ChaoticVolt BLE-I2S Bridge

Bluetooth A2DP and BLE GATT bridge for the ChaoticVolt 42dB platform, relaying audio over I2S and control over UART to the DSP engine.

This repository contains the bridge layer that sits between the user-facing control side of the 42dB platform and the downstream DSP engine. It handles Bluetooth audio ingress, BLE-facing control, transport-side coordination, and control forwarding toward ChaoticVolt-42dB-DSP-Engine.

ChaoticVolt Website

Bluetooth A2DP Sink — Receive high-quality audio from phones, tablets, and computers
I2S Audio Output — Streams received audio to I2S DAC or STM32 DSP engine
BLE GATT Control — Receives DSP commands wirelessly from the 42 Decibels iOS/Watch app
UART DSP Relay — Forwards BLE commands to external STM32 DSP engine via UART (GPIO4/GPIO5 @ 115200)
GalacticStatus — Extended status reporting with periodic BLE notifications (2×/sec)
OTA Firmware Updates — Over-the-air updates via BLE provisioning + WiFi download
Persistent Settings — Settings survive power cycles via NVS flash storage
iOS Compatible — Secure Simple Pairing (SSP) for seamless iOS pairing
BT RSSI Monitoring — Reads Bluetooth signal strength every 10s when A2DP is active

What this repository is

ESP32-WROOM-32 or compatible module
I2S DAC (e.g., MAX98357A, PCM5102A)
Speaker/amplifier

This repository is the transport-facing and control-bridging component of the 42dB platform.

Its role is to:

receive Bluetooth audio
forward audio over I2S toward the DSP engine
expose the BLE GATT control interface
relay control updates over UART toward the DSP engine
reflect device state back toward the companion app
manage settings persistence and update-related coordination where needed

UART to External DSP (STM32)

Signal	GPIO	Description
TX	GPIO4	To STM32 USART2 RX (PD6)
RX	GPIO5	From STM32 USART2 TX (PD5)

GATT commands received via BLE are echoed as GATT:CTRL:<hex>\r\n on GPIO4 @ 115200 baud.

Software Requirements

This repository is not the main DSP implementation and is not the companion app.

Role in the 42dB platform

The 42dB platform is split into a few clear layers:

ChaoticVolt-42dB-Companion-App — user-facing control on iPhone, iPad, and Apple Watch
ChaoticVolt-BLE-I2S-Bridge — BLE control bridge and audio transport-facing component
ChaoticVolt-42dB-DSP-Engine — real-time DSP processing layer
downstream DAC / output stage — conversion and playback

This repository exists to keep the transport and protocol-facing responsibilities separate from the DSP engine itself.

At a glance

Role: bridge layer for Bluetooth audio ingress and BLE control
Audio path: Bluetooth audio -> bridge -> I2S -> DSP engine
Control path: companion app -> BLE GATT -> bridge -> UART -> DSP engine
Downstream integration: ChaoticVolt-42dB-DSP-Engine
Status: active platform component

Core responsibilities

Bluetooth audio ingress

The bridge receives Bluetooth audio and forwards the digital audio stream toward the DSP engine over I2S.

Power on the ESP32
Search for "42 Decibels" in your device's Bluetooth settings
Pair and connect
Play audio — it streams to the I2S DAC

BLE GATT control

The bridge exposes the BLE GATT interface used by the companion app for settings, control, and state synchronization.

