5 min read
AVI to MP3: pulling audio out of legacy footage
AVI is the format archives keep handing you. Its audio can be pristine PCM or a compressed codec from 2003, and the difference matters.
AVI predates most of what a modern post workflow assumes. It has no proper timecode structure, weak metadata and a loose relationship with variable frame rates. What it does have, quite often, is clean uncompressed audio from a DV or early digital transfer.
Identify before you convert
- PCM audio: extract straight to WAV, nothing is lost.
- MP3 or AC3 inside AVI: already lossy, decode once and keep the result.
- 48 kHz or 32 kHz: DV era material often sits at 32 kHz and must be resampled deliberately.
The 32 kHz trap
DV cameras recorded 12 bit 32 kHz audio in four channel mode. Dropping that into a 48 kHz timeline without a proper sample rate conversion produces a pitch and duration error that grows across the clip. Convert explicitly, do not let the timeline guess.
Variable frame rate footage
Old screen captures and consumer camcorder AVIs sometimes carry a variable frame rate. The audio is continuous but the picture is not, so an extract that looks correct in isolation drifts against the conformed video. Check the length of the extracted audio against the reported duration before trusting it.
What to keep
Archive the extracted WAV, not the AVI's audio stream alone. A named, documented WAV with its sample rate written into the filename saves the next person a forensic session.