Add stereo and bass enhance APIs via PEQ overlay.

Expose POST /api/audio/stereo-enhance and bass-enhance with 0–100 levels,
persist enhancements in AudioProfile, and document the virtual EQ mapping in
the manual and SigmaStudio chapter.

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

Software/components/core/include/core/AudioEnhancements.hpp

@@ -0,0 +1,47 @@
+/**
+ * @file    AudioEnhancements.hpp
+ * @brief   Stereo width and bass boost enhancement levels.
+ *
+ * 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/EnhanceLevel.hpp"
+
+namespace core {
+
+/**
+ * @brief    AudioEnhancements — psychoacoustic EQ overlays (0–100 each).
+ *
+ * @dname    AudioEnhancements
+ * @return   n/a (type)
+ * @pubstate Mapped to PEQ bands at runtime (Chapter~\ref{ch:sigmastudio}).
+ *           No dedicated SigmaStudio blocks; see \texttt{applyEnhancementsToEq}.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+struct AudioEnhancements {
+    EnhanceLevel stereo; ///< Stereo depth / presence (PEQ bands 3–5).
+    EnhanceLevel bass;   ///< Bass emphasis (PEQ bands 1–2).
+
+    /**
+     * @brief    factoryDefault — both enhancements off.
+     *
+     * @dname    factoryDefault
+     * @return   AudioEnhancements at level 0.
+     * @pubstate none
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] static AudioEnhancements factoryDefault() noexcept;
+};
+
+} // namespace core

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

@@ -12,6 +12,7 @@
  */
 #pragma once
 
+#include "core/AudioEnhancements.hpp"
 #include "core/EqProfile.hpp"
 #include "core/GainDb.hpp"
 #include "core/MixerState.hpp"
@@ -32,8 +33,9 @@ namespace core {
 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.
+    GainDb masterLeft;           ///< Multiple 1 master volume, left.
+    GainDb masterRight;          ///< Multiple 1 master volume, right.
+    AudioEnhancements enhancements; ///< Stereo depth and bass boost (PEQ overlay).
 
     /**
      * @brief    factoryDefault — factory-flat audio path (0 dB everywhere).

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

@@ -15,6 +15,8 @@
 #include "core/AudioProfile.hpp"
 #include "core/ParseError.hpp"
 
+#include "core/EnhanceLevel.hpp"
+
 #include <expected>
 #include <string>
 #include <string_view>
@@ -74,4 +76,18 @@ namespace core {
  */
 [[nodiscard]] std::string serializeAudioErrorJson(std::string_view reason);
 
+/**
+ * @brief    parseEnhanceLevelJson — parse POST body \texttt{\{"level":0..100\}}.
+ *
+ * @dname    parseEnhanceLevelJson
+ * @param    json  Untrusted request body.
+ * @return   EnhanceLevel on success, or ParseError.
+ * @pubstate none
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+[[nodiscard]] std::expected<EnhanceLevel, ParseError> parseEnhanceLevelJson(
+    std::string_view json);
+
 } // namespace core

Software/components/core/include/core/EnhanceLevel.hpp

@@ -0,0 +1,93 @@
+/**
+ * @file    EnhanceLevel.hpp
+ * @brief   Strong type for 0–100 enhancement intensity (stereo / bass).
+ *
+ * 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    EnhanceLevel — validated enhancement intensity (0 = off, 100 = max).
+ *
+ * @dname    EnhanceLevel
+ * @return   n/a (type)
+ * @pubstate Owns level_ in 0..100 after construction.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+class EnhanceLevel {
+public:
+    /** Maximum enhancement intensity. */
+    static constexpr std::uint8_t kMax = 100U;
+
+    /**
+     * @brief    tryFromLevel — validate an enhancement level at the boundary.
+     *
+     * @dname    tryFromLevel
+     * @param    level  Untrusted 0..100 value from JSON.
+     * @return   EnhanceLevel on success, or ParseError::MissingField.
+     * @pubstate none
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] static std::expected<EnhanceLevel, ParseError> tryFromLevel(
+        std::uint32_t level) noexcept;
+
+    /**
+     * @brief    zero — enhancement disabled.
+     *
+     * @dname    zero
+     * @return   EnhanceLevel at 0.
+     * @pubstate none
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] static EnhanceLevel zero() noexcept;
+
+    /**
+     * @brief    value — read the stored level.
+     *
+     * @dname    value
+     * @return   Level 0..100.
+     * @pubstate reads level_.
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] std::uint8_t value() const noexcept;
+
+    /**
+     * @brief    fraction — normalised intensity in 0.0..1.0.
+     *
+     * @dname    fraction
+     * @return   level / 100 as float.
+     * @pubstate none
+     *
+     * @author   Michele Bigi
+     * @date     2026-07-06
+     */
+    [[nodiscard]] float fraction() const noexcept;
+
+private:
+    explicit EnhanceLevel(std::uint8_t level) noexcept;
+
+    std::uint8_t level_;
+};
+
+} // namespace core

Software/components/core/include/core/EnhancementsDesign.hpp

@@ -0,0 +1,39 @@
+/**
+ * @file    EnhancementsDesign.hpp
+ * @brief   Map enhancement levels onto Param EQ1 bands (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/AudioEnhancements.hpp"
+#include "core/EqProfile.hpp"
+
+namespace core {
+
+/**
+ * @brief    applyEnhancementsToEq — merge enhancement overlays into an EQ profile.
+ *
+ * @dname    applyEnhancementsToEq
+ * @param    base         User EQ settings (bands not touched stay unchanged).
+ * @param    enhancements Stereo and bass levels (0 = use base band only).
+ * @return   EqProfile with affected PEQ bands updated for safeload.
+ * @pubstate none
+ *
+ * Stereo (bands 3–5): slight 1\,kHz cut plus 3/8\,kHz lift for depth.
+ * Bass (bands 1–2): 100\,Hz and 400\,Hz peaking boost.
+ * Band 0 (high-pass) is never modified.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+[[nodiscard]] EqProfile applyEnhancementsToEq(
+    const EqProfile& base, const AudioEnhancements& enhancements) noexcept;
+
+} // namespace core