refactor: align voice-call TTS with core config

extensions/voice-call/src/telephony-audio.ts

@@ -0,0 +1,88 @@
+const TELEPHONY_SAMPLE_RATE = 8000;
+
+function clamp16(value: number): number {
+  return Math.max(-32768, Math.min(32767, value));
+}
+
+/**
+ * Resample 16-bit PCM (little-endian mono) to 8kHz using linear interpolation.
+ */
+export function resamplePcmTo8k(input: Buffer, inputSampleRate: number): Buffer {
+  if (inputSampleRate === TELEPHONY_SAMPLE_RATE) return input;
+  const inputSamples = Math.floor(input.length / 2);
+  if (inputSamples === 0) return Buffer.alloc(0);
+
+  const ratio = inputSampleRate / TELEPHONY_SAMPLE_RATE;
+  const outputSamples = Math.floor(inputSamples / ratio);
+  const output = Buffer.alloc(outputSamples * 2);
+
+  for (let i = 0; i < outputSamples; i++) {
+    const srcPos = i * ratio;
+    const srcIndex = Math.floor(srcPos);
+    const frac = srcPos - srcIndex;
+
+    const s0 = input.readInt16LE(srcIndex * 2);
+    const s1Index = Math.min(srcIndex + 1, inputSamples - 1);
+    const s1 = input.readInt16LE(s1Index * 2);
+
+    const sample = Math.round(s0 + frac * (s1 - s0));
+    output.writeInt16LE(clamp16(sample), i * 2);
+  }
+
+  return output;
+}
+
+/**
+ * Convert 16-bit PCM to 8-bit mu-law (G.711).
+ */
+export function pcmToMulaw(pcm: Buffer): Buffer {
+  const samples = Math.floor(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;
+}
+
+export function convertPcmToMulaw8k(
+  pcm: Buffer,
+  inputSampleRate: number,
+): Buffer {
+  const pcm8k = resamplePcmTo8k(pcm, inputSampleRate);
+  return pcmToMulaw(pcm8k);
+}
+
+/**
+ * Chunk audio buffer into 20ms frames for streaming (8kHz mono 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));
+    }
+  })();
+}
+
+function linearToMulaw(sample: number): number {
+  const BIAS = 132;
+  const CLIP = 32635;
+
+  const sign = sample < 0 ? 0x80 : 0;
+  if (sample < 0) sample = -sample;
+  if (sample > CLIP) sample = CLIP;
+
+  sample += BIAS;
+  let exponent = 7;
+  for (let expMask = 0x4000; (sample & expMask) === 0 && exponent > 0; exponent--) {
+    expMask >>= 1;
+  }
+
+  const mantissa = (sample >> (exponent + 3)) & 0x0f;
+  return ~(sign | (exponent << 4) | mantissa) & 0xff;
+}