_[index.value()] = settings;
+}
+
+const std::array<EqBandSettings, EqBandIndex::kBandCount>& EqProfile::bands()
+    const noexcept
+{
+    return bands_;
+}
+
+} // namespace core

Software/components/core/src/FrequencyHz.cpp

@@ -0,0 +1,37 @@
+/**
+ * @file    FrequencyHz.cpp
+ * @brief   FrequencyHz implementation.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "core/FrequencyHz.hpp"
+
+namespace core {
+
+FrequencyHz::FrequencyHz(std::uint32_t hz) noexcept
+    : hz_(hz)
+{
+}
+
+std::expected<FrequencyHz, ParseError> FrequencyHz::tryFromHz(
+    std::uint32_t hz) noexcept
+{
+    if (hz < kMinHz || hz > kMaxHz) {
+        return std::unexpected(ParseError::MissingField);
+    }
+    return FrequencyHz(hz);
+}
+
+std::uint32_t FrequencyHz::value() const noexcept
+{
+    return hz_;
+}
+
+} // namespace core

Software/components/core/src/GainDb.cpp

@@ -0,0 +1,46 @@
+/**
+ * @file    GainDb.cpp
+ * @brief   GainDb implementation.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "core/GainDb.hpp"
+
+namespace core {
+
+GainDb::GainDb(float db) noexcept
+    : db_(db)
+{
+}
+
+std::expected<GainDb, ParseError> GainDb::tryFromDb(float db) noexcept
+{
+    if (db < kMinDb || db > kMaxDb) {
+        return std::unexpected(ParseError::MissingField);
+    }
+    return GainDb(db);
+}
+
+GainDb GainDb::zero() noexcept
+{
+    return GainDb(0.0F);
+}
+
+float GainDb::value() const noexcept
+{
+    return db_;
+}
+
+float GainDb::linear() const noexcept
+{
+    return std::pow(10.0F, db_ / 20.0F);
+}
+
+} // namespace core

Software/components/core/src/MixerState.cpp

@@ -0,0 +1,31 @@
+/**
+ * @file    MixerState.cpp
+ * @brief   MixerState factory defaults.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "core/MixerState.hpp"
+
+namespace core {
+
+MixerState MixerState::factoryDefault() noexcept
+{
+    const GainDb unity = GainDb::zero();
+    return MixerState{
+        .si4684Left = unity,
+        .si4684Right = unity,
+        .esp32Left = unity,
+        .esp32Right = unity,
+        .mixLeft = unity,
+        .mixRight = unity,
+    };
+}
+
+} // namespace core

Software/components/core/test/CMakeLists.txt

@@ -22,6 +22,15 @@ add_library(digiradio_core STATIC
     "${CORE_SRC_DIR}/EmbeddedBlobReader.cpp"
     "${CORE_SRC_DIR}/TunerJson.cpp"
     "${CORE_SRC_DIR}/FrequencyKHz.cpp"
+    "${CORE_SRC_DIR}/GainDb.cpp"
+    "${CORE_SRC_DIR}/FrequencyHz.cpp"
+    "${CORE_SRC_DIR}/EqBandIndex.cpp"
+    "${CORE_SRC_DIR}/BiquadCoefficients.cpp"
+    "${CORE_SRC_DIR}/BiquadDesign.cpp"
+    "${CORE_SRC_DIR}/MixerState.cpp"
+    "${CORE_SRC_DIR}/EqProfile.cpp"
+    "${CORE_SRC_DIR}/AudioProfile.cpp"
+    "${CORE_SRC_DIR}/AudioProfileJson.cpp"
 )
 target_include_directories(digiradio_core PUBLIC
     "${CMAKE_CURRENT_SOURCE_DIR}/../include"
@@ -46,3 +55,11 @@ add_test(NAME tuner_json_test COMMAND tuner_json_test)
 add_executable(frequency_khz_test frequency_khz_test.cpp)
 target_link_libraries(frequency_khz_test PRIVATE digiradio_core)
 add_test(NAME frequency_khz_test COMMAND frequency_khz_test)
+
+add_executable(biquad_design_test biquad_design_test.cpp)
+target_link_libraries(biquad_design_test PRIVATE digiradio_core)
+add_test(NAME biquad_design_test COMMAND biquad_design_test)
+
+add_executable(audio_profile_json_test audio_profile_json_test.cpp)
+target_link_libraries(audio_profile_json_test PRIVATE digiradio_core)
+add_test(NAME audio_profile_json_test COMMAND audio_profile_json_test)

Software/components/core/test/audio_profile_json_test.cpp

@@ -0,0 +1,66 @@
+/**
+ * @file    audio_profile_json_test.cpp
+ * @brief   Host tests for AudioProfile JSON round-trip.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "core/AudioProfile.hpp"
+#include "core/AudioProfileJson.hpp"
+#include "core/EqBandIndex.hpp"
+#include "core/FrequencyHz.hpp"
+#include "core/GainDb.hpp"
+
+#include <cstdlib>
+#include <iostream>
+
+namespace {
+
+[[nodiscard]] int runRoundTripTest()
+{
+    core::AudioProfile profile = core::AudioProfile::factoryDefault();
+    const auto band = core::EqBandIndex::tryFromIndex(2U);
+    const auto gain = core::GainDb::tryFromDb(3.0F);
+    const auto center = core::FrequencyHz::tryFromHz(500U);
+    if (!band || !gain || !center) {
+        std::cerr << "test setup failed\n";
+        return EXIT_FAILURE;
+    }
+    profile.eq.setBand(*band,
+                       core::EqBandSettings{
+                           .gain = *gain,
+                           .center = *center,
+                           .q = 2.0F,
+                       });
+
+    const std::string json = core::serializeAudioProfileJson(profile);
+    const auto parsed = core::parseAudioProfileJson(json);
+    if (!parsed) {
+        std::cerr << "parse failed\n";
+        return EXIT_FAILURE;
+    }
+
+    const auto& b = parsed->eq.band(*band);
+    if (b.gain.value() != 3.0F || b.center.value() != 500U
+        || b.q != 2.0F) {
+        std::cerr << "round-trip mismatch\n";
+        return EXIT_FAILURE;
+    }
+    return EXIT_SUCCESS;
+}
+
+} // namespace
+
+int main()
+{
+    if (runRoundTripTest() != EXIT_SUCCESS) {
+        return EXIT_FAILURE;
+    }
+    return EXIT_SUCCESS;
+}

Software/components/core/test/biquad_design_test.cpp

@@ -0,0 +1,68 @@
+/**
+ * @file    biquad_design_test.cpp
+ * @brief   Host tests for ADAU biquad design and fixpoint conversion.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "core/BiquadDesign.hpp"
+#include "core/FrequencyHz.hpp"
+#include "core/GainDb.hpp"
+
+#include <cmath>
+#include <cstdlib>
+#include <iostream>
+
+namespace {
+
+[[nodiscard]] int runUnityGainFixpointTest()
+{
+    const auto gain = core::GainDb::zero();
+    const std::int32_t fix = core::gainDbToLinearFixpoint(gain);
+    if (fix != 0x00800000) {
+        std::cerr << "expected 0 dB fixpoint 0x00800000, got 0x"
+                  << std::hex << fix << std::dec << '\n';
+        return EXIT_FAILURE;
+    }
+    return EXIT_SUCCESS;
+}
+
+[[nodiscard]] int runFlatBiquadTest()
+{
+    const core::BiquadCoefficients flat = core::designFlatEq();
+    if (std::fabs(flat.b0 - 1.0F) > 0.001F || std::fabs(flat.b1) > 0.001F
+        || std::fabs(flat.b2) > 0.001F || std::fabs(flat.a0) > 0.001F
+        || std::fabs(flat.a1) > 0.001F) {
+        std::cerr << "flat biquad coefficients unexpected\n";
+        return EXIT_FAILURE;
+    }
+
+    const auto center = core::FrequencyHz::tryFromHz(1000U);
+    const auto zero = core::GainDb::zero();
+    const core::BiquadCoefficients peaking =
+        core::designPeakingEq(*center, zero, 1.414F);
+    if (std::fabs(peaking.b0 - 1.0F) > 0.001F) {
+        std::cerr << "0 dB peaking should match flat b0\n";
+        return EXIT_FAILURE;
+    }
+    return EXIT_SUCCESS;
+}
+
+} // namespace
+
+int main()
+{
+    if (runUnityGainFixpointTest() != EXIT_SUCCESS) {
+        return EXIT_FAILURE;
+    }
+    if (runFlatBiquadTest() != EXIT_SUCCESS) {
+        return EXIT_FAILURE;
+    }
+    return EXIT_SUCCESS;
+}

Software/components/drivers/adau1701/CMakeLists.txt

@@ -3,12 +3,13 @@ set(ADAU_FW_DIR "${CMAKE_CURRENT_LIST_DIR}/../../../Firmware/ADAU1701-Firmware")
 idf_component_register(
     SRCS
         "src/Adau1701Driver.cpp"
+        "src/Adau1701Dsp.cpp"
         "src/SigmaStudioFW.c"
         "src/adau1701_program.c"
     INCLUDE_DIRS
         "include"
         "${ADAU_FW_DIR}"
-    REQUIRES driver esp_driver_gpio esp_driver_i2c
+    REQUIRES core driver esp_driver_gpio esp_driver_i2c
 )
 
 target_compile_features(${COMPONENT_LIB} PUBLIC cxx_std_23)

Software/components/drivers/adau1701/include/adau1701/Adau1701Driver.hpp

@@ -1,6 +1,6 @@
 /**
  * @file    Adau1701Driver.hpp
- * @brief   ADAU1701 SigmaDSP driver — RAM boot on every power-up.
+ * @brief   ADAU1701 SigmaDSP driver — RAM boot and runtime safeload control.
  *
  * DigiRadio firmware — https://github.com/manvalan/DigiRadio
  *
@@ -14,6 +14,15 @@
 
