support google tts configuraiton (#149)

lib/synth-audio.js

@@ -383,6 +383,32 @@ const synthPolly = async(createHash, retrieveHash, logger,
   }
 };
 
+/* google AudioConfig settings we support, as [google camelCase name, freeswitch param name] */
+const GOOGLE_AUDIO_SETTINGS = [
+  ['speakingRate', 'speaking_rate'],
+  ['pitch', 'pitch'],
+  ['volumeGainDb', 'volume_gain_db']
+];
+
+/**
+ * Extract google AudioConfig settings from the synthesizer options.  They may be supplied
+ * nested under an audioConfig property (mirroring google's AudioConfig object) or flat at
+ * the top level, and either google's camelCase or snake_case names are accepted.
+ * @see https://cloud.google.com/text-to-speech/docs/reference/rest/v1/text/synthesize#AudioConfig
+ * @returns object keyed by google's camelCase names, holding only valid numeric settings
+ */
+const googleAudioConfig = (options) => {
+  const provided = {...options, ...(options?.audioConfig || {})};
+  const audioConfig = {};
+  for (const [name, snakeName] of GOOGLE_AUDIO_SETTINGS) {
+    const value = provided[name] ?? provided[snakeName];
+    /* note: 0 is meaningful for pitch and volumeGainDb, so check for absence explicitly */
+    if (value === undefined || value === null || value === '') continue;
+    const num = Number(value);
+    if (Number.isFinite(num)) audioConfig[name] = num;
+  }
+  return audioConfig;
+};
 
 const synthGoogle = async(logger, {
   credentials, stats, language, voice, gender, key, text, model, options, instructions,
@@ -418,6 +444,15 @@ const synthGoogle = async(logger, {
     // comma is used to separate parameters in freeswitch tts module
     const prompt = options?.prompt || instructions;
     if (prompt) params += `,prompt=${prompt.replace(/\n/g, ' ').replace(/,/g, ';')}`;
+    /**
+     * AudioConfig settings.  Note google only honors these in some api modes:
+     * tts applies all of them, live (HD voices) applies speakingRate only,
+     * and gemini ignores them entirely (use prompt instead for style control).
+     */
+    const audioSettings = googleAudioConfig(options);
+    for (const [name, snakeName] of GOOGLE_AUDIO_SETTINGS) {
+      if (name in audioSettings) params += `,${snakeName}=${audioSettings[name]}`;
+    }
     params += '}';
 
     return {
@@ -486,6 +521,9 @@ const synthGoogle = async(logger, {
     sampleRate = 8000;
   }
 
+  /* gemini voices do not support the AudioConfig settings; they use prompt for style control */
+  if (!isGemini) Object.assign(audioConfig, googleAudioConfig(options));
+
   const opts = { input, voice: voiceParams, audioConfig };
 
   try {