private com.google.protobuf.SingleFieldBuilderV3<
            com.google.cloud.texttospeech.v1.CustomVoiceParams,
            com.google.cloud.texttospeech.v1.CustomVoiceParams.Builder,
            com.google.cloud.texttospeech.v1.CustomVoiceParamsOrBuilder>
        getCustomVoiceFieldBuilder() {
      if (customVoiceBuilder_ == null) {
        customVoiceBuilder_ =
            new com.google.protobuf.SingleFieldBuilderV3<
                com.google.cloud.texttospeech.v1.CustomVoiceParams,
                com.google.cloud.texttospeech.v1.CustomVoiceParams.Builder,
                com.google.cloud.texttospeech.v1.CustomVoiceParamsOrBuilder>(
                getCustomVoice(), getParentForChildren(), isClean());
        customVoice_ = null;
      }
      return customVoiceBuilder_;
    }
<pre>
The configuration for a custom voice. If [CustomVoiceParams.model] is set,
the service will choose the custom voice matching the specified
configuration.
</pre>

<code>.google.cloud.texttospeech.v1.CustomVoiceParams custom_voice = 4;</code>
java-texttospeech/proto-google-cloud-texttospeech-v1/src/main/java/com/google/cloud/texttospeech/v1/VoiceSelectionParams.java