Playback speed

Control speaking speed in Rime synthesis using speedAlpha and inlineSpeedAlpha.

Speed controls use different parameters and value directions by model. Use this table before changing a request:

Scope	Models	Parameter	Faster speech	Slower speech
Entire response	Coda, Mist v3	timeScaleFactor	Below 1.0	Above 1.0
Entire response, compatibility	Coda, Mist v3	speedAlpha	Above 1.0	Below 1.0
Selected words, HTTP only	Mist v3	inlineSpeedAlpha	Below 1.0	Above 1.0

Adjusting the overall speed

Coda and Mist v3

Use the timeScaleFactor parameter. A value above 1.0 slows the audio, and a value below 1.0 speeds it up.

{
    ...otherParams,
    "text": "Hello, world!",
    "modelId": "coda",
    "timeScaleFactor": 0.85
}

Coda and Mist v3 also accept speedAlpha, but it works in the opposite direction from timeScaleFactor (higher than 1.0 is faster, lower is slower). Use timeScaleFactor for these models.

timeScaleFactor works over both HTTP and WebSocket endpoints. For /ws and /ws3, pass it as a query parameter when opening the connection.

Adjusting the speed of individual words

Per-word speed adjustment with inlineSpeedAlpha works on Mist v3 over HTTP. Coda doesn’t support it, and the WebSocket endpoints don’t read it.

To adjust the speed of individual words or phrases on Mist v3, use the inlineSpeedAlpha parameter. It takes a comma-separated list of speed values applied to words in square brackets.

On Mist v3, values < 1.0 speed up speech and values > 1.0 slow it down.

On Mist v3, inlineSpeedAlpha and speedAlpha go in opposite directions. speedAlpha is higher-is-faster, like Coda, while inlineSpeedAlpha is lower-is-faster. Don’t infer one from the other.

For example: “This sentence is [really] [fast]” with inlineSpeedAlpha “0.5, 3” will make “really” fast and “fast” slow on Mist v3.

{
    ...otherParams,
    "modelId": "mistv3",
    "text": "This sentence is [really] [fast]",
    "inlineSpeedAlpha": "0.5, 3"
}

Evaluate the result

Start near 1.0 and evaluate representative audio before increasing the adjustment. Large changes can reduce naturalness and clarity.