Use any BLE app (nRF Connect, LightBlue) or the 42 Decibels iOS app:

Scan for BLE devices
Connect to "42 Decibels"
Find the DSP Control Service (UUID: 00000001-1234-5678-9ABC-DEF012345678)
Write commands to the Control characteristic (UUID ends in ...0002)

BLE Protocol

Service UUID

DSP Control Service: 00000001-1234-5678-9ABC-DEF012345678

Characteristics

Characteristic	UUID	Properties	Size
Control Write	`00000002-1234-5678-9ABC-DEF012345678`	Write, Write Without Response	2 bytes
Status Notify	`00000003-1234-5678-9ABC-DEF012345678`	Read, Notify	4 bytes
GalacticStatus	`00000004-1234-5678-9ABC-DEF012345678`	Read, Notify	7 bytes
OTA Credentials	`00000005-1234-5678-9ABC-DEF012345678`	Write	98 bytes
OTA URL	`00000006-1234-5678-9ABC-DEF012345678`	Write	258 bytes
OTA Control	`00000007-1234-5678-9ABC-DEF012345678`	Write	2 bytes
OTA Status	`00000008-1234-5678-9ABC-DEF012345678`	Read, Notify	8 bytes

Command Format

Commands are 2 bytes: [CMD] [VALUE]

CMD	Name	Value	Description
`0x01`	SET_PRESET	`0x00-0x03`	OFFICE / FULL / NIGHT / SPEECH
`0x02`	SET_LOUDNESS	`0x00`/`0x01`	Loudness OFF/ON
`0x03`	GET_STATUS	`0x00`	Request status notification
`0x04`	SET_MUTE	`0x00`/`0x01`	Unmute/Mute audio
`0x05`	SET_AUDIO_DUCK	`0x00`/`0x01`	Audio Duck OFF/ON (-12dB panic)
`0x06`	SET_NORMALIZER	`0x00`/`0x01`	Normalizer (DRC) OFF/ON
`0x07`	SET_VOLUME	`0x00-0x64`	Volume trim 0–100 (0=mute, 100=full)
`0x08`	SET_BYPASS	`0x00`/`0x01`	DSP Bypass OFF/ON (skip EQ, keep safety)
`0x09`	SET_BASS_BOOST	`0x00`/`0x01`	Bass Boost OFF/ON (+8dB @ 100Hz)
`0x0A`	SET_SINE_TEST	`0x00`/`0x01`	Sine Test OFF/ON (1kHz internal tone)

Preset Values

Value	Preset	Description
`0x00`	OFFICE	Mild EQ for background/office listening
`0x01`	FULL	Rich bass and treble enhancement
`0x02`	NIGHT	Balanced for low volume (volume capped at 60%)
`0x03`	SPEECH	Voice clarity for podcasts and calls

Status Notification Format (4 bytes)

Format: [VERSION] [PRESET] [LOUDNESS] [FLAGS]

Byte	Description
0	VERSION — Protocol version (0x01)
1	PRESET — Current preset (0–3)
2	LOUDNESS — Loudness state (0/1)
3	FLAGS — Status bitfield (see below)

FLAGS bitfield (byte 3):

Bit	Mask	Field	Description
0	0x01	Limiter	Always 1 (limiter is always active)
1	0x02	Clipping	Clipping detected
2	0x04	Thermal	Thermal warning
3	0x08	Muted	Audio is muted
4	0x10	Duck	Audio Duck active
5	0x20	Normalizer	DRC active
6	0x40	Bypass	DSP Bypass active

Note: FLAGS byte 3 has its own bit layout — it is not a copy of the internal s_dsp_flags variable. Correctly remapped since v2.4.3.

GalacticStatus Format (7 bytes)

Extended status with periodic notifications (every 500ms when subscribed).

Format: [VER] [PRESET] [FLAGS] [ENERGY] [VOLUME] [BATTERY] [LAST_CONTACT]

Byte	Field	Description
0	VER	Protocol version (always 0x42)
1	PRESET	Current preset (0–3)
2	FLAGS	Shield status bitfield
3	ENERGY	Reserved (0–100)
4	VOLUME	Effective volume level (0–100)
5	BATTERY	Reserved (0–100)
6	LAST_CONTACT	Seconds since last BLE communication (0–255)

Shield Status FLAGS (byte 2):

Bit	Mask	Field	Description
0	0x01	Muted	Audio is muted
1	0x02	Audio Duck	Volume reduced (panic mode)
2	0x04	Loudness	Loudness compensation enabled
3	0x08	Normalizer	DRC enabled
4	0x10	Bypass	DSP Bypass active (corrected v2.4.3)
5	0x20	Bass Boost	Bass Boost enabled (corrected v2.4.3)

