WhisperLive/Audio-Transcription-Chrome/audiopreprocessor.js

class AudioPreProcessor extends AudioWorkletProcessor {
  constructor() {
    super();
    this.sampleRate = sampleRate || 48000;
    this.targetSampleRate = 16000;
    this.inputSamplesNeeded = this.sampleRate * 0.5; // 0.5s
    this.inputBuffer = new Float32Array(this.inputSamplesNeeded);
    this.inputWriteOffset = 0;
    this.processCount = 0;
    this.audioDetectedCount = 0;
  }

  process(inputs, outputs) {
    this.processCount++;
    
    const input = inputs[0];
    const output = outputs[0];
   
    if (!input || input.length === 0) {
      return true;
    }

    for (let channel = 0; channel < Math.min(input.length, output.length); channel++) {
      if (input[channel] && output[channel]) {
        output[channel].set(input[channel]);
      }
    }

    let monoInput;
    if (input.length === 1) {
      monoInput = input[0];
    } else if (input.length >= 2) {
      monoInput = new Float32Array(input[0].length);
      for (let i = 0; i < input[0].length; i++) {
        monoInput[i] = (input[0][i] + (input[1] ? input[1][i] : 0)) * 0.5;
      }
    } else {
      return true;
    }

    if (!monoInput || monoInput.length === 0) {
      return true;
    }

    let inputOffset = 0;
    while (inputOffset < monoInput.length) {
      const remainingBuffer = this.inputSamplesNeeded - this.inputWriteOffset;
      const toCopy = Math.min(remainingBuffer, monoInput.length - inputOffset);
      this.inputBuffer.set(monoInput.subarray(inputOffset, inputOffset + toCopy), this.inputWriteOffset);

      this.inputWriteOffset += toCopy;
      inputOffset += toCopy;

      if (this.inputWriteOffset === this.inputSamplesNeeded) {
        const downsampled = this.downsampleTo16kHz(this.inputBuffer);
        this.port.postMessage(downsampled);

        this.inputWriteOffset = 0;
      }
    }

    return true;
  }

  downsampleTo16kHz(inputBuffer) {
    const ratio = this.sampleRate / this.targetSampleRate;
    const length = Math.floor(inputBuffer.length / ratio);
    const result = new Float32Array(length);
    for (let i = 0; i < length; i++) {
      const idx = Math.floor(i * ratio);
      result[i] = inputBuffer[idx];
    }
    return result;
  }
}

registerProcessor('audiopreprocessor', AudioPreProcessor);