AVI to MP3

The one container where MP3 really is lossless

Every other page on this site has to explain that MP3 is a second encode. AVI is the exception. The format dates from the era when MP3 was the audio codec, so a DivX or Xvid film from a camcorder, a download from the 2000s or an old family video almost always has an MP3 stream inside it. Asking for an MP3 then means opening the container, taking that stream out and giving it a two-byte header — the audio is bit-for-bit what it was.

You can see this in the panel: it says “Nothing is re-encoded” and the extraction finishes in about a second. It is the same trick that makes MP4 to M4A take seconds, applied to the container that got there first.

When an AVI has something else inside

Not all of them do. Older or stranger AVIs can carry MS ADPCM, uncompressed PCM, AC-3 or MP2 audio, and a few carry the DivX-era audio codecs that no longer exist anywhere. The extractor reads what is really there and tells you: if it is not MP3, the MP3 you asked for is an encode like anywhere else, and the panel says so before it starts.

PCM inside an AVI is worth a second look — that is uncompressed audio, usually from a capture card or an old camcorder in its highest mode. WAV or FLAC keeps every sample of it; MP3 is the first thing that has ever thrown any of it away.

Damaged and half-downloaded files

AVIs are old enough that a lot of them are broken. A file that stops halfway, an index that was never written, a download that was abandoned — all of these are common, and all of them usually still play in VLC, which is forgiving. FFmpeg is stricter. If the extractor refuses a file that VLC plays, the audio stream is likely intact but the index is not; there is no fix for that here, and a desktop FFmpeg with the recovery flags is the tool for it.