Example Commands (Hex)

0100  - Set preset to OFFICE
0101  - Set preset to FULL
0102  - Set preset to NIGHT (volume capped at 60%)
0103  - Set preset to SPEECH
0200  - Disable loudness
0201  - Enable loudness
0300  - Request status
0400  - Unmute
0401  - Mute
0500  - Audio Duck OFF (normal volume)
0501  - Audio Duck ON (panic mode, -12dB)
0600  - Normalizer OFF
0601  - Normalizer ON
0764  - Set volume to 100%
073C  - Set volume to 60%
0700  - Set volume to 0%
0800  - DSP Bypass OFF (full DSP processing)
0801  - DSP Bypass ON (skip EQ, keep safety)
0900  - Bass Boost OFF
0901  - Bass Boost ON (+8dB @ 100Hz)
0A00  - Sine Test OFF
0A01  - Sine Test ON (1kHz internal tone)

OTA Commands (Write to OTA Control characteristic)

CMD	Name	Description
`0x10`	START	Start OTA download process
`0x11`	CANCEL	Cancel active OTA
`0x12`	REBOOT	Reboot to new firmware
`0x13`	GET_VERSION	Get current firmware version
`0x14`	ROLLBACK	Rollback to previous firmware
`0x15`	VALIDATE	Mark new firmware as valid

OTA Status Format (8 bytes)

Format: [STATE] [ERROR] [PROGRESS] [DL_KB_L] [DL_KB_H] [TOTAL_KB_L] [TOTAL_KB_H] [RSSI]

Byte	Field	Description
0	STATE	OTA state (0x00=Idle, 0x05=Downloading, 0x07=Success, 0xFF=Error)
1	ERROR	Error code (0=none)
2	PROGRESS	Download progress 0–100%
3-4	DOWNLOADED_KB	Bytes downloaded (little-endian KB)
5-6	TOTAL_KB	Total firmware size (little-endian KB)
7	RSSI	WiFi signal strength (signed dBm)

Legacy: ESP32-side DSP (≤ v2.3.1)

Up to and including v2.3.1, this firmware contained a full DSP processing chain running on the ESP32 itself (dsp_processor.c/h). This is useful as a reference or for standalone use without the STM32 DSP engine.

Last tag with ESP32-side DSP: v2.3.1

The v2.3.1 signal chain was:

Input → Pre-gain (-3dB) → HPF (95Hz) → Preset EQ → Loudness → Bass Boost → Normalizer → Limiter → Volume → Duck → Mute → Output

DSP components in v2.3.1:

Component	Description
Pre-gain	-3 dB headroom (always active)
High-pass Filter	2nd-order Butterworth @ 95 Hz
Preset EQ	4-band parametric EQ (OFFICE / FULL / NIGHT / SPEECH)
Loudness	Bass +12dB @ 80Hz, treble +6dB @ 12kHz
Bass Boost	Low-shelf +8dB @ 100Hz
Normalizer	Block DRC — threshold -20dB, ratio 4:1, +6dB makeup
Limiter	Peak limiter @ -1 dBFS (3ms attack, 120ms release)
Volume	Device-side trim 0–100
Audio Duck	-12dB panic reduction
Mute	Final output gate

The DSP module was removed in v2.4.0 ("Release v2.4.0: V4 architecture — A2DP sink + BLE GATT dual mode") when DSP processing was migrated to the external STM32 DSP engine.

