The mic uplink handed the host pure digital silence on a multi-channel
interface: AVAudioConverter's N→stereo downmix takes channels 0/1, but a
pro interface puts the mic on ONE higher discrete channel. Fold the input
to a mono bus ourselves instead — pick the mic's channel (or sum all) and
resample that to the encoder's 48 kHz stereo, so the silent 0/1 downmix
never happens.
- New "Microphone channel" setting (macOS): Auto (sum every channel — a
lone hot mic passes at full level) or pin 1-based channel N. Picker
appears only for multi-channel devices, driven by the device's input
channel count.
- Diagnostics that make this class of failure self-naming next session:
log the actual live capture device + format + fold mode, warn on a
silent UID fallback, and a one-shot silence tripwire on the EXTRACTED
signal (WARN on 10 s of zeros, else peak dBFS).
- foldToMono extracted as a pure, unit-tested helper (pin / sum-clamp x
interleaved / deinterleaved / mono / out-of-range).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
