HubAudio/docs/adr/ADR-003-I2S-Audio-Routing-Architecture.md

ADR-003: I2S Audio Routing Architecture

• Status: Accepted
• Date: 2026-08-01
• Decision Type: Audio Architecture

Context

HubAudio integrates multiple digital audio sources and destinations:

Sources:

• System Controller streaming subsystem
• Radio Receiver radio receiver
• Bluetooth receiver
• Optical digital input

Destinations:

• Analog audio output through CODEC/DAC
• Bluetooth transmission
• Optical digital output

A deterministic and scalable digital audio routing architecture is required.

Decision

The Audio Processor is the central routing element of the Audio Domain.

Each major audio device is assigned to a dedicated serial audio interface.

The preferred architecture is:

One audio device = One dedicated I2S interface

The Audio Processor operates as the audio timing reference.

External audio devices operate as I2S slaves whenever supported.

Audio Input Allocation

Audio Processor Port	Device	Function
SDATA_IN0	System Controller	Network audio stream
SDATA_IN1	Radio Receiver	Radio audio
SDATA_IN2	DECODEC	Optical digital input
SDATA_IN3	Bluetooth RX	Wireless audio input

System Controller -------- Radio Receiver --------- DECODEC ----------+---- Audio Processor BT RX ---------/

Audio Output Allocation

Audio Processor Port	Device	Function
SDATA_OUT0	CODEC/DAC	Analog audio output
SDATA_OUT1	Bluetooth TX	Wireless audio output
SDATA_OUT2	ENCODEC	Optical digital output
SDATA_OUT3	Reserved	Future expansion

         Audio Processor

             |
+------------+------------+
|            |            |

CODEC BTTX ENCODEC

Clocking

The Audio Processor provides the master audio timing reference.

The audio clock domain consists of:

• MCLK
• BCLK
• LRCLK

All connected audio peripherals must operate synchronized to this clock domain.

Consequences

Positive

• Deterministic audio timing
• Simple routing model
• Independent audio channels
• Easier debugging
• Future expansion capability

Negative

• Higher pin usage
• More PCB routing resources required
• Peripheral selection must consider I2S slave capability

Rationale

HubAudio prioritizes signal integrity, maintainability and deterministic audio behavior over maximum bus utilization.

The Audio Processor acts as the central audio router of the system.