Project Structure

├── CMakeLists.txt
├── README.md
├── Protocol.md                      # Full BLE protocol specification
├── FSD-DSP-001_ESP32_BLE_GATT.md    # Functional Specification Document
├── partitions_ota.csv               # OTA partition table
├── sdkconfig.defaults
└── main/
    ├── CMakeLists.txt
    ├── main.c                       # Application entry point + A2DP/I2S handling
    ├── ble_gatt_dsp.h/.c            # BLE GATT service + UART relay to STM32
    ├── nvs_settings.h/.c            # Persistent storage (NVS)
    ├── ota_manager.h/.c             # OTA state machine and download logic
    └── wifi_manager.h/.c            # WiFi STA mode for OTA downloads

Configuration

sdkconfig.defaults

Key configuration options:

# Dual-mode Bluetooth (Classic + BLE)
CONFIG_BTDM_CTRL_MODE_BTDM=y
CONFIG_BT_CLASSIC_ENABLED=y
CONFIG_BT_A2DP_ENABLE=y
CONFIG_BT_BLE_ENABLED=y
CONFIG_BT_GATTS_ENABLE=y

# Performance optimization
CONFIG_COMPILER_OPTIMIZATION_PERF=y

Customization

Device Name: Change BT_DEVICE_NAME in main.c
I2S Pins: Modify I2S_BCK_PIN, I2S_WS_PIN, I2S_DATA_PIN in main.c
UART DSP Relay: GPIO4/GPIO5 @ 115200 baud — always enabled when STM32 DSP engine is wired

Changelog

v2.4.3 (2026-02-18)

Fixed: Bypass and Bass Boost bits were swapped in GalacticStatus Shield byte (bit 4 was Bass Boost, bit 5 was Bypass — reversed from Protocol.md). Now bit 4 (0x10) = Bypass, bit 5 (0x20) = Bass Boost.
Fixed: Status Notify FLAGS byte (characteristic 0x0003, byte 3) was a raw copy of s_dsp_flags. Now correctly remapped to Protocol.md layout: bit 0 = limiter always 1, bit 3 = mute, bit 4 = duck, bit 5 = normalizer, bit 6 = bypass.

v2.4.2 (2026-02-16)

Fixed: Transient DSP state (duck, mute) was not reflected immediately in BLE status notifications.

v2.4.1 (2026-02-14)

New: Sine Test mode (command 0x0A) — generates a 1kHz internal tone for DAC/amp verification
Fixed: Audio path ring buffer with 50ms pre-buffering prevents DMA underrun at startup
Fixed: I2S output stuttering on fast connect/disconnect cycles

v2.4.0 (2026-02-10)

New: V4 architecture — dual Classic BT A2DP + BLE GATT in same firmware
New: UART echo of BLE GATT commands to external STM32 DSP engine (GPIO4/GPIO5 @ 115200)
New: BT RSSI monitoring and SBC codec quality logging
Removed: dsp_processor module — DSP processing migrated to external STM32 DSP engine

v2.3.0 (2026-01-28)

New: Bass Boost feature (+8dB @ 100Hz) — command 0x09
New: DSP Bypass command (0x08) — skips EQ, keeps safety processing
Fixed: Bypass mode now keeps pre-gain, limiter, volume (prevents hot tracks from clipping)
Fixed: Audio Duck debug log was printing "Bypass" incorrectly
Fixed: Sample rate detection for multi-bit SBC bitmasks (fallback to 44.1kHz)

v2.2.0 (2026-01-26)

DSP optimizations with block-based normalizer
Added initial DSP Bypass mode
Pre-gain adjusted to -3dB

v2.1.0 (2026-01-23)

OTA firmware update support (BLE + WiFi hybrid)
GalacticStatus periodic notifications every 500ms

v2.0.0 (2026-01-22)

Volume control with preset-based caps
Normalizer/DRC feature
Audio Duck panic button

v1.1.0 (2026-01-21)

Initial release with DSP presets and loudness

Related Projects

This firmware is one of three projects that form the 42dB audio system:

Project	Repository	Role
42dB STM32 DSP Engine	ChaoticVolt-42dB_STM32_DSP_engine	Real-time audio DSP processor
42dB iPhone & Apple Watch App	ChaoticVolt-42_Decibels-iPhone-and-WatchOS-app	BLE GATT control interface
ESP32 BLE GATT / A2DP firmware (this repo)	ChoticVolt-ESP32_I2S_Master_with_BLE_GATT	A2DP Bluetooth sink + GATT relay

