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Audio & Voice Input Guide

This guide covers how to use the onboard RV1103B analog codec (acodec) to record from the Omega4 microphone input. The physical connection is Mic In P/N, which maps to the differential left ADC path (DiffadcL).

1. Prerequisites​

  • Firmware with audio enabled.
  • alsa-utils package.

Install tools if needed:

opkg update
opkg install alsa-utils

2. Quick start (record in 4 steps)​

2.1 Enable mic bias and select the ADC path​

amixer -c 0 sset 'ADC Main MICBIAS' On
amixer -c 0 sset 'ADC MICBIAS Voltage' VREFx0_975
amixer -c 0 sset 'ADC Mode' DiffadcL
amixer -c 0 sset 'ADC MIC Left' Work
amixer -c 0 sset 'ADC MIC Right' Work

2.2 Apply safe gain defaults​

amixer -c 0 sset 'ADC MIC Left Gain' 2
amixer -c 0 sset 'ADC MIC Right Gain' 2
amixer -c 0 sset 'ADC ALC Left' 24
amixer -c 0 sset 'ADC ALC Right' 24
amixer -c 0 sset 'ADC Digital Left' 195
amixer -c 0 sset 'ADC Digital Right' 195

2.3 Record a mono WAV file​

arecord -D hw:0,0 -f S16_LE -r 16000 -c 1 -d 5 /tmp/mic.wav

2.4 Copy the file to your host (optional)​

scp root@<dut-ip>:/tmp/mic.wav .

3. Controls reference (acodec on card 0)​

List all controls:

amixer -c 0 scontrols

Key controls:

  • ADC Main MICBIAS (Off/On): enables mic bias (required for electret mics).
  • ADC MICBIAS Voltage: choose a bias voltage; VREFx0_975 is the tested default.
  • ADC Mode: DiffadcL / SingadcL / DiffadcR / SingadcR / SingadcLR / DiffadcLR. For Mic In P/N on Omega4 use DiffadcL (mono).
  • ADC MIC Left/Right (Work/Mute): enable the ADC paths.
  • ADC MIC Left Gain / ADC MIC Right Gain (0..3): mic PGA gain.
    • 0x01 -> 0 dB
    • 0x02 -> 20 dB
    • 0x03 -> 12 dB
  • ADC ALC Left/Right (0..31): analog ALC gain, from -9 dB (0) to +37.5 dB (31) in 1.5 dB steps. 24 (~+27 dB) is a balanced starting point.
  • ADC Digital Left/Right (0..255): digital gain. 0 dB is 0xc3 (195), which is the baseline used here.
  • ADC HPF Cut-off: high-pass filter control; leave default unless you need to remove DC or very low frequency noise.

4. Gain tuning notes​

  • If the signal is too low, increase ALC first (for example 24 -> 28), then raise digital gain (for example 195 -> 210).
  • If the signal clips (flat peaks), reduce ALC and/or digital gain.
  • Clipping was observed when ALC was very high with additional digital gain.

5. Verification commands​

Capture 3 seconds at 16 kHz mono:

arecord -D hw:0,0 -f S16_LE -r 16000 -c 1 -d 3 /tmp/mic-test.wav

Check current mixer values:

amixer -c 0 sget 'ADC Main MICBIAS'
amixer -c 0 sget 'ADC MICBIAS Voltage'
amixer -c 0 sget 'ADC Mode'
amixer -c 0 sget 'ADC MIC Left'
amixer -c 0 sget 'ADC MIC Right'
amixer -c 0 sget 'ADC MIC Left Gain'
amixer -c 0 sget 'ADC MIC Right Gain'
amixer -c 0 sget 'ADC ALC Left'
amixer -c 0 sget 'ADC ALC Right'
amixer -c 0 sget 'ADC Digital Left'
amixer -c 0 sget 'ADC Digital Right'

6. Quick speech-to-text demo (cloud)​

Record audio and send it to a cloud STT API. Replace the API key placeholder before running.

API_KEY="YOUR_API_KEY"
arecord -D hw:0,0 -f S16_LE -r 16000 -c 1 -d 5 /tmp/mic.wav

curl -s https://api.openai.com/v1/audio/transcriptions \
  -H "Authorization: Bearer $API_KEY" \
  -F model=whisper-1 \
  -F file=@/tmp/mic.wav

7. Troubleshooting​

  • arecord shows no devices: confirm the audio codec and I2S/SAI interface are enabled in your firmware.
  • Audio is noisy or silent: verify the mic wiring, bias settings, and gain levels.
  • Playback fails: check that a speaker/amp is connected and aplay -l lists an output device.