 #include "adau1701/Adau1701Error.hpp"
 
+#include "core/AudioProfile.hpp"
+#include "core/EqBandIndex.hpp"
+#include "core/EqProfile.hpp"
+#include "core/FrequencyHz.hpp"
+#include "core/GainDb.hpp"
+#include "core/MixSource.hpp"
+#include "core/MixerState.hpp"
+
+#include <cstdint>
 #include <expected>
 
 namespace adau1701 {
@@ -36,7 +45,7 @@ struct Adau1701Pins {
 };
 
 /**
- * @brief    Adau1701Driver — owns I2C + reset and loads SigmaStudio RAM.
+ * @brief    Adau1701Driver — owns I2C + reset, boot, and safeload runtime.
  *
  * @dname    Adau1701Driver
  * @param    pins  Board wiring for I2C and RESET#.
@@ -45,7 +54,8 @@ struct Adau1701Pins {
  *           successful default_download replay.
  *
  * Writes the SigmaStudio export from Firmware/ADAU1701-Firmware on every
- * boot (no EEPROM self-boot on DigiRadio).
+ * boot (no EEPROM self-boot on DigiRadio). Runtime EQ and mixer changes
+ * use the ADAU1701 safeload mechanism.
  *
  * @author   Michele Bigi
  * @date     2026-07-06
@@ -102,7 +112,104 @@ public:
      */
     [[nodiscard]] bool isBooted() const noexcept;
 
+    /**
+     * @brief    applyProfile — safeload mixer, EQ, and master from snapshot.
+     *
+     * @dname    applyProfile
+     * @param    profile  User audio configuration.
+     * @return   Ok on success, or Adau1701Error.
+     * @pubstate writes parameter RAM via safeload.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] std::expected<void, Adau1701Error> applyProfile(
+        const core::AudioProfile& profile);
+
+    /**
+     * @brief    applyMixer — safeload input and stereo-mixer gains.
+     *
+     * @dname    applyMixer
+     * @param    mixer  Per-source and St Mixer1 levels.
+     * @return   Ok on success, or Adau1701Error.
+     * @pubstate writes parameter RAM via safeload.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] std::expected<void, Adau1701Error> applyMixer(
+        const core::MixerState& mixer);
+
+    /**
+     * @brief    applyEq — safeload all six PEQ bands.
+     *
+     * @dname    applyEq
+     * @param    eq  Six-band parametric EQ settings.
+     * @return   Ok on success, or Adau1701Error.
+     * @pubstate writes parameter RAM via safeload.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] std::expected<void, Adau1701Error> applyEq(
+        const core::EqProfile& eq);
+
+    /**
+     * @brief    setInputVolume — safeload one input path gain.
+     *
+     * @dname    setInputVolume
+     * @param    source  Si4684 or ESP32 I2S path.
+     * @param    left    Left channel gain.
+     * @param    right   Right channel gain.
+     * @return   Ok on success, or Adau1701Error.
+     * @pubstate writes parameter RAM via safeload.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] std::expected<void, Adau1701Error> setInputVolume(
+        core::MixSource source, core::GainDb left, core::GainDb right);
+
+    /**
+     * @brief    setMasterVolume — safeload Multiple 1 master output gain.
+     *
+     * @dname    setMasterVolume
+     * @param    left   Left master gain.
+     * @param    right  Right master gain.
+     * @return   Ok on success, or Adau1701Error.
+     * @pubstate writes parameter RAM via safeload.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] std::expected<void, Adau1701Error> setMasterVolume(
+        core::GainDb left, core::GainDb right);
+
+    /**
+     * @brief    setEqBand — design and safeload one PEQ band.
+     *
+     * @dname    setEqBand
+     * @param    band   Band index 0..5.
+     * @param    gain   Band gain in dB.
+     * @param    center Centre frequency.
+     * @param    q      Quality factor.
+     * @return   Ok on success, or Adau1701Error.
+     * @pubstate writes five coefficients in one safeload transfer.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] std::expected<void, Adau1701Error> setEqBand(
+        core::EqBandIndex band, core::GainDb gain, core::FrequencyHz center,
+        float q);
+
 private:
+    [[nodiscard]] std::expected<void, Adau1701Error> ensureBooted() const;
+    [[nodiscard]] std::expected<void, Adau1701Error> safeloadGain(
+        unsigned paramAddr, core::GainDb gain);
+    [[nodiscard]] std::expected<void, Adau1701Error> safeloadFixpoint(
+        unsigned paramAddr, std::int32_t fixpoint);
+
     Adau1701Pins pins_;
     bool booted_;
     void* i2cBus_;

Software/components/drivers/adau1701/include/adau1701/Adau1701Dsp.hpp

@@ -0,0 +1,70 @@
+/**
+ * @file    Adau1701Dsp.hpp
+ * @brief   core::IDsp adapter over Adau1701Driver.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include "adau1701/Adau1701Driver.hpp"
+
+#include "core/IDsp.hpp"
+
+namespace adau1701 {
+
+/**
+ * @brief    Adau1701Dsp — maps core::IDsp to Adau1701Driver safeload API.
+ *
+ * @dname    Adau1701Dsp
+ * @return   n/a (type)
+ * @pubstate Borrows Adau1701Driver for the process lifetime.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+class Adau1701Dsp final : public core::IDsp {
+public:
+    /**
+     * @brief    Adau1701Dsp — bind to the board driver instance.
+     *
+     * @dname    Adau1701Dsp
+     * @param    driver  Booted ADAU1701 driver (must outlive this adapter).
+     * @pubstate stores driver reference.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    explicit Adau1701Dsp(Adau1701Driver& driver);
+
+    [[nodiscard]] std::expected<void, core::DspError> applyProfile(
+        const core::AudioProfile& profile) override;
+
+    [[nodiscard]] std::expected<void, core::DspError> applyMixer(
+        const core::MixerState& mixer) override;
+
+    [[nodiscard]] std::expected<void, core::DspError> applyEq(
+        const core::EqProfile& eq) override;
+
+    [[nodiscard]] std::expected<void, core::DspError> setInputVolume(
+        core::MixSource source, core::GainDb left, core::GainDb right) override;
+
+    [[nodiscard]] std::expected<void, core::DspError> setMasterVolume(
+        core::GainDb left, core::GainDb right) override;
+
+    [[nodiscard]] std::expected<void, core::DspError> setEqBand(
+        core::EqBandIndex band, core::GainDb gain, core::FrequencyHz center,
+        float q) override;
+
+private:
+    [[nodiscard]] static core::DspError mapError(Adau1701Error error) noexcept;
+
+    Adau1701Driver& driver_;
+};
+
+} // namespace adau1701

Software/components/drivers/adau1701/include/adau1701/Adau1701Error.hpp

@@ -28,6 +28,8 @@ enum class Adau1701Error {
     I2cInitFailed,
     ResetFailed,
     DownloadFailed,
+    NotBooted,
+    SafeloadFailed,
 };
 
 } // namespace adau1701

Software/components/drivers/adau1701/include/adau1701/Adau1701ParamMap.hpp

@@ -0,0 +1,77 @@
+/**
+ * @file    Adau1701ParamMap.hpp
+ * @brief   SigmaStudio parameter RAM addresses for DigiRadio runtime control.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include "DigiRadio_IC_1_PARAM.h"
+
+#include "core/EqBandIndex.hpp"
+#include "core/MixSource.hpp"
+
+#include <cstdint>
+
+namespace adau1701 {
+
+/**
+ * @brief    paramAddrEqBandBase — first coefficient address for a PEQ band.
+ *
+ * @dname    paramAddrEqBandBase
+ * @param    band  Band index 0..5.
+ * @return   ADDR_PARAMEQ1_STn_B0 from the SigmaStudio export.
+ * @pubstate none
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+[[nodiscard]] inline unsigned paramAddrEqBandBase(std::uint8_t band) noexcept
+{
+    return static_cast<unsigned>(ADDR_PARAMEQ1_ST0_B0)
+           + static_cast<unsigned>(band) * 5U;
+}
+
+/**
+ * @brief    paramAddrInputLeft — left volume address for an input source.
+ *
+ * @dname    paramAddrInputLeft
+ * @param    source  Si4684 or ESP32 path.
+ * @return   Parameter RAM address for the left channel gain cell.
+ * @pubstate none
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+[[nodiscard]] inline unsigned paramAddrInputLeft(core::MixSource source) noexcept
+{
+    return source == core::MixSource::Si4684
+               ? static_cast<unsigned>(ADDR_SI4674)
+               : static_cast<unsigned>(ADDR_ESP32);
+}
+
+/**
+ * @brief    paramAddrInputRight — right volume address for an input source.
+ *
+ * @dname    paramAddrInputRight
+ * @param    source  Si4684 or ESP32 path.
+ * @return   Parameter RAM address for the right channel gain cell.
+ * @pubstate none
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+[[nodiscard]] inline unsigned paramAddrInputRight(core::MixSource source) noexcept
+{
+    return source == core::MixSource::Si4684
+               ? static_cast<unsigned>(ADDR_SI4674_1)
+               : static_cast<unsigned>(ADDR_ESP32_1);
+}
+
+} // namespace adau1701

Software/components/drivers/adau1701/src/Adau1701Driver.cpp

@@ -13,6 +13,11 @@
 
 #include "adau1701/Adau1701Driver.hpp"
 
+#include "adau1701/Adau1701ParamMap.hpp"
+
+#include "core/BiquadDesign.hpp"
+
+#include "DigiRadio_IC_1_PARAM.h"
 #include "SigmaStudioFW.h"
 
 #include "driver/gpio.h"
@@ -115,4 +120,139 @@ bool Adau1701Driver::isBooted() const noexcept
     return booted_;
 }
 
