feat: restore voice-call plugin parity

extensions/voice-call/src/providers/tts-openai.ts

@@ -0,0 +1,264 @@
+/**
+ * OpenAI TTS Provider
+ *
+ * Generates speech audio using OpenAI's text-to-speech API.
+ * Handles audio format conversion for telephony (mu-law 8kHz).
+ *
+ * Best practices from OpenAI docs:
+ * - Use gpt-4o-mini-tts for intelligent realtime applications (supports instructions)
+ * - Use tts-1 for lower latency, tts-1-hd for higher quality
+ * - Use marin or cedar voices for best quality
+ * - Use pcm or wav format for fastest response times
+ *
+ * @see https://platform.openai.com/docs/guides/text-to-speech
+ */
+
+/**
+ * OpenAI TTS configuration.
+ */
+export interface OpenAITTSConfig {
+  /** OpenAI API key (uses OPENAI_API_KEY env if not set) */
+  apiKey?: string;
+  /**
+   * TTS model:
+   * - gpt-4o-mini-tts: newest, supports instructions for tone/style control (recommended)
+   * - tts-1: lower latency
+   * - tts-1-hd: higher quality
+   */
+  model?: string;
+  /**
+   * Voice to use. For best quality, use marin or cedar.
+   * All 13 voices: alloy, ash, ballad, coral, echo, fable, nova, onyx, sage, shimmer, verse, marin, cedar
+   * Note: tts-1/tts-1-hd only support: alloy, ash, coral, echo, fable, onyx, nova, sage, shimmer
+   */
+  voice?: string;
+  /** Speed multiplier (0.25 to 4.0) */
+  speed?: number;
+  /**
+   * Instructions for speech style (only works with gpt-4o-mini-tts model).
+   * Examples: "Speak in a cheerful tone", "Talk like a sympathetic customer service agent"
+   */
+  instructions?: string;
+}
+
+/**
+ * Supported OpenAI TTS voices (all 13 built-in voices).
+ * For best quality, use marin or cedar.
+ * Note: tts-1 and tts-1-hd support a smaller set.
+ */
+export const OPENAI_TTS_VOICES = [
+  "alloy",
+  "ash",
+  "ballad",
+  "coral",
+  "echo",
+  "fable",
+  "nova",
+  "onyx",
+  "sage",
+  "shimmer",
+  "verse",
+  "marin",
+  "cedar",
+] as const;
+
+export type OpenAITTSVoice = (typeof OPENAI_TTS_VOICES)[number];
+
+/**
+ * OpenAI TTS Provider for generating speech audio.
+ */
+export class OpenAITTSProvider {
+  private apiKey: string;
+  private model: string;
+  private voice: OpenAITTSVoice;
+  private speed: number;
+  private instructions?: string;
+
+  constructor(config: OpenAITTSConfig = {}) {
+    this.apiKey = config.apiKey || process.env.OPENAI_API_KEY || "";
+    // Default to gpt-4o-mini-tts for intelligent realtime applications
+    this.model = config.model || "gpt-4o-mini-tts";
+    // Default to coral - good balance of quality and natural tone
+    this.voice = (config.voice as OpenAITTSVoice) || "coral";
+    this.speed = config.speed || 1.0;
+    this.instructions = config.instructions;
+
+    if (!this.apiKey) {
+      throw new Error(
+        "OpenAI API key required (set OPENAI_API_KEY or pass apiKey)",
+      );
+    }
+  }
+
+  /**
+   * Generate speech audio from text.
+   * Returns raw PCM audio data (24kHz, mono, 16-bit).
+   */
+  async synthesize(text: string, instructions?: string): Promise<Buffer> {
+    // Build request body
+    const body: Record<string, unknown> = {
+      model: this.model,
+      input: text,
+      voice: this.voice,
+      response_format: "pcm", // Raw PCM audio (24kHz, mono, 16-bit signed LE)
+      speed: this.speed,
+    };
+
+    // Add instructions if using gpt-4o-mini-tts model
+    const effectiveInstructions = instructions || this.instructions;
+    if (effectiveInstructions && this.model.includes("gpt-4o-mini-tts")) {
+      body.instructions = effectiveInstructions;
+    }
+
+    const response = await fetch("https://api.openai.com/v1/audio/speech", {
+      method: "POST",
+      headers: {
+        Authorization: `Bearer ${this.apiKey}`,
+        "Content-Type": "application/json",
+      },
+      body: JSON.stringify(body),
+    });
+
+    if (!response.ok) {
+      const error = await response.text();
+      throw new Error(`OpenAI TTS failed: ${response.status} - ${error}`);
+    }
+
+    const arrayBuffer = await response.arrayBuffer();
+    return Buffer.from(arrayBuffer);
+  }
+
+  /**
+   * Generate speech and convert to mu-law format for Twilio.
