Build with AssemblyAISDK ReferencesC#

Streaming Speech-to-Text

AssemblyAI’s Streaming Speech-to-Text (STT) allows you to transcribe live audio streams with high accuracy and low latency. By streaming your audio data to our secure WebSocket API, you can receive transcripts back within a few hundred milliseconds.

Supported languages

Streaming Speech-to-Text is only available for English.

Audio requirements

The audio format must conform to the following requirements:

  • PCM16 or Mu-law encoding (See Specify the encoding)
  • A sample rate that matches the value of the supplied sample_rate parameter
  • Single-channel
  • 100 to 2000 milliseconds of audio per message

Audio segments with a duration between 100 ms and 450 ms produce the best results in transcription accuracy.

Specify the encoding

By default, transcriptions expect PCM16 encoding. If you want to use Mu-law encoding, you must set the Encoding parameter to AudioEncoding.PcmMulaw:

1await using var transcriber = new RealtimeTranscriber(new RealtimeTranscriberOptions
2{
3    ...
4    Encoding = AudioEncoding.PcmMulaw
5});
EncodingSDK ParameterDescription
PCM16 (default)AudioEncoding.PcmS16lePCM signed 16-bit little-endian.
Mu-lawAudioEncoding.PcmMulawPCM Mu-law.

Add custom vocabulary

You can add up to 2500 characters of custom vocabulary to boost their transcription probability.

For this, create an array of strings and set the WordBoost parameter:

1await using var transcriber = new RealtimeTranscriber(new RealtimeTranscriberOptions
2{
3    ...,
4    WordBoost = ["aws", "azure", "google cloud"]
5});

If you’re not using one of the SDKs, you must ensure that the word_boost parameter is a JSON array that is URL encoded. See this code example.

Authenticate with a temporary token

If you need to authenticate on the client, you can avoid exposing your API key by using temporary authentication tokens. You should generate this token on your server and pass it to the client.

1

To generate a temporary token, call client.Realtime.CreateTemporaryTokenAsync().

Use the expires_in parameter to specify how long the token should be valid for, in seconds.

1var tokenResponse = await client.Realtime.CreateTemporaryTokenAsync(expiresIn: 60);
The expiration time must be a value between 60 and 360000 seconds.
2

The client should retrieve the token from the server and use the token to authenticate the transcriber.

Each token has a one-time use restriction and can only be used for a single session.

1To use it, specify the `token` parameter when initializing the streaming transcriber.
2
3await using var transcriber = new RealtimeTranscriber(new RealtimeTranscriberOptions
4{
5    Token = tokenResponse.Token,
6    ...
7});

Manually end current utterance

To manually end an utterance, call ForceEndUtteranceAsync():

1await transcriber.ForceEndUtteranceAsync();

Manually ending an utterance immediately produces a final transcript.

Configure the threshold for automatic utterance detection

You can configure the threshold for how long to wait before ending an utterance.

To change the threshold, call ConfigureEndUtteranceThresholdAsync() while the transcriber is connected.

1await transcriber.ConfigureEndUtteranceThresholdAsync(500);

By default, Streaming Speech-to-Text ends an utterance after 700 milliseconds of silence. You can configure the duration threshold any number of times during a session after the session has started. The valid range is between 0 and 20000.

Disable partial transcripts

If you’re only using the final transcript, you can disable partial transcripts to reduce network traffic.

To disable partial transcripts, set the DisablePartialTranscripts parameter to true.

1await using var transcriber = new RealtimeTranscriber(new RealtimeTranscriberOptions
2{
3    ...,
4    DisablePartialTranscripts = true
5});

Enable extra session information

The client receives a SessionInformation message right before receiving the session termination message. Subscribe to the SessionInformationReceived event to receive the message.

1transcriber.SessionInformationReceived.Subscribe(info =>
2{
3    Console.WriteLine("Session information:\n- duration: {0}", info.AudioDurationSeconds);
4});

For best practices, see the Best Practices section in the Streaming guide.