Version Compatibility

The three projects communicate over a shared UART/BLE protocol (command format GATT:CTRL:<hex>). When the protocol changes, all three must be updated together:

Protocol	STM32 DSP Engine	ESP32 Firmware	iPhone/Watch App
v1	v0.3.0 – v0.5.x	v2.4.1 – v2.4.3	v1.0.x

License

PolyForm Noncommercial 1.0.0 — See LICENSE. Commercial use requires explicit written permission.

Author

Robin Kluit

References

Audio EQ Cookbook — Robert Bristow-Johnson's biquad filter formulas
ESP-IDF Programming Guide — Espressif Systems

Maintenance

This repo currently does not accept external pull requests. Please use Issues or Discussions for reports and suggestions.

UART control relay

Control updates destined for the DSP engine are translated and forwarded over UART or an equivalent low-level control path.

State reflection

The bridge helps keep the user-facing control layer aligned with device reality by reflecting relevant status back toward BLE clients.

Settings and update coordination

Where needed, the bridge may also participate in settings persistence, OTA-related coordination, and other platform glue responsibilities.

Architectural intent

This repository represents an architectural transition away from older all-in-one implementations.

Earlier iterations may have concentrated more roles inside one ESP32-based codebase, including DSP-adjacent behavior. The current role of this project is narrower and cleaner:

audio transport-facing integration
BLE control handling
bridge logic
state synchronization
downstream handoff to the DSP engine

The DSP engine now lives in its own dedicated repository:

ChaoticVolt-42dB-DSP-Engine

That separation is intentional and should remain visible throughout the repo.

Platform relationship

This repository makes the most sense when viewed together with:

ChaoticVolt-42dB-Companion-App
ChaoticVolt-42dB-DSP-Engine

Together, these repositories form the user-facing control path, the bridge layer, and the real-time DSP path of the 42dB platform.

Documentation

See the docs/ directory for deeper technical documentation.

Notes

This repository is a bridge and integration component.

It should be presented as a stable architectural role within the 42dB platform, even if the exact hardware partitioning and transport-side implementation details continue to evolve.

Name		Name	Last commit message	Last commit date
Latest commit History 23 Commits
docs		docs
main		main
.gitignore		.gitignore
BUILDING.md		BUILDING.md
CMakeLists.txt		CMakeLists.txt
CODE_OF_CONDUCT.md		CODE_OF_CONDUCT.md
LICENSE.txt		LICENSE.txt
README.md		README.md
partitions_ota.csv		partitions_ota.csv
sdkconfig		sdkconfig
sdkconfig.defaults		sdkconfig.defaults

Folders and files

Latest commit

History

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ChaoticVolt BLE-I2S Bridge

What this repository is

UART to External DSP (STM32)

Software Requirements

Role in the 42dB platform

At a glance

Core responsibilities

Bluetooth audio ingress

BLE GATT control

BLE Protocol

Service UUID

Characteristics

Command Format

Preset Values

Status Notification Format (4 bytes)

GalacticStatus Format (7 bytes)

Example Commands (Hex)

OTA Commands (Write to OTA Control characteristic)

OTA Status Format (8 bytes)

Legacy: ESP32-side DSP (≤ v2.3.1)

Project Structure

Configuration

sdkconfig.defaults

Customization

Changelog

v2.4.3 (2026-02-18)

v2.4.2 (2026-02-16)

v2.4.1 (2026-02-14)

v2.4.0 (2026-02-10)

v2.3.0 (2026-01-28)

v2.2.0 (2026-01-26)

v2.1.0 (2026-01-23)

v2.0.0 (2026-01-22)

v1.1.0 (2026-01-21)

Related Projects

Version Compatibility

License

Author

References

Maintenance

UART control relay

State reflection

Settings and update coordination

Architectural intent

Platform relationship

Documentation

Notes

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