+std::expected<void, Adau1701Error> Adau1701Driver::ensureBooted() const
+{
+    if (!booted_) {
+        return std::unexpected(Adau1701Error::NotBooted);
+    }
+    return {};
+}
+
+std::expected<void, Adau1701Error> Adau1701Driver::safeloadFixpoint(
+    unsigned paramAddr, std::int32_t fixpoint)
+{
+    if (sigma_safeload_param(paramAddr, fixpoint) != 0) {
+        return std::unexpected(Adau1701Error::SafeloadFailed);
+    }
+    return {};
+}
+
+std::expected<void, Adau1701Error> Adau1701Driver::safeloadGain(
+    unsigned paramAddr, core::GainDb gain)
+{
+    return safeloadFixpoint(paramAddr, core::gainDbToLinearFixpoint(gain));
+}
+
+std::expected<void, Adau1701Error> Adau1701Driver::setInputVolume(
+    core::MixSource source, core::GainDb left, core::GainDb right)
+{
+    if (auto ready = ensureBooted(); !ready) {
+        return ready;
+    }
+    if (auto result = safeloadGain(paramAddrInputLeft(source), left); !result) {
+        return result;
+    }
+    return safeloadGain(paramAddrInputRight(source), right);
+}
+
+std::expected<void, Adau1701Error> Adau1701Driver::setMasterVolume(
+    core::GainDb left, core::GainDb right)
+{
+    if (auto ready = ensureBooted(); !ready) {
+        return ready;
+    }
+    if (auto result = safeloadGain(static_cast<unsigned>(ADDR_MULTIPLE1), left);
+        !result) {
+        return result;
+    }
+    return safeloadGain(static_cast<unsigned>(ADDR_MULTIPLE1_1), right);
+}
+
+std::expected<void, Adau1701Error> Adau1701Driver::applyMixer(
+    const core::MixerState& mixer)
+{
+    if (auto ready = ensureBooted(); !ready) {
+        return ready;
+    }
+    if (auto result = setInputVolume(core::MixSource::Si4684, mixer.si4684Left,
+                                     mixer.si4684Right);
+        !result) {
+        return result;
+    }
+    if (auto result = setInputVolume(core::MixSource::Esp32, mixer.esp32Left,
+                                     mixer.esp32Right);
+        !result) {
+        return result;
+    }
+    if (auto result =
+            safeloadGain(static_cast<unsigned>(ADDR_STMIXER1_ST0_VOLUME),
+                         mixer.mixLeft);
+        !result) {
+        return result;
+    }
+    return safeloadGain(static_cast<unsigned>(ADDR_STMIXER1_ST1_VOLUME),
+                        mixer.mixRight);
+}
+
+std::expected<void, Adau1701Error> Adau1701Driver::setEqBand(
+    core::EqBandIndex band, core::GainDb gain, core::FrequencyHz center, float q)
+{
+    if (auto ready = ensureBooted(); !ready) {
+        return ready;
+    }
+
+    const core::BiquadCoefficients coeffs =
+        core::designPeakingEq(center, gain, q);
+    const auto fixpoints = coeffs.toFixpoint823();
+    const unsigned baseAddr = paramAddrEqBandBase(band.value());
+
+    unsigned addrs[5U];
+    int values[5U];
+    for (unsigned i = 0U; i < 5U; ++i) {
+        addrs[i] = baseAddr + i;
+        values[i] = fixpoints[i];
+    }
+
+    if (sigma_safeload_block(5U, addrs, values) != 0) {
+        return std::unexpected(Adau1701Error::SafeloadFailed);
+    }
+    return {};
+}
+
+std::expected<void, Adau1701Error> Adau1701Driver::applyEq(
+    const core::EqProfile& eq)
+{
+    if (auto ready = ensureBooted(); !ready) {
+        return ready;
+    }
+
+    for (std::uint8_t i = 0; i < core::EqBandIndex::kBandCount; ++i) {
+        const auto index = core::EqBandIndex::tryFromIndex(i);
+        if (!index) {
+            return std::unexpected(Adau1701Error::SafeloadFailed);
+        }
+        const core::EqBandSettings& band = eq.band(*index);
+        if (auto result = setEqBand(*index, band.gain, band.center, band.q);
+            !result) {
+            return result;
+        }
+    }
+    return {};
+}
+
+std::expected<void, Adau1701Error> Adau1701Driver::applyProfile(
+    const core::AudioProfile& profile)
+{
+    if (auto ready = ensureBooted(); !ready) {
+        return ready;
+    }
+    if (auto result = applyMixer(profile.mixer); !result) {
+        return result;
+    }
+    if (auto result = applyEq(profile.eq); !result) {
+        return result;
+    }
+    return setMasterVolume(profile.masterLeft, profile.masterRight);
+}
+
 } // namespace adau1701

Software/components/drivers/adau1701/src/Adau1701Dsp.cpp

@@ -0,0 +1,90 @@
+/**
+ * @file    Adau1701Dsp.cpp
+ * @brief   Adau1701Dsp implementation.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "adau1701/Adau1701Dsp.hpp"
+
+namespace adau1701 {
+
+Adau1701Dsp::Adau1701Dsp(Adau1701Driver& driver)
+    : driver_(driver)
+{
+}
+
+core::DspError Adau1701Dsp::mapError(Adau1701Error error) noexcept
+{
+    switch (error) {
+    case Adau1701Error::NotBooted:
+        return core::DspError::NotBooted;
+    case Adau1701Error::SafeloadFailed:
+        return core::DspError::SafeloadFailed;
+    default:
+        return core::DspError::SafeloadFailed;
+    }
+}
+
+std::expected<void, core::DspError> Adau1701Dsp::applyProfile(
+    const core::AudioProfile& profile)
+{
+    if (auto result = driver_.applyProfile(profile); !result) {
+        return std::unexpected(mapError(result.error()));
+    }
+    return {};
+}
+
+std::expected<void, core::DspError> Adau1701Dsp::applyMixer(
+    const core::MixerState& mixer)
+{
+    if (auto result = driver_.applyMixer(mixer); !result) {
+        return std::unexpected(mapError(result.error()));
+    }
+    return {};
+}
+
+std::expected<void, core::DspError> Adau1701Dsp::applyEq(
+    const core::EqProfile& eq)
+{
+    if (auto result = driver_.applyEq(eq); !result) {
+        return std::unexpected(mapError(result.error()));
+    }
+    return {};
+}
+
+std::expected<void, core::DspError> Adau1701Dsp::setInputVolume(
+    core::MixSource source, core::GainDb left, core::GainDb right)
+{
+    if (auto result = driver_.setInputVolume(source, left, right); !result) {
+        return std::unexpected(mapError(result.error()));
+    }
+    return {};
+}
+
+std::expected<void, core::DspError> Adau1701Dsp::setMasterVolume(
+    core::GainDb left, core::GainDb right)
+{
+    if (auto result = driver_.setMasterVolume(left, right); !result) {
+        return std::unexpected(mapError(result.error()));
+    }
+    return {};
+}
+
+std::expected<void, core::DspError> Adau1701Dsp::setEqBand(
+    core::EqBandIndex band, core::GainDb gain, core::FrequencyHz center,
+    float q)
+{
+    if (auto result = driver_.setEqBand(band, gain, center, q); !result) {
+        return std::unexpected(mapError(result.error()));
+    }
+    return {};
+}
+
+} // namespace adau1701

Software/components/drivers/adau1701/src/SigmaStudioFW.c

@@ -58,3 +58,78 @@ void SIGMA_WRITE_REGISTER_BLOCK(unsigned char devAddress,
         remaining = (unsigned char)(remaining - chunk);
     }
 }
+
+static int sigma_i2c_write(unsigned int reg, const unsigned char* data,
+                           unsigned char length)
+{
+    if (s_dev == NULL || data == NULL || length == 0U) {
+        return -1;
+    }
+
+    unsigned char buf[2U + 4U];
+    if ((size_t)length + 2U > sizeof(buf)) {
+        return -1;
+    }
+    buf[0] = (unsigned char)((reg >> 8) & 0xFFU);
+    buf[1] = (unsigned char)(reg & 0xFFU);
+    memcpy(buf + 2U, data, length);
+    const esp_err_t err =
+        i2c_master_transmit(s_dev, buf, (size_t)(2U + length), 1000);
+    return err == ESP_OK ? 0 : -1;
+}
+
+static int sigma_write_fixpoint_reg(unsigned int reg, int fixpoint)
+{
+    unsigned char payload[4U];
+    payload[0] = 0U;
+    payload[1] = (unsigned char)((fixpoint >> 16) & 0xFFU);
+    payload[2] = (unsigned char)((fixpoint >> 8) & 0xFFU);
+    payload[3] = (unsigned char)(fixpoint & 0xFFU);
+    return sigma_i2c_write(reg, payload, 4U);
+}
+
+static int sigma_write_param_addr(unsigned int reg, unsigned int paramAddr)
+{
+    unsigned char payload[2U];
+    payload[0] = (unsigned char)((paramAddr >> 8) & 0xFFU);
+    payload[1] = (unsigned char)(paramAddr & 0xFFU);
+    return sigma_i2c_write(reg, payload, 2U);
+}
+
+static int sigma_trigger_safeload(void)
+{
+    unsigned char payload[2U];
+    payload[0] = (unsigned char)((ADAU1701_CORE_IST_TRIGGER >> 8) & 0xFFU);
+    payload[1] = (unsigned char)(ADAU1701_CORE_IST_TRIGGER & 0xFFU);
+    return sigma_i2c_write(ADAU1701_CORE_CONTROL_REG, payload, 2U);
+}
+
+int sigma_safeload_param(unsigned int paramAddr, int fixpoint)
+{
+    const unsigned int addrs[1U] = {paramAddr};
+    const int values[1U] = {fixpoint};
+    return sigma_safeload_block(1U, addrs, values);
+}
+
+int sigma_safeload_block(unsigned char count, const unsigned int* paramAddrs,
+                         const int* fixpoints)
+{
+    if (count == 0U || count > 5U || paramAddrs == NULL || fixpoints == NULL) {
+        return -1;
+    }
+
+    for (unsigned char i = 0U; i < count; ++i) {
+        const unsigned int dataReg =
+            ADAU1701_SAFELOAD_DATA_BASE + (unsigned int)i;
+        const unsigned int addrReg =
+            ADAU1701_SAFELOAD_ADDR_BASE + (unsigned int)i;
+        if (sigma_write_fixpoint_reg(dataReg, fixpoints[i]) != 0) {
+            return -1;
+        }
+        if (sigma_write_param_addr(addrReg, paramAddrs[i]) != 0) {
+            return -1;
+        }
+    }
+
+    return sigma_trigger_safeload();
+}

