Extract Audio

## Format Decision Tree

| User wants | Format | Flags | Why |
|------------|--------|-------|-----|
| Music, archive quality | FLAC | `-c:a flac` | Lossless, no quality loss |
| Music, small + transparent | MP3 VBR | `-c:a libmp3lame -q:a 0` | ~200kbps avg, perceptually lossless |
| Podcast / voice | MP3 128k CBR | `-c:a libmp3lame -b:a 128k` | Sufficient for speech, universally compatible |
| Mobile / streaming | AAC 192k | `-c:a aac -b:a 192k` | Better than MP3 at equivalent bitrate |
| DAW / editing | WAV | `-c:a pcm_s16le -ar 44100` | No encoding loss, widest DAW support |
| Source already target format | Copy | `-c:a copy` | No re-encode, instant, lossless |

## Process

### 1. Probe audio streams

```bash
ffprobe -v quiet -print_format json -show_streams "$INPUT" | \
  python3 -c "
import json, sys
streams = [s for s in json.load(sys.stdin)['streams'] if s['codec_type']=='audio']
for i, s in enumerate(streams):
    print(f'Stream {i}: {s[\"codec_name\"]} {s.get(\"bit_rate\",\"?\")} bps {s.get(\"channel_layout\",\"?\")}')
"
```

### 2. Determine format

Apply the decision tree above if the user didn't specify. If the source audio codec already matches the target, use `-c:a copy` to avoid transcoding.

If multiple audio streams exist, ask the user which to extract — or use `-map 0:a` to extract all. Once the user responds, apply `-map 0:a:N` (where N is the zero-based stream index they chose) or `-map 0:a` for all streams in the Phase 3 command.

### 3. Construct command

```bash
# General pattern (-vn drops the video stream entirely):
ffmpeg -i "$INPUT" -vn [FORMAT_FLAGS] "$OUTPUT"

# Examples:
ffmpeg -i video.mp4 -vn -c:a libmp3lame -q:a 0 audio.mp3        # MP3 VBR best quality
ffmpeg -i video.mp4 -vn -c:a libmp3lame -b:a 128k podcast.mp3   # MP3 128k CBR
ffmpeg -i video.mp4 -vn -c:a flac archive.flac                   # FLAC lossless
ffmpeg -i video.mp4 -vn -c:a aac -b:a 192k mobile.aac           # AAC
ffmpeg -i video.mp4 -vn -c:a pcm_s16le -ar 44100 edit.wav       # WAV for DAW
ffmpeg -i video.mp4 -vn -c:a copy original.m4a                  # Copy audio stream
```

### 4. Confirm and run

Show: detected source codec and bitrate, chosen output format, output path. Wait for approval, then run.

Report output file size and duration: `ffprobe -v quiet -show_format "$OUTPUT" | grep -E "duration|size"`

## Key Decisions

Preserve generation quality: avoid transcoding chains that degrade source fidelity. Each decision below is an application of this principle.

- **Lossy-to-lossy warning**: if the source is already lossy (MP3, AAC, OGG) and the user wants a different lossy format, warn them that re-encoding degrades quality. Recommend keeping the source format or using `-c:a copy` where container compatibility allows.
- For files >1 hour, ask whether the user wants the full file or a specific range — trimming can be added with `-ss` and `-to` before `-vn`.
- M4A vs AAC: AAC is the codec, M4A is the container. Use `.m4a` extension for Apple device compatibility; use `.aac` for a raw stream.