# Live Streaming Microphone Transcription

## Summary

Add a `--stream` mode to `transcribe.py` that continuously captures audio from the microphone, detects speech segments using energy-based VAD, and transcribes each segment in near-real-time using the Cohere ASR model. Output scrolls as timestamped lines in the terminal. Ctrl+C stops the session.

## Context

- **Model**: CohereLabs/cohere-transcribe-03-2026, max 35s audio clips, 5s overlap for auto-chunking
- **Inference speed**: ~0.4s for 5-10s audio on GPU (0.04-0.08x real-time)
- **Microphone**: PD200X Podcast Microphone via PipeWire, 16kHz mono
- **Existing code**: `transcribe.py` has `--mic` (fixed duration) and demo file modes

## Architecture

### Audio Capture

`sounddevice.InputStream` with a callback streams 16kHz mono float32 audio into a thread-safe buffer. The callback appends raw samples; a separate consumer reads them.

### Voice Activity Detection

Energy-based VAD using RMS amplitude over 50ms frames (800 samples at 16kHz):

- **Threshold**: Calibrated from ~0.5s of ambient silence at startup, with a sensible fallback (~-40 dBFS)
- **State machine**: `SILENCE -> SPEAKING -> SILENCE`
  - SILENCE -> SPEAKING: RMS exceeds threshold for >= 3 consecutive frames (~150ms)
  - SPEAKING -> SILENCE: RMS stays below threshold for >= 0.8s
- **Pre-roll**: ~0.3s of audio before speech onset is included to avoid clipping word beginnings
- **Safety cap**: If speech exceeds 30s without a pause, force a chunk boundary (model max is 35s)

### Threading Model

Two threads communicating via `queue.Queue`:

1. **Audio thread** (sounddevice callback + VAD logic): captures audio, runs VAD state machine, pushes completed speech segments onto the queue
2. **Transcription thread**: pulls segments from the queue, runs `processor() -> model.generate() -> processor.decode()`, prints results

No state carried between segments. Each is transcribed independently.

### Output

Timestamped lines printed to stdout as each segment is transcribed:

```
[00:03] Good morning, this is a test of the live captioning system.
[00:08] The model seems to be picking up my voice pretty well.
```

### Shutdown

Ctrl+C sets a stop flag via signal handler. The audio stream stops, any buffered speech is flushed and transcribed, then the program exits cleanly.

## CLI Interface

```
uv run python transcribe.py --stream            # stream, default language (en)
uv run python transcribe.py --stream --lang ja   # stream in Japanese
uv run python transcribe.py --mic [duration]     # existing fixed-duration mode
uv run python transcribe.py                      # existing demo file mode
```

### Startup Sequence

1. Print "Loading model..." and load model
2. Record ~0.5s of ambient audio, compute silence threshold
3. Print threshold info and "Listening... (Ctrl+C to stop)"
4. Begin streaming

## Dependencies

No new dependencies. Uses: `sounddevice`, `numpy`, `threading`, `queue`, `signal`, `time` (all already available).

## Code Organization

All new logic in `transcribe.py`. File grows from ~50 to ~150-180 lines. No new files.

## Constraints

- Model max input: 35s per chunk (safety cap at 30s)
- Sampling rate must be 16kHz
- Single-channel (mono) audio only
