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

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

# 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.