Software/components/net/CMakeLists.txt

@@ -7,7 +7,7 @@ idf_component_register(
         "src/NetBootstrap.cpp"
     INCLUDE_DIRS "include"
     EMBED_FILES "www/index.html.gz"
-    REQUIRES core esp_wifi esp_netif esp_event nvs_flash esp_http_server tuner
+    REQUIRES core esp_wifi esp_netif esp_event nvs_flash esp_http_server tuner audio
 )
 
 target_compile_features(${COMPONENT_LIB} PUBLIC cxx_std_23)

Software/components/net/include/net/NetBootstrap.hpp

@@ -27,6 +27,10 @@
 #include <expected>
 #include <optional>
 
+namespace audio {
+class AudioService;
+} // namespace audio
+
 namespace tuner {
 class TunerService;
 } // namespace tuner
@@ -52,6 +56,7 @@ public:
      * @dname    start
      * @param    store  Secure store consulted for saved STA credentials.
      * @param    tuner  Tuner service exposed by the HTTP API.
+     * @param    audio  Audio service exposed by the HTTP API.
      * @return   NetBootstrap on success, or a NetError.
      * @pubstate none
      *
@@ -59,7 +64,8 @@ public:
      * @date     2026-07-06
      */
     [[nodiscard]] static std::expected<NetBootstrap, NetError>
-    start(core::ISecureStore& store, tuner::TunerService& tuner);
+    start(core::ISecureStore& store, tuner::TunerService& tuner,
+          audio::AudioService& audio);
 
     NetBootstrap(const NetBootstrap&) = delete;
     NetBootstrap& operator=(const NetBootstrap&) = delete;

Software/components/net/include/net/SetupWebServer.hpp

@@ -25,6 +25,10 @@
 
 struct httpd_req;
 
+namespace audio {
+class AudioService;
+} // namespace audio
+
 namespace tuner {
 class TunerService;
 } // namespace tuner
@@ -47,6 +51,7 @@ namespace net {
 struct HttpRouteContext {
     core::ISecureStore* store;        ///< Secure store for Wi-Fi provisioning.
     tuner::TunerService* tuner;       ///< Tuner service for tuner REST routes.
+    audio::AudioService* audio;       ///< Audio service for ADAU1701 REST routes.
 };
 
 /**
@@ -119,21 +124,24 @@ public:
      * @param    store     Secure store for POST /api/wifi persistence.
      * @param    netState  Active network phase exposed to handlers.
      * @param    tuner     Tuner service for the tuner REST routes.
+     * @param    audio     Audio service for the audio REST routes.
      * @return   Ok on success, or NetError::HttpServerStartFailed.
-     * @pubstate writes server_, store_, netState_, and tuner_ on success.
+     * @pubstate writes server_, store_, netState_, tuner_, and audio_ on success.
      *
      * @author   Michele Bigi
      * @date     2026-07-06
      */
     [[nodiscard]] std::expected<void, NetError> start(core::ISecureStore& store,
                                                       NetState netState,
-                                                      tuner::TunerService& tuner);
+                                                      tuner::TunerService& tuner,
+                                                      audio::AudioService& audio);
 
 private:
     httpd_handle* server_;
     core::ISecureStore* store_;
     NetState netState_;
     tuner::TunerService* tuner_;
+    audio::AudioService* audio_;
     HttpRouteContext routeContext_;
 };
 

Software/components/net/src/NetBootstrap.cpp

@@ -23,6 +23,7 @@
 #include "esp_netif.h"
 #include "esp_wifi.h"
 #include "nvs_flash.h"
+#include "audio/AudioService.hpp"
 #include "tuner/TunerService.hpp"
 
 namespace net {
@@ -98,7 +99,8 @@ constexpr char kTag[] = "NetBootstrap";
  * @date     2026-07-06
  */
 [[nodiscard]] std::expected<NetBootstrap, NetError>
-startSetupMode(core::ISecureStore& store, tuner::TunerService& tuner)
+startSetupMode(core::ISecureStore& store, tuner::TunerService& tuner,
+               audio::AudioService& audio)
 {
     esp_netif_create_default_wifi_ap();
 
@@ -108,7 +110,8 @@ startSetupMode(core::ISecureStore& store, tuner::TunerService& tuner)
     }
 
     SetupWebServer webServer;
-    if (auto webResult = webServer.start(store, NetState::SoftApSetup, tuner);
+    if (auto webResult =
+            webServer.start(store, NetState::SoftApSetup, tuner, audio);
         !webResult) {
         return std::unexpected(webResult.error());
     }
@@ -130,7 +133,8 @@ startSetupMode(core::ISecureStore& store, tuner::TunerService& tuner)
  * @date     2026-07-06
  */
 [[nodiscard]] std::expected<NetBootstrap, NetError>
-startStaMode(core::ISecureStore& store, tuner::TunerService& tuner)
+startStaMode(core::ISecureStore& store, tuner::TunerService& tuner,
+             audio::AudioService& audio)
 {
     auto credsResult = store.loadWifiCredentials();
     if (!credsResult) {
@@ -146,7 +150,8 @@ startStaMode(core::ISecureStore& store, tuner::TunerService& tuner)
     }
 
     SetupWebServer webServer;
-    if (auto webResult = webServer.start(store, NetState::StaConnected, tuner);
+    if (auto webResult =
+            webServer.start(store, NetState::StaConnected, tuner, audio);
         !webResult) {
         return std::unexpected(webResult.error());
     }
@@ -159,7 +164,8 @@ startStaMode(core::ISecureStore& store, tuner::TunerService& tuner)
 } // namespace
 
 std::expected<NetBootstrap, NetError>
-NetBootstrap::start(core::ISecureStore& store, tuner::TunerService& tuner)
+NetBootstrap::start(core::ISecureStore& store, tuner::TunerService& tuner,
+                    audio::AudioService& audio)
 {
     if (auto platform = initPlatform(); !platform) {
         return std::unexpected(platform.error());
@@ -170,14 +176,14 @@ NetBootstrap::start(core::ISecureStore& store, tuner::TunerService& tuner)
     }
 
     if (store.hasWifiCredentials()) {
-        auto staResult = startStaMode(store, tuner);
+        auto staResult = startStaMode(store, tuner, audio);
         if (staResult) {
             return staResult;
         }
         ESP_LOGW(kTag, "STA join failed — falling back to setup SoftAP");
     }
 
-    return startSetupMode(store, tuner);
+    return startSetupMode(store, tuner, audio);
 }
 
 NetBootstrap::NetBootstrap(std::optional<SoftApHost> softAp,

Software/components/net/src/SetupWebServer.cpp

@@ -18,6 +18,8 @@
 
 #include "net/SetupWebServer.hpp"
 
+#include "core/AudioProfile.hpp"
+#include "core/AudioProfileJson.hpp"
 #include "core/FirmwareVersion.hpp"
 #include "core/HealthStatus.hpp"
 #include "core/HealthStatusJson.hpp"
@@ -26,6 +28,7 @@
 #include "core/TunerJson.hpp"
 #include "core/WifiProvisionJson.hpp"
 #include "tuner/TunerService.hpp"
+#include "audio/AudioService.hpp"
 
 #include "esp_http_server.h"
 #include "esp_log.h"
@@ -40,7 +43,7 @@ namespace net {
 
 namespace {
 constexpr char kTag[] = "SetupWebServer";
-constexpr char kFirmwareVersion[] = "0.4.0";
+constexpr char kFirmwareVersion[] = "0.5.0";
 constexpr unsigned kRebootDelaySec = 3;
 
 extern const uint8_t www_index_html_gz_start[] asm(
@@ -363,6 +366,109 @@ esp_err_t tunerSeekPostHandler(httpd_req_t* req)
     return httpd_resp_send(req, json.c_str(), json.size());
 }
 
