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Audio Pipeline

Audio capture, ring buffer, and processing.

Data Flow

Microphone (Oboe / AVAudioEngine)
  → Uint8List (PCM16 little-endian, 32 kHz mono)
  → pcm16ToFloat32 (normalized −1.0 … 1.0)
  → RingBuffer.write
  → downstream consumers (spectrogram, inference, recording)

Ring Buffer

RingBuffer is a fixed-size circular buffer of Float32List samples. Multiple consumers can read from it simultaneously:

  • Spectrogram: reads the latest fftSize samples each frame
  • Inference: reads windowDuration * sampleRate samples each cycle
  • Recording: periodically flushes new samples to a WAV file

Audio Capture Service

AudioCaptureService wraps the platform microphone and pushes PCM16 data into the ring buffer. It also exposes an RMS level stream (~15 Hz) for UI metering.

Microphone-loss handling

A lost microphone (stalled stream or stream error) is retried with a capped exponential back-off while the app is in the foreground. In the background the watchdog only retries when something legitimately keeps the app capturing: on Android a mode must own the shared microphone foreground service (ForegroundServiceGuard.owner != null), and the retry cadence drops to a probe every 30 s to 2 min so a contended microphone is never hammered (that wedged the audio HAL on some devices); on iOS the audio background mode keeps the app alive and the record plugin pauses on an interruption without resuming, so the normal cadence applies. Without a reason to capture, the recorder is released once and reclaimed on the next foreground transition.

Recording Service

RecordingService supports two modes:

  • Full: Continuously flushes the ring buffer to an audio file (WAV or FLAC, depending on user settings) at 1-second intervals via AudioFileWriter. The service writes through IsolateAudioFileWriter, which hosts the real WavWriter / FlacEncoder on a worker isolate and forwards each call: the FLAC encoder is pure Dart, and the UI isolate is also the platform thread on Android and iOS, so a second of encoding every second must not run there. Each call completes when the worker has finished it, which preserves the flush loop's back-pressure.
  • Detections Only: Saves audio clips centered on detection timestamps (pre-context + inference window + post-context) in the chosen format (WAV or FLAC). Every clip of a detection round is encoded from one snapshot in a single Isolate.run (writeClipFiles).

On iOS, voice memos automatically fall back to .wav (PCM16) format to avoid Apple CoreAudio AAC compression collisions, while preserving full file-sharing and packaging compatibility.