Balena Audio POC

This repo presents a simple POC of a MAX98357A I2S Amplifier configured with a Raspberry Pi 3 running on Balena.

This is built as a standalone container, based around SimpleAudio, with project audio files pre-included. These can be played via a simple MQTT interface. Audio can also be stopped on the same interface.

Project audio files should be stored in audio dir, alongside the main audio_manager.py program.

Wiring

The diagram below shows the RPi header with the connections to the MAX98357A I2S Amplifier breakout board.

          .___.
      --1-|O X|--2-/-Vin
      --3-|O O|--4-
      --5-|O O|--6-/-GND
      --7-|O O|--8-
      --9-|O O|-10-
      -11-|O O|-12-/-BCLK
      -13-|O O|-14-
      -15-|O O|-16-
      -17-|O O|-18-
      -19-|O O|-20-
      -21-|O O|-22-
      -23-|O O|-24-
      -25-|O O|-26-
      -27-|O O|-28-
      -29-|O O|-30-
      -31-|O O|-32-
      -33-|O O|-34-
 LRC-/-35-|O O|-36-
      -37-|O O|-38-
GAIN-/-39-|O O|-40-/-DIN
          '---'
        RPi Header

In this case, the GAIN pin is connected to a Ground, this provides 12dB of gain. Gain can be tweaked as per table below:

100K resistor is connected between GAIN and GND	15dB
GAIN connected to GND	12dB
GAIN is not connected to anything	9dB
GAIN is connected to VIN	6dB
100K resistor is connected between GAIN and VIN	3dB

Note, distrotion was experienced when testing at 15dB.

Balena Settings

In either Fleet Configuration or Device Configuration in the Balena Project dashboard, the following settings need to be applied.

• Define DT parameters must be set to "i2c_arm=on","spi=on","audio=off"
• Under Custom configuration variables, add BALENA_HOST_CONFIG_dtoverlay with hifiberry-dac

MQTT Protocol

The MQTT topics that the Audio Container is subscribed to are customisable at runtime but the defaults are:

• Play topic: audio/play
• Stop topic: audio/stop

The play topic is used to trigger playback of a <filename> from the audio directory and it takes a JSON message as follows:

{
    'file': <filename>
}

The stop topic will imediately stop any playback that is running and it requires no payload.

Logs

Audio Container logs can be viewed by ssh'ing to a device and running:

$ tail -f /data/audio.log

Demo

This POC contains example .wav files and in scripts there is a basic python script that will cycle through each of these files and request playback via MQTT.

After pushing the POC to a Balena device, run the following (replace host with device hostname) from a python3 environment on the same network:

$ pip install -r demo_requirements.txt
$ python audio_demo.py -mqtt_host bda395a.local