+/**
+ * @brief    audioProfileGetHandler — serve GET /api/audio/profile JSON.
+ *
+ * @dname    audioProfileGetHandler
+ * @param    req  HTTP request handle from esp_http_server.
+ * @return   ESP_OK on success, or an esp_err_t error code.
+ * @pubstate reads route context audio service snapshot.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+esp_err_t audioProfileGetHandler(httpd_req_t* req)
+{
+    auto* ctx = routeContextFrom(req);
+    if (ctx == nullptr || ctx->audio == nullptr) {
+        httpd_resp_set_status(req, "503 Service Unavailable");
+        return httpd_resp_send(req, nullptr, 0);
+    }
+    const std::string json =
+        core::serializeAudioProfileJson(ctx->audio->currentProfile());
+    httpd_resp_set_type(req, "application/json");
+    return httpd_resp_send(req, json.c_str(), json.size());
+}
+
+/**
+ * @brief    audioProfilePutHandler — accept PUT /api/audio/profile JSON.
+ *
+ * @dname    audioProfilePutHandler
+ * @param    req  HTTP request handle from esp_http_server.
+ * @return   ESP_OK on success, or an esp_err_t error code.
+ * @pubstate parses profile, safeloads ADAU1701, persists to NVS.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+esp_err_t audioProfilePutHandler(httpd_req_t* req)
+{
+    auto* ctx = routeContextFrom(req);
+    if (ctx == nullptr || ctx->audio == nullptr) {
+        httpd_resp_set_status(req, "503 Service Unavailable");
+        return httpd_resp_send(req, nullptr, 0);
+    }
+
+    std::array<char, 2048> body{};
+    if (!readRequestBody(req, body)) {
+        httpd_resp_set_status(req, "400 Bad Request");
+        return httpd_resp_send(req, nullptr, 0);
+    }
+
+    const auto parsed =
+        core::parseAudioProfileJson(std::string_view(body.data()));
+    if (!parsed) {
+        const std::string json =
+            core::serializeAudioErrorJson(parseErrorToken(parsed.error()));
+        httpd_resp_set_status(req, "400 Bad Request");
+        httpd_resp_set_type(req, "application/json");
+        return httpd_resp_send(req, json.c_str(), json.size());
+    }
+
+    if (auto applied = ctx->audio->applyProfile(*parsed, true); !applied) {
+        const std::string json = core::serializeAudioErrorJson("store_failed");
+        httpd_resp_set_status(req, "500 Internal Server Error");
+        httpd_resp_set_type(req, "application/json");
+        return httpd_resp_send(req, json.c_str(), json.size());
+    }
+
+    const std::string json = core::serializeAudioSavedJson();
+    httpd_resp_set_type(req, "application/json");
+    return httpd_resp_send(req, json.c_str(), json.size());
+}
+
+/**
+ * @brief    audioResetPostHandler — restore factory-flat profile.
+ *
+ * @dname    audioResetPostHandler
+ * @param    req  HTTP request handle from esp_http_server.
+ * @return   ESP_OK on success, or an esp_err_t error code.
+ * @pubstate applies AudioProfile::factoryDefault() and persists to NVS.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+esp_err_t audioResetPostHandler(httpd_req_t* req)
+{
+    auto* ctx = routeContextFrom(req);
+    if (ctx == nullptr || ctx->audio == nullptr) {
+        httpd_resp_set_status(req, "503 Service Unavailable");
+        return httpd_resp_send(req, nullptr, 0);
+    }
+
+    const core::AudioProfile defaults = core::AudioProfile::factoryDefault();
+    if (auto applied = ctx->audio->applyProfile(defaults, true); !applied) {
+        const std::string json = core::serializeAudioErrorJson("store_failed");
+        httpd_resp_set_status(req, "500 Internal Server Error");
+        httpd_resp_set_type(req, "application/json");
+        return httpd_resp_send(req, json.c_str(), json.size());
+    }
+
+    const std::string json = core::serializeAudioSavedJson();
+    httpd_resp_set_type(req, "application/json");
+    return httpd_resp_send(req, json.c_str(), json.size());
+}
+
 /**
  * @brief    indexGetHandler — serve gzipped setup page from flash.
  *
@@ -450,7 +556,8 @@ SetupWebServer::SetupWebServer()
     , store_(nullptr)
     , netState_(NetState::Uninitialized)
     , tuner_(nullptr)
-    , routeContext_{nullptr, nullptr}
+    , audio_(nullptr)
+    , routeContext_{nullptr, nullptr, nullptr}
 {
 }
 
@@ -459,13 +566,15 @@ SetupWebServer::SetupWebServer(SetupWebServer&& other) noexcept
     , store_(other.store_)
     , netState_(other.netState_)
     , tuner_(other.tuner_)
+    , audio_(other.audio_)
     , routeContext_(other.routeContext_)
 {
     other.server_ = nullptr;
     other.store_ = nullptr;
     other.netState_ = NetState::Uninitialized;
     other.tuner_ = nullptr;
-    other.routeContext_ = {nullptr, nullptr};
+    other.audio_ = nullptr;
+    other.routeContext_ = {nullptr, nullptr, nullptr};
 }
 
 SetupWebServer& SetupWebServer::operator=(SetupWebServer&& other) noexcept
@@ -478,12 +587,14 @@ SetupWebServer& SetupWebServer::operator=(SetupWebServer&& other) noexcept
         store_ = other.store_;
         netState_ = other.netState_;
         tuner_ = other.tuner_;
+        audio_ = other.audio_;
         routeContext_ = other.routeContext_;
         other.server_ = nullptr;
         other.store_ = nullptr;
         other.netState_ = NetState::Uninitialized;
         other.tuner_ = nullptr;
-        other.routeContext_ = {nullptr, nullptr};
+        other.audio_ = nullptr;
+        other.routeContext_ = {nullptr, nullptr, nullptr};
     }
     return *this;
 }
@@ -494,13 +605,14 @@ SetupWebServer::~SetupWebServer()
         httpd_stop(server_);
         server_ = nullptr;
     }
-    routeContext_ = {nullptr, nullptr};
+    routeContext_ = {nullptr, nullptr, nullptr};
 }
 
 std::expected<void, NetError> SetupWebServer::start(
     core::ISecureStore& store,
     NetState netState,
-    tuner::TunerService& tuner)
+    tuner::TunerService& tuner,
+    audio::AudioService& audio)
 {
     if (server_ != nullptr) {
         return {};
@@ -509,8 +621,10 @@ std::expected<void, NetError> SetupWebServer::start(
     store_ = &store;
     netState_ = netState;
     tuner_ = &tuner;
+    audio_ = &audio;
     routeContext_.store = &store;
     routeContext_.tuner = &tuner;
+    routeContext_.audio = &audio;
 
     httpd_config_t config = HTTPD_DEFAULT_CONFIG();
     config.server_port = 80;
@@ -518,7 +632,7 @@ std::expected<void, NetError> SetupWebServer::start(
 
     if (httpd_start(&server_, &config) != ESP_OK) {
         ESP_LOGE(kTag, "httpd_start failed");
-        routeContext_ = {nullptr, nullptr};
+        routeContext_ = {nullptr, nullptr, nullptr};
         return std::unexpected(NetError::HttpServerStartFailed);
     }
 
@@ -588,6 +702,30 @@ std::expected<void, NetError> SetupWebServer::start(
     };
     httpd_register_uri_handler(server_, &tunerSeekUri);
 
+    const httpd_uri_t audioProfileGetUri = {
+        .uri = "/api/audio/profile",
+        .method = HTTP_GET,
+        .handler = audioProfileGetHandler,
+        .user_ctx = routeCtx,
+    };
+    httpd_register_uri_handler(server_, &audioProfileGetUri);
+
+    const httpd_uri_t audioProfilePutUri = {
+        .uri = "/api/audio/profile",
+        .method = HTTP_PUT,
+        .handler = audioProfilePutHandler,
+        .user_ctx = routeCtx,
+    };
+    httpd_register_uri_handler(server_, &audioProfilePutUri);
+
+    const httpd_uri_t audioResetUri = {
+        .uri = "/api/audio/reset",
+        .method = HTTP_POST,
+        .handler = audioResetPostHandler,
+        .user_ctx = routeCtx,
+    };
+    httpd_register_uri_handler(server_, &audioResetUri);
+
     ESP_LOGI(kTag, "HTTP server listening on port 80");
     return {};
 }

Software/components/net/www/index.html

@@ -61,6 +61,7 @@
       color: var(--text);
       font: inherit;
     }
+    input[type="range"] { padding: 0; }
     button {
       width: 100%;
       padding: var(--space-2);
@@ -196,6 +197,22 @@
 
       <p class="msg" id="tuner-msg" aria-live="polite"></p>
     </section>
+
+    <section id="audio-section">
+      <h2>Audio</h2>
+      <p>ADAU1701 mixer and master volume (safeload at runtime).</p>
+      <label for="master-db">Master (dB)</label>
+      <input id="master-db" type="range" min="-60" max="12" step="0.5" value="0">
+      <label for="si4684-db">Radio path Si4684 (dB)</label>
+      <input id="si4684-db" type="range" min="-60" max="12" step="0.5" value="0">
+      <label for="esp32-db">ESP32 path (dB)</label>
+      <input id="esp32-db" type="range" min="-60" max="12" step="0.5" value="0">
+      <div class="row">
+        <button type="button" id="save-audio">Save profile</button>
+        <button type="button" class="secondary" id="reset-audio">Reset flat</button>
+      </div>
+      <p class="msg" id="audio-msg" aria-live="polite"></p>
+    </section>
   </main>
   <script>
     function showMsg(el, text, ok) {
@@ -397,6 +414,70 @@
     });
 
     refreshTunerStatus();
+
+    var audioProfile = null;
+
+    function loadAudioProfile() {
+      var msg = document.getElementById("audio-msg");
+      return fetch("/api/audio/profile")
+        .then(function (r) { return r.json(); })
+        .then(function (d) {
+          audioProfile = d;
+          document.getElementById("master-db").value = d.master.left_db;
+          document.getElementById("si4684-db").value = d.mixer.si4684_left_db;
+          document.getElementById("esp32-db").value = d.mixer.esp32_left_db;
+          showMsg(msg, "", true);
+        })
+        .catch(function () { showMsg(msg, "Audio profile load failed.", false); });
+    }
+
+    document.getElementById("save-audio").addEventListener("click", function () {
+      var msg = document.getElementById("audio-msg");
+      if (!audioProfile) {
+        showMsg(msg, "Profile not loaded.", false);
+        return;
+      }
+      var master = parseFloat(document.getElementById("master-db").value, 10);
+      var si4684 = parseFloat(document.getElementById("si4684-db").value, 10);
+      var esp32 = parseFloat(document.getElementById("esp32-db").value, 10);
+      audioProfile.master.left_db = master;
+      audioProfile.master.right_db = master;
+      audioProfile.mixer.si4684_left_db = si4684;
+      audioProfile.mixer.si4684_right_db = si4684;
+      audioProfile.mixer.esp32_left_db = esp32;
+      audioProfile.mixer.esp32_right_db = esp32;
+      fetch("/api/audio/profile", {
+        method: "PUT",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify(audioProfile)
+      })
+        .then(function (r) { return r.json().then(function (d) { return { ok: r.ok, d: d }; }); })
+        .then(function (res) {
+          if (res.ok && res.d.status === "saved") {
+            showMsg(msg, "Audio profile saved.", true);
+          } else {
+            showMsg(msg, "Save failed: " + (res.d.reason || "unknown"), false);
+          }
+        })
+        .catch(function () { showMsg(msg, "Save request failed.", false); });
+    });
+
+    document.getElementById("reset-audio").addEventListener("click", function () {
+      var msg = document.getElementById("audio-msg");
+      fetch("/api/audio/reset", { method: "POST" })
+        .then(function (r) { return r.json().then(function (d) { return { ok: r.ok, d: d }; }); })
+        .then(function (res) {
+          if (res.ok && res.d.status === "saved") {
+            showMsg(msg, "Factory-flat profile applied.", true);
+            loadAudioProfile();
+          } else {
+            showMsg(msg, "Reset failed: " + (res.d.reason || "unknown"), false);
+          }
+        })
+        .catch(function () { showMsg(msg, "Reset request failed.", false); });
+    });
+
+    loadAudioProfile();
   </script>
 </body>
 </html>

