Summary
On a multichannel audio interface (Universal Audio Apollo, Focusrite, MOTU, etc.), Amical records near-silence if the microphone is on any input other than channel 1. Whisper then returns "no words detected", even though the interface, the mic, and all OS permissions are correct.
Environment
- macOS (Apple Silicon), Amical 1.9.x, Local / Whisper Medium
- Audio interface: Universal Audio Apollo Twin (2 preamps; front Hi-Z instrument = ch1, rear Mic 2 = ch2)
- Microphone (SM58) on rear input 2 → Core Audio channel 2
What happens
- Amical history shows repeated "no words detected".
- The recorded WAVs (
$TMPDIR/amical-audio/*.wav) are full length but ~-66 dBFS (digital silence).
- Capturing the interface directly and measuring per channel shows the voice is on channel 2, while channel 1 is silent (it's the empty Hi-Z instrument input).
- Zoom / Google Meet / other dictation apps capture the voice fine (they take stereo / the correct channel).
Root cause
The recorder requests a mono stream:
getUserMedia({ audio: { channelCount: 1, sampleRate, echoCancellation: false, noiseSuppression: false, autoGainControl: false } })and the audio-recorder-processor AudioWorklet consumes only channel 0:
const channelData = input[0]; // channel 0 == interface channel 1
With channelCount: 1, Chromium hands over only the device's first channel, so any mic on channel 2+ is dropped. (Confirmed empirically: the recorded audio is bit-for-bit the interface's silent channel 1, not a downmix.)
Impact
Anyone using a pro audio interface with the mic on an input other than #1 — an extremely common studio configuration — gets unusable, silent dictation with no error explaining why.
Suggested fix
Capture at least two channels and let the worklet pick the channel that carries signal (or expose a channel selector). A minimal, backward-compatible change:
channelCount: 1 → channelCount: 2- worklet:
const channelData = input[0]; → const channelData = input[1] || input[0];
The || input[0] keeps ordinary mono microphones working unchanged. A more complete fix could auto-select the loudest channel or add a UI channel picker. PR with the minimal version to follow.
Summary
On a multichannel audio interface (Universal Audio Apollo, Focusrite, MOTU, etc.), Amical records near-silence if the microphone is on any input other than channel 1. Whisper then returns "no words detected", even though the interface, the mic, and all OS permissions are correct.
Environment
What happens
$TMPDIR/amical-audio/*.wav) are full length but ~-66 dBFS (digital silence).Root cause
The recorder requests a mono stream:
and the
audio-recorder-processorAudioWorklet consumes only channel 0:With
channelCount: 1, Chromium hands over only the device's first channel, so any mic on channel 2+ is dropped. (Confirmed empirically: the recorded audio is bit-for-bit the interface's silent channel 1, not a downmix.)Impact
Anyone using a pro audio interface with the mic on an input other than #1 — an extremely common studio configuration — gets unusable, silent dictation with no error explaining why.
Suggested fix
Capture at least two channels and let the worklet pick the channel that carries signal (or expose a channel selector). A minimal, backward-compatible change:
channelCount: 1→channelCount: 2const channelData = input[0];→const channelData = input[1] || input[0];The
|| input[0]keeps ordinary mono microphones working unchanged. A more complete fix could auto-select the loudest channel or add a UI channel picker. PR with the minimal version to follow.