Text to speech mulaw 8000 Hz: Gemini 3.8 and Sume TTS

Sume TTS 1.0 can output mu-law or A-law at 8000 Hz: set output_format to a wav or raw container, encoding to pcm_mulaw or pcm_alaw, and sample_rate to 8000. The default is mp3, so you have to ask. Gemini 3.8 TTS offers the same codecs through audio/mulaw and audio/alaw mime types.

Gemini facts are from its speech-generation guide; Sume facts from the API reference. Read 2026-10-01.

How does Gemini ask for telephony audio?

The guide lists audio/mulaw (8-bit G.711 mu-law, common in North American and Japanese telephony) and audio/alaw (8-bit G.711 A-law, common in European and international telephony), set in response_format.mime_type, with an optional sample_rate such as 8000.

What does the Sume request look like?

Only the output_format object changes. Encoding applies to wav and raw containers, so an mp3 request cannot carry mu-law.

{
  "output_format": {
    "container": "wav",
    "encoding": "pcm_mulaw",
    "sample_rate": 8000
  }
}

What should I check before using it on a phone line?

Play the file through your carrier or PBX once; the reference does not describe telephony testing. Pick mu-law or A-law to match your trunk. For prompt content and structure, see IVR and voicemail prompts with text to speech.