Reference for supported audio formats across all Velma-2 endpoints, with guidance on format selection, conversion, and the special requirements of the streaming Synthetic Voice Detection (SVD) endpoint.
Every endpoint advertises its accepted audio_format values in its spec. The spec is the authoritative list — check the API reference for each endpoint to confirm exact format support, since formats are added over time. Broadly:
- HTTP batch endpoints (STT Batch, STT English VFast, SVD Batch, Redaction Batch) accept common container formats; the file extension determines decoding. See each spec for the current list.
- STT Streaming and Redaction Streaming accept self-describing container formats (auto-detected from headers) and raw PCM / mu-law / A-law (which require
audio_format, sample_rate, and num_channels).
- SVD Streaming has the broadest container support plus raw PCM, with required
audio_format and conditional sample_rate / num_channels for raw formats.
Recommended maximum file size for all HTTP batch endpoints: 100 MB.
audio_format query parameter.
Container formats include metadata — sample rate, channel count, codec — within the stream itself. When using a container format, sample_rate and num_channels are not required. See the SVD Streaming spec’s audio_format enum for the full list of accepted container values.
wss://...?api_key=YOUR_API_KEY&audio_format=webm
sample_rate and num_channels are required query parameters when using any raw format.
wss://...?api_key=YOUR_API_KEY&audio_format=s16le&sample_rate=16000&num_channels=1
s8, s16le, s16be, s24le, s24be, s32le, s32be, u8, u16le, u16be, u24le, u24be, u32le, u32be, f32le, f32be, f64le, f64be, mulaw, alaw
| Use case | audio_format | sample_rate | num_channels |
|---|
| Default / native app | s16le | 16000 | 1 |
Web Audio API (AudioWorklet) | f32le | 48000 | 1 |
| Native stereo capture | s16le | 48000 | 2 |
| Telephony (mu-law) | mulaw | 8000 | 1 |
| Telephony (A-law) | alaw | 8000 | 1 |
The s16le passthrough optimization
When the input is s16le at 16 kHz mono, no format conversion is performed before analysis. This is the most efficient configuration for the SVD Streaming endpoint.
All other formats are decoded and resampled to 16 kHz mono before analysis. There is no functional difference in output, but the passthrough path avoids the conversion overhead.
If you control the audio capture pipeline and are integrating with the SVD Streaming endpoint, capture in s16le at 16 kHz mono to take advantage of zero-cost passthrough.
Supported sample_rate values
8000, 11025, 16000, 22050, 32000, 44100, 48000, 96000
num_channels range
1–8. Multi-channel audio is downmixed to mono before analysis.
| Endpoint | Status / code | Cause |
|---|
| STT English VFast | 400 | Unsupported file extension, empty file, or decode error |
| STT Batch | 400 | Unsupported format or empty file |
| SVD Batch | 400 | Empty file or unsupported format |
| SVD Batch | 422 | Audio shorter than 0.5 seconds |
| SVD Streaming | Close code 1003 | Invalid audio_format, sample_rate, or num_channels query parameter |
| SVD Streaming | Close code 4002 | Audio could not be decoded or does not match the declared format |
The SVD Streaming endpoint validates format parameters at connection time (close code 1003) and again when it attempts to decode audio (close code 4002). This covers both undecodable audio (e.g. a truncated or corrupt stream) and format mismatches (e.g. declaring audio_format=s16le but sending WebM data).