Software/components/secure_store/CMakeLists.txt

@@ -1,5 +1,7 @@
 idf_component_register(
-    SRCS "src/NvsSecureStore.cpp"
+    SRCS
+        "src/NvsSecureStore.cpp"
+        "src/NvsAudioProfileStore.cpp"
     INCLUDE_DIRS "include"
     REQUIRES core nvs_flash
 )

Software/components/secure_store/include/secure_store/NvsAudioProfileStore.hpp

@@ -0,0 +1,42 @@
+/**
+ * @file    NvsAudioProfileStore.hpp
+ * @brief   NVS-backed IAudioProfileStore for user audio settings.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include "core/IAudioProfileStore.hpp"
+
+namespace secure_store {
+
+/**
+ * @brief    NvsAudioProfileStore — persists AudioProfile JSON in NVS.
+ *
+ * @dname    NvsAudioProfileStore
+ * @return   n/a (type)
+ * @pubstate Uses namespace digiradio, key audio_profile_json.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+class NvsAudioProfileStore final : public core::IAudioProfileStore {
+public:
+    [[nodiscard]] bool hasProfile() const override;
+
+    [[nodiscard]] std::expected<void, core::StoreError> saveProfile(
+        const core::AudioProfile& profile) override;
+
+    [[nodiscard]] std::expected<core::AudioProfile, core::StoreError>
+    loadProfile() const override;
+
+    [[nodiscard]] std::expected<void, core::StoreError> clearProfile() override;
+};
+
+} // namespace secure_store

Software/components/secure_store/src/NvsAudioProfileStore.cpp

@@ -0,0 +1,111 @@
+/**
+ * @file    NvsAudioProfileStore.cpp
+ * @brief   NvsAudioProfileStore implementation.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "secure_store/NvsAudioProfileStore.hpp"
+
+#include "core/AudioProfileJson.hpp"
+
+#include "nvs.h"
+
+#include <string>
+#include <vector>
+
+namespace secure_store {
+
+namespace {
+constexpr char kNamespace[] = "digiradio";
+constexpr char kProfileKey[] = "audio_profile_json";
+} // namespace
+
+bool NvsAudioProfileStore::hasProfile() const
+{
+    nvs_handle_t handle = 0;
+    if (nvs_open(kNamespace, NVS_READONLY, &handle) != ESP_OK) {
+        return false;
+    }
+
+    std::size_t len = 0;
+    const esp_err_t err = nvs_get_str(handle, kProfileKey, nullptr, &len);
+    nvs_close(handle);
+    return err == ESP_OK && len > 1U;
+}
+
+std::expected<void, core::StoreError> NvsAudioProfileStore::saveProfile(
+    const core::AudioProfile& profile)
+{
+    const std::string json = core::serializeAudioProfileJson(profile);
+
+    nvs_handle_t handle = 0;
+    if (nvs_open(kNamespace, NVS_READWRITE, &handle) != ESP_OK) {
+        return std::unexpected(core::StoreError::IoFailed);
+    }
+
+    esp_err_t err = nvs_set_str(handle, kProfileKey, json.c_str());
+    if (err == ESP_OK) {
+        err = nvs_commit(handle);
+    }
+    nvs_close(handle);
+
+    if (err != ESP_OK) {
+        return std::unexpected(core::StoreError::IoFailed);
+    }
+    return {};
+}
+
+std::expected<core::AudioProfile, core::StoreError>
+NvsAudioProfileStore::loadProfile() const
+{
+    nvs_handle_t handle = 0;
+    if (nvs_open(kNamespace, NVS_READONLY, &handle) != ESP_OK) {
+        return std::unexpected(core::StoreError::NotFound);
+    }
+
+    std::size_t len = 0;
+    if (nvs_get_str(handle, kProfileKey, nullptr, &len) != ESP_OK || len == 0U) {
+        nvs_close(handle);
+        return std::unexpected(core::StoreError::NotFound);
+    }
+
+    std::vector<char> buf(len);
+    if (nvs_get_str(handle, kProfileKey, buf.data(), &len) != ESP_OK) {
+        nvs_close(handle);
+        return std::unexpected(core::StoreError::IoFailed);
+    }
+    nvs_close(handle);
+
+    if (auto parsed = core::parseAudioProfileJson(buf.data()); parsed) {
+        return *parsed;
+    }
+    return std::unexpected(core::StoreError::InvalidData);
+}
+
+std::expected<void, core::StoreError> NvsAudioProfileStore::clearProfile()
+{
+    nvs_handle_t handle = 0;
+    if (nvs_open(kNamespace, NVS_READWRITE, &handle) != ESP_OK) {
+        return std::unexpected(core::StoreError::IoFailed);
+    }
+
+    esp_err_t err = nvs_erase_key(handle, kProfileKey);
+    if (err == ESP_OK || err == ESP_ERR_NVS_NOT_FOUND) {
+        err = nvs_commit(handle);
+    }
+    nvs_close(handle);
+
+    if (err != ESP_OK) {
+        return std::unexpected(core::StoreError::IoFailed);
+    }
+    return {};
+}
+
+} // namespace secure_store

Software/components/services/audio/CMakeLists.txt

@@ -0,0 +1,8 @@
+idf_component_register(
+    SRCS
+        "src/AudioService.cpp"
+    INCLUDE_DIRS "include"
+    REQUIRES core
+)
+
+target_compile_features(${COMPONENT_LIB} PUBLIC cxx_std_23)

Software/components/services/audio/include/audio/AudioService.hpp

@@ -0,0 +1,158 @@
+/**
+ * @file    AudioService.hpp
+ * @brief   Application service for ADAU1701 mixer/EQ/master configuration.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include "core/AudioProfile.hpp"
+#include "core/DspError.hpp"
+#include "core/EqBandIndex.hpp"
+#include "core/FrequencyHz.hpp"
+#include "core/GainDb.hpp"
+#include "core/IAudioProfileStore.hpp"
+#include "core/IDsp.hpp"
+#include "core/MixSource.hpp"
+#include "core/StoreError.hpp"
+
+#include <expected>
+
+namespace audio {
+
+/**
+ * @brief    AudioService — intent-level audio path control for HTTP/UI.
+ *
+ * @dname    AudioService
+ * @return   n/a (type)
+ * @pubstate Borrows core::IDsp; optionally persists via IAudioProfileStore.
+ *           Tracks the last applied profile in RAM.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+class AudioService {
+public:
+    /**
+     * @brief    AudioService — bind DSP and optional profile store.
+     *
+     * @dname    AudioService
+     * @param    dsp    ADAU1701 adapter (must outlive this service).
+     * @param    store  Optional NVS store; nullptr skips persistence.
+     * @pubstate initialises profile_ to factoryDefault().
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    AudioService(core::IDsp& dsp, core::IAudioProfileStore* store);
+
+    /**
+     * @brief    loadAndApply — restore saved profile or factory default.
+     *
+     * @dname    loadAndApply
+     * @return   Ok on success, or DspError from IDsp.
+     * @pubstate updates profile_ and safeloads the ADAU1701.
+     *
+     * Call once after ADAU1701 boot. When no profile is stored, applies
+     * AudioProfile::factoryDefault() without writing NVS.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] std::expected<void, core::DspError> loadAndApply();
+
+    /**
+     * @brief    currentProfile — read the in-memory profile snapshot.
+     *
+     * @dname    currentProfile
+     * @return   Last applied or pending profile.
+     * @pubstate reads profile_.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] const core::AudioProfile& currentProfile() const noexcept;
+
+    /**
+     * @brief    applyProfile — safeload a full profile and optionally save.
+     *
+     * @dname    applyProfile
+     * @param    profile  Validated user configuration.
+     * @param    persist  When true and store is set, write NVS.
+     * @return   Ok on success, DspError, or StoreError::IoFailed.
+     * @pubstate updates profile_; may persist via store_.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] std::expected<void, core::StoreError> applyProfile(
+        const core::AudioProfile& profile, bool persist);
+
+    /**
+     * @brief    setInputVolume — update one input path and apply live.
+     *
+     * @dname    setInputVolume
+     * @param    source  Si4684 or ESP32 path.
+     * @param    left    Left gain.
+     * @param    right   Right gain.
+     * @param    persist When true and store is set, write NVS.
+     * @return   Ok on success, or StoreError wrapping DspError/IoFailed.
+     * @pubstate updates profile_.mixer and safeloads.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] std::expected<void, core::StoreError> setInputVolume(
+        core::MixSource source, core::GainDb left, core::GainDb right,
+        bool persist);
+
+    /**
+     * @brief    setMasterVolume — update master output and apply live.
+     *
+     * @dname    setMasterVolume
+     * @param    left    Left master gain.
+     * @param    right   Right master gain.
+     * @param    persist When true and store is set, write NVS.
+     * @return   Ok on success, or StoreError.
+     * @pubstate updates profile_ master fields and safeloads.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] std::expected<void, core::StoreError> setMasterVolume(
+        core::GainDb left, core::GainDb right, bool persist);
+
+    /**
+     * @brief    setEqBand — update one PEQ band and apply live.
+     *
+     * @dname    setEqBand
+     * @param    band    Band index 0..5.
+     * @param    gain    Band gain in dB.
+     * @param    center  Centre frequency.
+     * @param    q       Quality factor.
+     * @param    persist When true and store is set, write NVS.
+     * @return   Ok on success, or StoreError.
+     * @pubstate updates profile_.eq and safeloads.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] std::expected<void, core::StoreError> setEqBand(
+        core::EqBandIndex band, core::GainDb gain, core::FrequencyHz center,
+        float q, bool persist);
+
+private:
+    [[nodiscard]] std::expected<void, core::StoreError> persistProfile() const;
+
+    core::IDsp& dsp_;
+    core::IAudioProfileStore* store_;
+    core::AudioProfile profile_;
+};
+
+} // namespace audio

