Telephony Audio Formats

Telephony providers commonly send 8 kHz mono audio encoded as mu-law or A-law. Gradium supports those formats directly for STT and can produce telephony-friendly TTS output.

Recommended Formats

Use case	Gradium format	Notes
Incoming PSTN audio to STT	`ulaw_8000` or `alaw_8000`	Use the encoding your telephony provider sends.
Incoming linear PCM to STT	`pcm_8000` or `pcm_16000`	16-bit signed little-endian mono PCM.
Outgoing audio to PSTN	`ulaw_8000` or `alaw_8000`	Avoid resampling in your telephony bridge when possible.
Higher quality app audio	`pcm_24000`, `pcm_48000`, `wav`, or `opus`	Use outside PSTN constraints.

STT WebSocket Setup

{
  "type": "setup",
  "model_name": "default",
  "input_format": "ulaw_8000",
  "json_config": {"language": "en"}
}

Then send base64-encoded audio payloads:

{"type": "audio", "audio": "base64_encoded_ulaw_chunk"}

TTS WebSocket Setup

{
  "type": "setup",
  "voice_id": "YTpq7expH9539ERJ",
  "output_format": "ulaw_8000"
}

The audio messages contain base64-encoded mu-law chunks that can be forwarded to a telephony media stream.

Chunk Size Guidance

Many telephony media streams send 20 ms frames. Gradium accepts small chunks, but batching to roughly 80 ms can reduce message overhead while keeping latency low:

Format	20 ms payload	80 ms payload
`ulaw_8000` / `alaw_8000`	160 bytes	640 bytes
`pcm_8000`	320 bytes	1280 bytes
`pcm_16000`	640 bytes	2560 bytes

Bridge Checklist

Preserve the provider’s stream/session ID in your own logs.
Use one Gradium STT session per caller audio stream.
Set input_format to the actual bytes you forward.
For TTS, request an output format your provider can play directly.
Use STT step messages or your provider’s VAD to decide when to send the user’s turn to an agent.
Close the WebSocket and start a new session if the call leg changes format.

Speech-to-Text WebSocket

Real-time transcription with VAD and flush.

Text-to-Speech WebSocket

Stream generated audio back to your telephony bridge.

Getting Started

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Telephony Audio Formats

Recommended Formats

STT WebSocket Setup

TTS WebSocket Setup

Chunk Size Guidance

Bridge Checklist

Speech-to-Text WebSocket

Text-to-Speech WebSocket

​Recommended Formats

​STT WebSocket Setup

​TTS WebSocket Setup

​Chunk Size Guidance

​Bridge Checklist

​Related

Speech-to-Text WebSocket

Text-to-Speech WebSocket

Recommended Formats

STT WebSocket Setup

TTS WebSocket Setup

Chunk Size Guidance

Bridge Checklist

Related