File formats and audio
Audio format
The file format the audio is stored in, for instance m4a, mp3 or wav.
The audio format is the way the recording is stored. For transcription the choice matters less than people think, with one exception: how hard the audio has been compressed.
The common ones
| Format | Type | Where you meet it |
|---|---|---|
| m4a | Compressed (AAC) | Standard on iPhone and Mac |
| mp3 | Compressed | Everywhere, older standard |
| wav | Uncompressed | Recording equipment, editing |
| flac | Compressed without loss | Archives |
| aac | Compressed | Streaming, video |
| ogg / opus | Compressed | Browser recording, video meetings |
| mp4 / mov | Video with an audio track | Screen recording, video meetings |
Nearly all transcription tools accept all of these. Video is unpacked to the audio track automatically.
Lossy and lossless compression
Lossless (wav, flac) keeps all the information from the recording.
Lossy (mp3, m4a, aac, opus) throws information away to save space. At ordinary settings this is not noticeable to the human ear, nor to the transcription.
The problem arises with hard compression. An mp3 at 32 kbit/s, or a speech recording that has been compressed twice because it passed through several stages, loses details the model needs. Consonants go first, and consonants are precisely what distinguishes words from one another.
Practical rule
The default setting on your phone or recorder is fine. Do not pick “low quality” to save space, and do not convert a file more times than necessary. One minute of speech in m4a at the default setting is under a megabyte.
What actually decides the result is not the format, but how close the microphone was to whoever was speaking.