DigiRadio

DAB+/FM Bluetooth Receiver with SigmaDSP Audio Processing

Open-source digital radio — Si4684 tuner · ADAU1701 SigmaDSP · Bluetooth aptX Adaptive · ESP32-S3

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Overview

DigiRadio is an open-source digital radio receiver that combines terrestrial broadcast reception (DAB+ / DAB / FM) with a programmable audio-processing stage and high-quality Bluetooth audio output.

Incoming broadcast audio is decoded by a Silicon Labs Si4684 digital-radio receiver, processed by an Analog Devices ADAU1701 SigmaDSP (equalisation, mixing, level control), and transmitted over Bluetooth by a Qualcomm QCC3056-based Feasycom FSC-BT1035 module supporting the aptX Adaptive codec. An Espressif ESP32-S3 acts as the host controller, orchestrating the three subsystems and providing Wi-Fi connectivity, encrypted credential storage, and a browser-based configuration UI.

The entire tuner → DSP → Bluetooth path is fully digital (I²S, 48 kHz / 24-bit) — no analogue conversions are introduced between the tuner and the wireless link.

 FM/DAB antenna → [ Si4684 ] --I²S--> [ ADAU1701 DSP ] --I²S--> [ FSC-BT1035 ] → aptX
                       ↑                     ↑                        ↑
                       └──── SPI ──── [ ESP32-S3 host ] ──── UART ────┘
                                             │  I²C
                                      Wi-Fi · NVS · HTTP UI

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Key Features

Hardware

• DAB+ / DAB / FM reception (Si4684) via external whip antenna (SMA)
• Fully digital audio path — no avoidable A/D–D/A conversions
• Programmable audio processing on the ADAU1701 SigmaDSP
• Bluetooth 5.2 output with aptX / aptX HD / aptX Adaptive
• ESP32-S3 host with native USB, Wi-Fi and BLE
• USB-C powered; single-rail 3.3 V + low-noise 1.8 V for the tuner
• 6-layer impedance-controlled PCB, 50 × 90 mm, two ground planes
• Three antennas (ESP32 2.4 GHz, BT1035 2.4 GHz, FM/DAB SMA) with proper keep-outs

Firmware (ESP-IDF, C++23 — fw 0.8.5)

Area	Capability
Boot & chips	Si4684 DAB+FM firmware load (HOST_LOAD blobs), ADAU1701 RAM program at every boot, BT1035 I2S slave init (AT+AUXCFG=3, AT+I2SCFG=67)
Tuner	FM tune/seek, DAB ensemble tune, service scan & play, RSQ/RDS, DAB event status
Now playing	FM RDS (PS, RadioText, PI, PTY) and DAB Dynamic Label (DLS) in status JSON and web UI
Audio	6-band EQ, input mixer, stereo/bass enhancement overlays, profile persist in encrypted NVS
Presets	Station list CRUD, reorder, recall with audio profile re-apply, last-preset restore at boot
Bluetooth	Discoverable pairing, A2DP status, disconnect
Network	SoftAP setup mode, STA provisioning, tabbed gzipped SPA (/), typed JSON REST API
Security	NVS + flash encryption at rest (dev mode); keys never in repo
Quality	13 host unit tests, 4 CI jobs (tests, Doxygen, manual sync, blob policy)

Architecture follows a functional core + imperative shell: pure domain logic (compilation, JSON, EQ design, RDS/DLS parsing) runs on the host under ctest; drivers and FreeRTOS live in thin ESP-IDF components. See Software/AGENTS.md.

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Firmware Progress

Vertical slices landed on main (newest first):

Version	Highlights
0.8.5	BT1035 I2S slave boot init (schematic-aligned); I2C pull-ups R1/R16 confirmed 2 kΩ; Hardware/DATASHEET/ bundle
0.8.4	Dual OTA + DSP blob updates, EEPROM identity, System tab OTA/DSP upload
0.8.3	NVS + flash encryption (development mode), initEncryptedStorage, security docs
0.8.2	Tabbed Web UI (now-playing, 6-band EQ, all REST); Si4684 blob CI policy
0.8.1	IntegrationService — boot preset recall, tune orchestration (tuner + audio + NVS last_preset); services stub removed
0.8.0	Preset reorder API/UI; broadcast metadata (RDS + DAB DLS); readDabServiceData driver path
0.7.1	CI workflow (host tests, Doxygen, manual sync); Doxygen warnings cleared
0.7.0	Station presets (NVS station_list), BT1035 pairing, full /api/stations/*
0.5–0.6	ADAU1701 runtime EQ/mixer, Si4684 tuning & DAB service list, BT1035 driver
0.3–0.4	Secure store, Wi-Fi provisioning, companion-chip boot, walking skeleton

Next: device HIL when the PCB arrives (Software/docs/security-flash-nvs.md, Software/docs/TODO.md). Contributing: CONTRIBUTING.md.

