> ## Documentation Index
> Fetch the complete documentation index at: https://assemblyai.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Audio requirements

> Duration, size, format, and sample-rate constraints for Sync STT audio.

The Sync API accepts short audio clips with the following constraints:

| Constraint        | Value                                             |
| ----------------- | ------------------------------------------------- |
| Minimum duration  | 80 ms                                             |
| Maximum duration  | 120 s                                             |
| Maximum file size | 40 MB                                             |
| Sample width      | 16-bit only                                       |
| Channels          | Mono or stereo (stereo is down-mixed internally)  |
| Sample rates      | 8000, 16000, 22050, 24000, 32000, 44100, 48000 Hz |
| Formats           | WAV (`audio/wav`) or raw PCM S16LE (`audio/pcm`)  |

For WAV, the sample rate and channel count are read from the file header. For raw PCM, pass `sample_rate` and `channels` in the `config` part — see [Sending raw PCM audio](/sync-stt/getting-started/transcribe-a-short-audio-file#sending-raw-pcm-audio).

Audio that violates a constraint is rejected with a `400`, `413`, or `415` — see [Error handling](/sync-stt/error-handling) for the full error table.

For audio longer than 120 seconds, use [Pre-recorded STT](/pre-recorded-audio/getting-started/transcribe-an-audio-file). For live audio, use [Real-time STT](/streaming/getting-started/transcribe-streaming-audio).