+   * Twilio Media Streams expect 8kHz mono mu-law audio.
+   */
+  async synthesizeForTwilio(text: string): Promise<Buffer> {
+    // Get raw PCM from OpenAI (24kHz, 16-bit signed LE, mono)
+    const pcm24k = await this.synthesize(text);
+
+    // Resample from 24kHz to 8kHz
+    const pcm8k = resample24kTo8k(pcm24k);
+
+    // Encode to mu-law
+    return pcmToMulaw(pcm8k);
+  }
+}
+
+/**
+ * Resample 24kHz PCM to 8kHz using linear interpolation.
+ * Input/output: 16-bit signed little-endian mono.
+ */
+function resample24kTo8k(input: Buffer): Buffer {
+  const inputSamples = input.length / 2;
+  const outputSamples = Math.floor(inputSamples / 3);
+  const output = Buffer.alloc(outputSamples * 2);
+
+  for (let i = 0; i < outputSamples; i++) {
+    // Calculate position in input (3:1 ratio)
+    const srcPos = i * 3;
+    const srcIdx = srcPos * 2;
+
+    if (srcIdx + 3 < input.length) {
+      // Linear interpolation between samples
+      const s0 = input.readInt16LE(srcIdx);
+      const s1 = input.readInt16LE(srcIdx + 2);
+      const frac = srcPos % 1 || 0;
+      const sample = Math.round(s0 + frac * (s1 - s0));
+      output.writeInt16LE(clamp16(sample), i * 2);
+    } else {
+      // Last sample
+      output.writeInt16LE(input.readInt16LE(srcIdx), i * 2);
+    }
+  }
+
+  return output;
+}
+
+/**
+ * Clamp value to 16-bit signed integer range.
+ */
+function clamp16(value: number): number {
+  return Math.max(-32768, Math.min(32767, value));
+}
+
+/**
+ * Convert 16-bit PCM to 8-bit mu-law.
+ * Standard G.711 mu-law encoding for telephony.
+ */
+function pcmToMulaw(pcm: Buffer): Buffer {
+  const samples = pcm.length / 2;
+  const mulaw = Buffer.alloc(samples);
+
+  for (let i = 0; i < samples; i++) {
+    const sample = pcm.readInt16LE(i * 2);
+    mulaw[i] = linearToMulaw(sample);
+  }
+
+  return mulaw;
+}
+
+/**
+ * Convert a single 16-bit linear sample to 8-bit mu-law.
+ * Implements ITU-T G.711 mu-law encoding.
+ */
+function linearToMulaw(sample: number): number {
+  const BIAS = 132;
+  const CLIP = 32635;
+
+  // Get sign bit
+  const sign = sample < 0 ? 0x80 : 0;
+  if (sample < 0) sample = -sample;
+
+  // Clip to prevent overflow
+  if (sample > CLIP) sample = CLIP;
+
+  // Add bias and find segment
+  sample += BIAS;
+  let exponent = 7;
+  for (
+    let expMask = 0x4000;
+    (sample & expMask) === 0 && exponent > 0;
+    exponent--, expMask >>= 1
+  ) {
+    // Find the segment (exponent)
+  }
+
+  // Extract mantissa bits
+  const mantissa = (sample >> (exponent + 3)) & 0x0f;
+
+  // Combine into mu-law byte (inverted for transmission)
+  return ~(sign | (exponent << 4) | mantissa) & 0xff;
+}
+
+/**
+ * Convert 8-bit mu-law to 16-bit linear PCM.
+ * Useful for decoding incoming audio.
+ */
+export function mulawToLinear(mulaw: number): number {
+  // mu-law is transmitted inverted
+  mulaw = ~mulaw & 0xff;
+
+  const sign = mulaw & 0x80;
+  const exponent = (mulaw >> 4) & 0x07;
+  const mantissa = mulaw & 0x0f;
+
+  let sample = ((mantissa << 3) + 132) << exponent;
+  sample -= 132;
+
+  return sign ? -sample : sample;
+}
+
+/**
+ * Chunk audio buffer into 20ms frames for streaming.
+ * At 8kHz mono, 20ms = 160 samples = 160 bytes (mu-law).
+ */
+export function chunkAudio(
+  audio: Buffer,
+  chunkSize = 160,
+): Generator<Buffer, void, unknown> {
+  return (function* () {
+    for (let i = 0; i < audio.length; i += chunkSize) {
+      yield audio.subarray(i, Math.min(i + chunkSize, audio.length));
+    }
+  })();
+}