Software/components/services/audio/src/AudioService.cpp

@@ -0,0 +1,118 @@
+/**
+ * @file    AudioService.cpp
+ * @brief   AudioService implementation.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "audio/AudioService.hpp"
+
+namespace audio {
+
+AudioService::AudioService(core::IDsp& dsp, core::IAudioProfileStore* store)
+    : dsp_(dsp)
+    , store_(store)
+    , profile_(core::AudioProfile::factoryDefault())
+{
+}
+
+std::expected<void, core::DspError> AudioService::loadAndApply()
+{
+    if (store_ != nullptr && store_->hasProfile()) {
+        if (auto loaded = store_->loadProfile(); loaded) {
+            profile_ = *loaded;
+        }
+    }
+
+    if (auto applied = dsp_.applyProfile(profile_); !applied) {
+        return applied;
+    }
+    return {};
+}
+
+const core::AudioProfile& AudioService::currentProfile() const noexcept
+{
+    return profile_;
+}
+
+std::expected<void, core::StoreError> AudioService::persistProfile() const
+{
+    if (store_ == nullptr) {
+        return {};
+    }
+    return store_->saveProfile(profile_);
+}
+
+std::expected<void, core::StoreError> AudioService::applyProfile(
+    const core::AudioProfile& profile, bool persist)
+{
+    if (auto applied = dsp_.applyProfile(profile); !applied) {
+        return std::unexpected(core::StoreError::IoFailed);
+    }
+    profile_ = profile;
+    if (persist) {
+        return persistProfile();
+    }
+    return {};
+}
+
+std::expected<void, core::StoreError> AudioService::setInputVolume(
+    core::MixSource source, core::GainDb left, core::GainDb right, bool persist)
+{
+    if (auto applied = dsp_.setInputVolume(source, left, right); !applied) {
+        return std::unexpected(core::StoreError::IoFailed);
+    }
+
+    if (source == core::MixSource::Si4684) {
+        profile_.mixer.si4684Left = left;
+        profile_.mixer.si4684Right = right;
+    } else {
+        profile_.mixer.esp32Left = left;
+        profile_.mixer.esp32Right = right;
+    }
+
+    if (persist) {
+        return persistProfile();
+    }
+    return {};
+}
+
+std::expected<void, core::StoreError> AudioService::setMasterVolume(
+    core::GainDb left, core::GainDb right, bool persist)
+{
+    if (auto applied = dsp_.setMasterVolume(left, right); !applied) {
+        return std::unexpected(core::StoreError::IoFailed);
+    }
+    profile_.masterLeft = left;
+    profile_.masterRight = right;
+    if (persist) {
+        return persistProfile();
+    }
+    return {};
+}
+
+std::expected<void, core::StoreError> AudioService::setEqBand(
+    core::EqBandIndex band, core::GainDb gain, core::FrequencyHz center, float q,
+    bool persist)
+{
+    if (auto applied = dsp_.setEqBand(band, gain, center, q); !applied) {
+        return std::unexpected(core::StoreError::IoFailed);
+    }
+    profile_.eq.setBand(band, core::EqBandSettings{
+                                 .gain = gain,
+                                 .center = center,
+                                 .q = q,
+                             });
+    if (persist) {
+        return persistProfile();
+    }
+    return {};
+}
+
+} // namespace audio

Software/docs/manual/ch-api.tex

@@ -6,7 +6,7 @@ implemented in \texttt{SetupWebServer}. Request bodies are parsed into
 domain types in the pure core (\texttt{components/core}) before any
 persistence or driver call. Exact C++ signatures live in the generated
 Doxygen output under \texttt{docs/api/}; this chapter documents the
-wire protocol and behaviour as shipped in firmware~0.4.0 (Slices~1--4).
+wire protocol and behaviour as shipped in firmware~0.5.0 (Slices~1--5).
 
 \section{Transport and reachability}
 
@@ -36,7 +36,7 @@ Returns a health-check DTO serialised by
 
 \begin{drnote}[Response schema]
 \begin{drcode}[JSON]
-{"status":"ok","fw":"0.4.0"}
+{"status":"ok","fw":"0.5.0"}
 \end{drcode}
 \begin{itemize}
   \item \texttt{status} --- coarse indicator; \texttt{ok} when the
@@ -172,10 +172,55 @@ Seeks FM upward (no request body). Returns
 \texttt{\{"frequency\_khz":...\}} on success. HTTP status: \textbf{200 OK};
 \textbf{409} on seek failure.
 
+\subsection{\texttt{GET /api/audio/profile}}
+\label{sec:api-audio-profile-get}
+
+Returns the current ADAU1701 audio snapshot serialised by
+\texttt{core::serializeAudioProfileJson()} from \texttt{core::AudioProfile}.
+The handler reads \texttt{audio::AudioService::currentProfile()}.
+
+\begin{drnote}[Response schema (excerpt)]
+\begin{drcode}[JSON]
+{"mixer":{"si4684_left_db":0,"si4684_right_db":0,"esp32_left_db":0,
+ "esp32_right_db":0,"mix_left_db":0,"mix_right_db":0},
+ "master":{"left_db":0,"right_db":0},
+ "eq":[{"gain_db":0,"center_hz":40,"q":1.414}, ...]}
+\end{drcode}
+Six EQ bands are always present (\texttt{eq} array length~6).
+\end{drnote}
+
+HTTP status: \textbf{200 OK}; \textbf{503} when the audio service is
+unavailable.
+
+\subsection{\texttt{PUT /api/audio/profile}}
+\label{sec:api-audio-profile-put}
+
+Applies a full audio profile. Body parsed by
+\texttt{core::parseAudioProfileJson()}; on success
+\texttt{audio::AudioService::applyProfile(..., persist=true)} safeloads
+the ADAU1701 and writes NVS key \texttt{audio\_profile\_json}.
+
+\begin{drnote}[Success response]
+\begin{drcode}[JSON]
+{"status":"saved"}
+\end{drcode}
+\end{drnote}
+
+HTTP status: \textbf{200 OK}; \textbf{400} for parse/validation failures;
+\textbf{500} when safeload or NVS persistence fails.
+
+\subsection{\texttt{POST /api/audio/reset}}
+\label{sec:api-audio-reset}
+
+Restores \texttt{AudioProfile::factoryDefault()} (flat EQ, 0\,dB gains),
+applies it to the DSP, and persists to NVS. Success response:
+\texttt{\{"status":"saved"\}}. HTTP status: \textbf{200 OK}; \textbf{500}
+on apply/persist failure.
+
 \section{Boot and network state machine}
 \label{sec:api-boot-flow}
 
-At boot, \texttt{net::NetBootstrap::start(store, tuner)} consults
+At boot, \texttt{net::NetBootstrap::start(store, tuner, audio)} consults
 \texttt{ISecureStore::hasWifiCredentials()}:
 
 \begin{enumerate}
@@ -193,7 +238,9 @@ This explicit \texttt{enum class NetState} replaces ad-hoc flags; see
 \label{sec:api-storage}
 
 Wi-Fi credentials are stored in NVS namespace \texttt{digiradio}, keys
-\texttt{wifi\_ssid} and \texttt{wifi\_pwd}. Passwords are wrapped in
+\texttt{wifi\_ssid} and \texttt{wifi\_pwd}. Audio profiles (non-secret) use
+the same namespace, key \texttt{audio\_profile\_json}, via
+\texttt{secure\_store::NvsAudioProfileStore}. Passwords are wrapped in
 \texttt{core::Secret} in RAM and are never logged or returned by the API.
 
 \begin{drcaution}[Encryption at rest]

Software/docs/manual/ch-classes.tex

@@ -56,16 +56,17 @@ configured SoftAP. Imperative shell; no business logic.
 
 \section{SetupWebServer}\label{cls:SetupWebServer}
 Minimal HTTP server: gzipped setup UI, \texttt{GET /api/health},
-\texttt{POST /api/wifi}, and tuner routes (\texttt{/api/tuner/*}).
+\texttt{POST /api/wifi}, tuner routes (\texttt{/api/tuner/*}), and audio
-JSON parsing and serialisation delegate to the pure core; credentials
+routes (\texttt{/api/audio/*}). JSON parsing and serialisation delegate to
-persist via \texttt{ISecureStore}; tuner operations via
+the pure core; credentials persist via \texttt{ISecureStore}; tuner via
-\texttt{tuner::TunerService}.
+\texttt{tuner::TunerService}; audio via \texttt{audio::AudioService}.
 
 \section{NetBootstrap}\label{cls:NetBootstrap}
 Owns network resources for setup or STA mode.
 \texttt{start(store, tuner)} initialises the platform, joins stored Wi-Fi
 when credentials exist, or falls back to the \texttt{DigiRadio-setup}
 SoftAP. Must outlive \texttt{app\_main} for the process lifetime.
+\texttt{start(store, tuner, audio)} also wires the audio REST routes.
 
