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Text-Speech Provider Module

Overview

The Text-To-Speech Provider module synthesizes Alexa speech on demand. Auto SDK supports one text-to-speech provider, which uses Alexa's voice as the default voice for speech synthesis.

The Text-To-Speech Provider module performs the following functions:

  • Generating speech from a text or SSML document to provide the speech to the TTS module.
  • Providing capabilities based on the properties of the TTS provider, such as available locales.

The TTS Provider module follows the existing AVS (Alexa Voice Service) protocol to carry out speech synthesis and requires connection to the Local Voice Control (LVC) service. If the device is disconnected from LVC, speech synthesis fails.

Note: The module can synthesize speech only in the current locale as set by the application.

Configuring the TTS Provider Module

The Text-To-Speech Provider module does not require Engine configuration.

Specifying the TTS Provider in AASB Messages

In all of the text-to-speech AASB messages that involve the provider parameter, use the string, "text-to-speech-provider", to specify the TTS provider.

Using TTS Provider Module with Different Input Types

How the TTS Provider module synthesizes speech depends on the input type and the requestPayload field in the options parameter of the PrepareSpeech message published to the Engine when request speech synthesis. The requestPayload structure is as follows:

{
        "requestPayload" : {
            "voiceId" : {VOICE_IDENTIFIER},
            "locale" : {LOCALE_STRING}
        }
}

Note: You do not need to specify the requestPayload in the options parameter because currently only the default voice (Alexa’s voice) is supported. Therefore, you can leave the options parameter unspecified or supply an empty string for it.

The following list explains how the TTS Provider module synthesizes speech depending on the input type and requestPayload:

  • If input is text and requestPayload is empty, the text is synthesized to speech with Alexa's voice and the current locale.
  • If input is text and requestPayload specifies the voice and locale, the text is synthesized to speech in the specified voice and locale. The locale must be the current locale.

  • If input is text and requestPayload contains one or more unsupported parameters, speech is not synthesized. The speech synthesis fails with the error VOICE_NOT_SUPPORTED or LOCALE_NOT_SUPPORTED, depending on the unsupported parameter.

  • If input is SSML and contains all the supported tags and requestPayload specifies voice and locale, the SSML document is synthesized to speech in the specified voice and locale. The locale must be the current locale.

  • If input is SSML and contains all the supported tags, requestPayload is empty, the SSML document is synthesized to speech with Alexa's voice and current locale.
  • If input is SSML and contains all the supported tags, and requestPayload contains one or more unsupported parameters, speech is not synthesized. The speech synthesis fails with the error VOICE_NOT_SUPPORTED or LOCALE_NOT_SUPPORTED, depending on the unsupported parameter.
  • If input is SSML and contains one or more unsupported tags, speech is synthesized but the unsupported tag is ignored. The text within the tag is synthesized normally.

TTS Capability Returned

To request the capabilities of the TTS provider being used, your application publishes the GetCapabilities message. The Engine publishes the GetCapabilitiesReply message with the following payload structure:

"text-to-speech-provider" : {

    "voices" : [    
            {
                 "voiceId": "Alexa",
                 "locales": [CURRENT_LOCALE]
             }
    ]
}

NOTE: The locale returned is always the current locale because you can load only one locale model at a time, which is the current locale.

SSML Examples

  • <speak> ETA <break time=\"3s\"/> three hours </speak>
  • <speak> Turn right <emphasis level=\"strong\">in twenty feet</emphasis> </speak>
  • <speak> Turn right on<lang xml:lang=\"fr-CA\">Moulin Rouge street.</lang> </speak>
  • <speak> <p>Turn left in 500ft.</p> <p>Then turn right.</p> </speak>
  • <speak>Turn left on, <phoneme alphabet=\"ipa\" ph=\"Bo.fort\">Beaufort</phoneme></speak>
  • <speak> Turn right onto <phoneme alphabet='nt-sampa' ph='*\"stAk|t@n \"strit'>Stockton Street</phoneme> </speak>
  • <speak> Your ETA is 5 minutes on <say-as interpret-as=\"date\" format=\"dmy\">12-10-2020</say-as>. </speak>
  • <speak> Take a <prosody volume=\"-6dB\">U turn.</prosody> </speak>
  • <speak> Take the next left onto <sub alias=\"John Doe\">JD</sub> street </speak>

Errors

The TTS provider defines its set of error strings or codes. The PrepareSpeechFailed message payload uses the reason parameter to send the error strings to the application.

The following list describes the error strings used by the Local TTS provider:

  • LOCALE_NOT_SUPPORTED occurs in any of the following situations:

    • The language model for the current locale is missing.
    • The locale specified in requestPayload is not the current locale.
    • The locale specified is invalid.

  • VOICE_NOT_SUPPORTED occurs when the application specifies in requestPayload an invalid voice or is not Alexa.

  • INTERNAL_ERROR is an internal error that signifies an error when the TTS or TTS Provider module processes a request.

  • PROVIDER_NOT_CONNECTED occurs when the provider is not connected to LVC and a speech synthesis request is made.

NOTE: If a speech synthesis request is made during an ongoing Alexa dialog, the speech is synthesized after the current Alexa dialog ends.