The proto file provides the following defaults for messages in SynthesisRequest. Mandatory fields are shown in bold.

Items in SynthesisRequest Default
    voice (Voice)  
    name Mandatory, for example, Evan
    model Mandatory, for example, enhanced
    age_group (EnumAgeGroup) ADULT
    gender (EnumGender) ANY
    audio_params (AudioParameters)  
    audio_format (AudioFormat) PCM 22.5kHz
    volume_percentage 80
    speaking_rate_factor 1.0
    audio_chunk_duration_ms 20000 (20 seconds)
    target_audio_length_ms 0, meaning no maximum duration
    disable_early_emission False: Send audio segments as soon as possible
    input (Input)  
    text (Text) Mandatory: one of text, tokenized_sequence, or ssml
    tokenized_sequence (TokenizedSequence)
    ssml (SSML)
      ssml_validation_mode (EnumSSMLValidationMode) STRICT
    escape_sequence \! and <ESC>
    resources (SynthesisResource)  
      type (EnumResourceType) USER_DICTIONARY
    lid_params (LanguageIdentificationParameters)
      disable False: LID is turned on
      languages Empty, meaning use all available languages
      always_use_ highest_confidence False: Use highest language with any confidence score
    download_params (DownloadParameters)  
      headers Empty
      refuse_cookies False: Accept cookies
      request_timeout_ms NVC server default, usually 30000 (30 seconds)
    event_params (EventParameters)  
    send_sentence_marker_events False: Do not send
    send_word_marker_events False: Do not send
    send_phoneme_marker_events False: Do not send
    send_bookmark_marker_events False: Do not send
    send_paragraph_marker_events False: Do not send
    send_visemes False: Do not send
    send_log_events False: Do not send
    suppress_input False: Include text and URIs in logs
    client_data Empty
    user_id Empty