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Repository Structure

DigiRadio/
├── .github/workflows/        CI — host tests, Doxygen, manual sync, Si4684 blob policy
├── Hardware/
│   ├── schematics/           Schematic (PDF)
│   ├── gerber/               Gerber + drill (fabrication)
│   ├── bom/                  Bill of Materials
│   ├── pick-and-place/       CPL / centroid
│   ├── easyeda/              EasyEDA source
│   └── 3d/                   Renders / STEP
├── Software/                 Firmware project root — open this in Cursor
│   ├── AGENTS.md             Authoritative coding rules for agents
│   ├── instructions.md       Agent kickoff & slice roadmap
│   ├── Doxyfile              API documentation gate
│   ├── components/
│   │   ├── core/             Pure domain (host-tested, no ESP-IDF)
│   │   ├── drivers/          Si4684, ADAU1701, BT1035
│   │   ├── services/         Tuner, audio, Bluetooth, station, integration
│   │   ├── secure_store/     NVS wrappers (Wi-Fi, profiles, presets)
│   │   └── net/              Wi-Fi, HTTP server, gzipped web UI
│   ├── docs/
│   │   ├── manual/           LaTeX technical manual (canonical)
│   │   ├── security-flash-nvs.md  NVS/flash encryption + HIL checklist
│   │   └── TODO.md           Agent backlog and completed tasks
│   ├── Firmware/             Si4684 blobs + ADAU1701 SigmaStudio export
│   ├── main/                 app_main, hardware bootstrap
│   └── tools/                fetch_si4684_firmware, check_*, gzip-www
├── CONTRIBUTING.md           human-facing dev guide
├── LICENSE                   CERN-OHL-S v2 (hardware)
└── README.md                 ← you are here

Build the manual from Software/docs/manual/:

cd Software/docs/manual && latexmk -lualatex manual.tex

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Getting Started

Firmware (developers)

Open Software/ as the Cursor project so AGENTS.md and .cursor/rules/ load.

cd Software
idf.py set-target esp32s3
idf.py build
idf.py erase-flash flash   # once when first enabling encryption (0.8.3+)
idf.py -p <port> monitor

Host unit tests (no hardware):

cd Software
cmake -S components/core/test -B build-host \
      -DCMAKE_CXX_COMPILER="$(brew --prefix llvm)/bin/clang++"   # macOS
cmake --build build-host
ctest --test-dir build-host --output-on-failure

Quality gates (also enforced in CI):

cd Software
doxygen Doxyfile
python3 tools/check-manual-sync.py
python3 tools/check_si4684_blobs.py

Full build notes: Software/README.md · security: Software/docs/security-flash-nvs.md.

First boot (device)

1. Power the board; ESP32 starts SoftAP setup mode (DigiRadio-setup — see manual).
2. Connect and open the gzipped web UI or call GET /api/health.
3. POST /api/wifi with STA credentials; device reboots into normal mode.
4. Tune via /api/tuner/*, manage presets via /api/stations/*, pair BT via /api/bluetooth/*.

Schemas, error tokens, and boot flow: Software/docs/manual/ch-api.tex.

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HTTP API (summary)

Method	Path	Purpose
GET	/api/health	Status, firmware version, companion-chip flags
POST	/api/wifi	Provision STA credentials
GET	/api/tuner/status	Tuner snapshot incl. RDS/DLS metadata
GET	/api/tuner/services	DAB service list
POST	/api/tuner/tune · /play · /seek	Tuner control
GET/PUT	/api/audio/profile	ADAU1701 mixer + EQ
POST	/api/audio/reset · /stereo-enhance · /bass-enhance	Audio overlays
GET	/api/bluetooth/status	BT1035 state
POST	/api/bluetooth/pair · /pair/stop · /disconnect	Bluetooth control
GET/POST	/api/stations · /remove · /reorder · /tune	Preset list & recall

Generated C++ API reference: run doxygen Doxyfile → Software/docs/api/html/index.html.

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Design Highlights

Topic	Choice	Rationale
Audio quality	QCC3056 / aptX Adaptive	Codec dominates wireless quality; DAC lives in the sink
Audio path	Fully digital I²S	Avoids unnecessary A/D–D/A between tuner and Bluetooth
I²S master	ADAU1701 (12.288 MHz)	Clean ×256 → 48 kHz; avoids sample-rate mismatch
Firmware style	C++23, std::expected, RAII	Typed errors, host-testable core, no primitive obsession
Stack-up	6-layer, 2× GND	Solid ground under RF, crystals and USB
USB routing	TOP, ref. GND	90 Ω diff at manufacturable geometry
Antennas	3 zones, diagonal 2.4 GHz	Coexistence separation + full-layer keep-outs

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Status

Layer	State
Schematic	Complete and reviewed
PCB layout	6-layer, DRC clean, plane continuity verified
BOM	Finalised (manufacturable / sourced)
Prototype	In fabrication (PCBWay)
Firmware	0.8.5 on main — BT1035 I2S boot init, dual OTA, encrypted NVS, full Web UI
Web UI	Tabbed SPA covering every REST endpoint
HIL	Device validation pending PCB (Software/docs/security-flash-nvs.md)
Agent backlog	Feature-complete on main (fw 0.8.5); see Software/docs/TODO.md

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Manufacturing

The board is manufactured with PCBWay as a 6-layer, impedance-controlled PCB with turnkey assembly. The FSC-BT1035 Bluetooth module is sourced from Feasycom (the footprint uses a BT806-compatible, pin-identical land pattern). Component datasheets for design review live in Hardware/DATASHEET/ (Si4684-A10, AN851, ADAU1701, FSC-BT1035). See the Technical Manual hardware chapter and manufacturing notes for fabrication settings and MSL-3 handling of the BT module.

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License

• Hardware (schematics, PCB, Gerbers, BOM): CERN-OHL-S v2 — see LICENSE
• Firmware / Software: Apache-2.0 — see Software/LICENSE
• Documentation: CC BY 4.0

Intellectual property of the design remains with the author. You are free to study, modify and build the hardware under the terms of the respective licenses.

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Author

Michele Bigi — Open-source hardware project.

Contributions, issues and questions are welcome via the issue tracker.