ESP32-S31 A2DP Audio Sink — Multi-Codec

A high-resolution Bluetooth A2DP audio receiver for the ESP32-S31 (N16R16V), built on a modified ESP-IDF v6.1 Bluedroid stack. Unlike stock ESP-IDF — which decodes only SBC — this project decodes the full range of A2DP codecs in-stack, streams the PCM straight to I2S, and runs entirely on internal SRAM (no PSRAM required).

Supported codecs

Codec	Rates / depth	Notes
LDAC	up to 96 kHz / 32-bit	660 / 909 / 990 kbps
LHDC v5	up to 192 kHz / 32-bit	400–1000 kbps
aptX / aptX HD / aptX LL	up to 48 kHz / 24-bit
Opus	48 kHz
LC3plus	up to 96 kHz
AAC	up to 48 kHz	Helix decoder
SBC	44.1 / 48 kHz	stock baseline

Highlights

• In-stack decoding. Decoders are integrated into the AOSP-style Bluedroid A2DP path (A2DP_GetDecoderInterface / tA2DP_DECODER_INTERFACE), the same approach used on my v5.5.2 build — not the v6.1 external-codec framework.
• Internal-SRAM only. The design fits in the ESP32-S31's internal RAM. Heavy codec tables live in flash (.rodata) and are lazily allocated to RAM only while that codec is active, then freed on codec switch or disconnect, so only the working set is ever resident.
• Dual-core scheduling. The audio decode task is pinned to core 1 (nearly alone), while the BT controller, host, and I2S render task share core 0. This keeps heavy paths (e.g. LHDC v5 at 1000 kbps) from starving the controller or overflowing the decode queue.
• Exact I2S pipeline. 32-bit Philips slots, APLL clocking (with a graceful fallback) for the high sample rates, and a prefetch/processing ring buffer ported from my working v5.5.2 sink.
• Clean stream teardown. I2S output stops and the ring buffer is cleared the instant the phone pauses or disconnects — no residual audio tail.

Hardware

• MCU: ESP32-S31 (RISC-V dual-core), N16R16V — 16 MB flash, 16 MB PSRAM (PSRAM unused).
• Output: external I2S DAC (BCLK / LRCK / DIN). Configure the pins in the app's Kconfig.

Repository layout

ESP32-S31-A2DP-Codecs/
├── S31_A2DP_Sink/          # the application project (main, sdkconfig.defaults, partitions)
└── esp-idf-v6.1-codecs/    # the forked ESP-IDF v6.1 with the modified Bluedroid codec stack

The fork is based on ESP-IDF release/v6.1. All of my changes live under esp-idf-v6.1-codecs/components/bt/ (the decoders, codec libraries, and A2DP plumbing) and the a2dp_utils example components (the I2S service).

Building

# 1. Set up the forked IDF as your IDF_PATH
cd esp-idf-v6.1-codecs
./install.ps1            # or install.sh
. ./export.ps1           # dot-source so idf.py is wired into the shell

# 2. Build & flash the app
cd ../S31_A2DP_Sink
idf.py set-target esp32s31
idf.py build
idf.py -p <PORT> flash monitor

The application must be built against this fork — the codec decoders are not present in a stock ESP-IDF checkout.

Codec licensing

This repository redistributes vendor codec sources (LDAC, aptX, LHDC, and others). These are subject to their respective owners' licenses. Ensure you have the necessary rights before using or redistributing them.