mka-opus-mixdown.js

const fs = require('fs');
const { exec, execSync } = require('child_process');

const getStartTime = (fileName) => {
  const ffprobeOutput = execSync(`ffprobe -v quiet -print_format json -show_entries format=start_time ${fileName}`).toString();
  const ffprobeJSON = JSON.parse(ffprobeOutput);
  return parseFloat(ffprobeJSON.format.start_time) * 1000;
};

const mixAudioTracks = (audioFiles) => {
  let aresampleFilters = [];
  let amixInputs = [];
  const referenceStartTime = Math.min(...audioFiles.map(file => file.startTime));

  audioFiles.forEach((file, index) => {
    const delay = file.startTime - referenceStartTime;
    if (delay > 0) {
      aresampleFilters.push(`[${index}]aresample=async=1,adelay=${delay}|${delay}[a${index}]`);
    } else {
      aresampleFilters.push(`[${index}]aresample=async=1[a${index}]`);
    }
    amixInputs.push(`[a${index}]`);
  });

  const filterComplex = `${aresampleFilters.join('; ')}; ${amixInputs.join('')}amix=inputs=${audioFiles.length}[audio]`;
  const inputFiles = audioFiles.map(file => `-acodec libopus -i ${file.name}`).join(' ');
  const ffmpegCommand = `ffmpeg ${inputFiles} -filter_complex "${filterComplex}" -map [audio] -acodec libopus output.mka`;
  console.log({ffmpegCommand});

  exec(ffmpegCommand, (error, stdout, stderr) => {
    if (error) {
      console.error(`Error executing ffmpeg: ${error}`);
      return;
    }
    console.log(`ffmpeg stdout: ${stdout}`);
    console.log(`ffmpeg stderr: ${stderr}`);
  });
};

const main = () => {
  const dirPath = '.';
  const fileNames = fs.readdirSync(dirPath).filter(fileName => fileName.endsWith('.mka'));
  const audioFiles = fileNames.map(fileName => ({ name: fileName, startTime: getStartTime(fileName) }));
  mixAudioTracks(audioFiles);
};

main();