 % ------------------------------------------------------------------
 % Domain core + secure store (Slice 2)
@@ -135,10 +136,47 @@ DTOs.
 RAII I2C driver for the ADAU1701 SigmaDSP. \texttt{boot()} asserts RESET\#,
 initialises the shared I2C bus, and replays the SigmaStudio export from
 \texttt{Firmware/ADAU1701-Firmware/} on every power-up (no EEPROM self-boot
-on DigiRadio).
+on DigiRadio). Runtime mixer, EQ, and master volume updates use the
+ADAU1701 safeload mechanism via \texttt{applyProfile()} and related methods.
+
+\section{Adau1701Dsp}\label{cls:Adau1701Dsp}
+\texttt{core::IDsp} adapter over \texttt{Adau1701Driver}. Maps domain-level
+audio control to safeload I2C transactions without exposing parameter RAM
+addresses to services.
 
 % ------------------------------------------------------------------
-% Application services (Slice 4)
+% Domain core — audio (Slice 5)
+% ------------------------------------------------------------------
+
+\section{GainDb}\label{cls:GainDb}
+Validated decibel gain/attenuation ($-96$\,dB to $+12$\,dB). Used for
+input volumes, master level, and EQ band gain. Converted to ADAU 8.23
+fixpoint via \texttt{gainDbToLinearFixpoint()} before safeload.
+
+\section{FrequencyHz}\label{cls:FrequencyHz}
+Validated PEQ centre frequency (20--20\,000\,Hz) for the 48\,kHz
+SigmaStudio project. Parsed at the HTTP boundary before coefficient design.
+
+\section{EqBandIndex}\label{cls:EqBandIndex}
+Strong index into the six-band \texttt{Param EQ1} module (0--5). Maps to
+parameter RAM via \texttt{adau1701::paramAddrEqBandBase()}.
+
+\section{EqProfile}\label{cls:EqProfile}
+Six-band parametric EQ snapshot with default centre frequencies
+(40\,Hz--12\,kHz). Designed in the pure core; coefficients safeloaded by
+\texttt{Adau1701Driver::applyEq()}.
+
+\section{IDsp}\label{cls:IDsp}
+Abstract ADAU1701 control boundary. Defines \texttt{applyProfile()},
+\texttt{setInputVolume()}, \texttt{setEqBand()}, and related intent-level
+operations without ESP-IDF types.
+
+\section{IAudioProfileStore}\label{cls:IAudioProfileStore}
+Persistence boundary for \texttt{AudioProfile} (mixer, EQ, master). Device
+implementation: \texttt{NvsAudioProfileStore}; host tests use fakes.
+
+% ------------------------------------------------------------------
+% Application services (Slice 4–5)
 % ------------------------------------------------------------------
 
 \section{ITuner}\label{cls:ITuner}
@@ -156,3 +194,13 @@ target and volume, and maps driver failures to \texttt{core::TunerError}.
 \texttt{ITuner} adapter over \texttt{Si4684Driver}. Translates domain calls
 into SPI commands and maps \texttt{Si4684Error} to \texttt{TunerError}.
 Constructed once in \texttt{HardwareBootstrap} alongside the driver.
+
+\section{AudioService}\label{cls:AudioService}
+Application service for ADAU1701 mixer, EQ, and master volume. Holds a
+reference to \texttt{core::IDsp}, tracks the in-memory \texttt{AudioProfile},
+loads from \texttt{IAudioProfileStore} after boot, and applies changes via
+safeload. Exposed on \texttt{/api/audio/*} and the web UI Audio section.
+
+\section{NvsAudioProfileStore}\label{cls:NvsAudioProfileStore}
+\texttt{IAudioProfileStore} implementation storing serialised
+\texttt{AudioProfile} JSON in NVS namespace \texttt{digiradio}.

Software/docs/manual/ch-firmware.tex

@@ -194,6 +194,9 @@ bring-up:
     (\texttt{DigiRadio\_IC\_1.h}) replayed through
     \texttt{SIGMA\_WRITE\_REGISTER\_BLOCK} after hardware reset. The DSP
     program lives in RAM only; download runs on every boot.
+  \item \textbf{Audio profile}: \texttt{audio::AudioService::loadAndApply()}
+    restores the saved \texttt{core::AudioProfile} from NVS (or factory
+    defaults) via ADAU1701 safeload before network bring-up.
 \end{enumerate}
 
 If either driver returns an error, the firmware logs the failure and stops

Software/instructions.md

@@ -49,7 +49,7 @@ Repository: https://github.com/manvalan/DigiRadio
 3. **Companion-chip boot** — done (Slice 3): Si4684 DAB + ADAU1701 RAM load.
 4. Station/frequency list model + persistence + UI.
 5. Si4684 tuning: RSQ, station list, DAB properties.
-6. ADAU1701 runtime: safeload EQ + input mixer.
+6. **ADAU1701 runtime** — done (Slice 5): safeload EQ + input mixer + HTTP.
 7. FSC-BT1035 driver: AT init (incl. `AT+AUXCFG=1`), audio out.
 8. Integration: TunerService + AudioService end to end.
 
@@ -119,3 +119,23 @@ Acceptance criteria:
 - [x] ADAU1701 reset + I2C + `default_download_IC_1()` replay.
 - [x] Host test for `EmbeddedBlobReader`; manual sync green.
 - [ ] Device flash verified (requires ESP-IDF toolchain on build host).
+
+## Slice 5 — ADAU1701 runtime + audio API (complete)
+
+Goal: safeload mixer/EQ/master on the ADAU1701 at runtime; persist user
+profiles in NVS; expose REST and web UI controls.
+
+Build on Slice 3–4:
+- Pure core: `GainDb`, `EqProfile`, `AudioProfile`, `IDsp`, biquad design,
+  `parseAudioProfileJson` / `serializeAudioProfileJson`.
+- Driver: `sigma_safeload_*`, extended `Adau1701Driver`, `Adau1701Dsp`.
+- Service: `audio::AudioService`, `secure_store::NvsAudioProfileStore`.
+- HTTP: `GET/PUT /api/audio/profile`, `POST /api/audio/reset`; Audio
+  section in the web UI. Firmware **0.5.0**.
+
+Acceptance criteria:
+- [x] Safeload volume/mixer/EQ without direct param RAM writes during audio.
+- [x] Profile load/apply after ADAU boot; NVS round-trip via JSON.
+- [x] Host tests for biquad fixpoint and audio profile JSON.
+- [x] Doxygen green; manual sync green; `ch-api.tex` documents audio routes.
+- [ ] Device flash verified on hardware.

Software/main/CMakeLists.txt

@@ -3,5 +3,5 @@ idf_component_register(
         "main.cpp"
         "hardware_bootstrap.cpp"
     INCLUDE_DIRS "."
-    REQUIRES core net secure_store adau1701 si4684 tuner
+    REQUIRES core net secure_store adau1701 si4684 tuner audio
 )

Software/main/hardware_bootstrap.cpp

@@ -14,7 +14,10 @@
 #include "hardware_bootstrap.hpp"
 
 #include "adau1701/Adau1701Driver.hpp"
+#include "adau1701/Adau1701Dsp.hpp"
+#include "audio/AudioService.hpp"
 #include "board_pins.hpp"
+#include "secure_store/NvsAudioProfileStore.hpp"
 #include "si4684/Si4684Band.hpp"
 #include "si4684/Si4684Driver.hpp"
 #include "si4684/Si4684EmbeddedImages.hpp"
@@ -51,6 +54,9 @@ adau1701::Adau1701Driver gAdau1701(
         .resetGpio = board::pins::Adau1701Reset,
         .i2cAddr7 = board::pins::Adau1701Addr,
     });
+adau1701::Adau1701Dsp gAdau1701Dsp(gAdau1701);
+secure_store::NvsAudioProfileStore gAudioStore;
+audio::AudioService gAudioService(gAdau1701Dsp, &gAudioStore);
 
 bool gReady = false;
 } // namespace
@@ -74,6 +80,10 @@ std::expected<void, HardwareBootError> HardwareBootstrap::boot()
         }
     }
 
+    if (auto audioResult = gAudioService.loadAndApply(); !audioResult) {
+        ESP_LOGW(kTag, "ADAU1701 profile apply failed");
+    }
+
     gReady = true;
     ESP_LOGI(kTag, "companion chips ready");
     return {};
@@ -84,4 +94,9 @@ si4684::Si4684Tuner& HardwareBootstrap::si4684Tuner()
     return gSi4684Tuner;
 }
 
+audio::AudioService& HardwareBootstrap::audioService()
+{
+    return gAudioService;
+}
+
 } // namespace hardware

Software/main/hardware_bootstrap.hpp

@@ -14,6 +14,10 @@
 
 #include <expected>
 
+namespace audio {
+class AudioService;
+} // namespace audio
+
 namespace si4684 {
 class Si4684Tuner;
 } // namespace si4684
@@ -75,6 +79,20 @@ public:
      * @date     2026-07-06
      */
     [[nodiscard]] static si4684::Si4684Tuner& si4684Tuner();
+
+    /**
+     * @brief    audioService — borrow the audio orchestration service after boot.
+     *
+     * @dname    audioService
+     * @return   Reference to the static AudioService instance.
+     * @pubstate reads static storage initialised by boot().
+     *
+     * Valid only after a successful boot() call in the same process.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] static audio::AudioService& audioService();
 };
 
 } // namespace hardware

Software/main/main.cpp

@@ -47,7 +47,7 @@ void heartbeatTask(void* arg)
  */
 extern "C" void app_main()
 {
-    ESP_LOGI(kTag, "DigiRadio firmware boot — Slice 4");
+    ESP_LOGI(kTag, "DigiRadio firmware boot — Slice 5");
 
     auto hwResult = hardware::HardwareBootstrap::boot();
     if (!hwResult) {
@@ -60,7 +60,8 @@ extern "C" void app_main()
 
     static secure_store::NvsSecureStore store;
 
-    auto netResult = net::NetBootstrap::start(store, tunerService);
+    auto netResult = net::NetBootstrap::start(
+        store, tunerService, hardware::HardwareBootstrap::audioService());
     if (!netResult) {
         ESP_LOGE(kTag, "network bootstrap failed");
         return;