An implementation of PSK31 encoding in Web Audio

psk31.js

const ENERGY_PER_BIT = 100;
const CARRIER_FREQUENCY = 440;
const PAYLOAD = 'Test Payload';

const getAsciiArrayFromChar = (character) => (character.charCodeAt(0) >>> 0).toString(2).split('');

// Parameter names come from the equation for BPSK here:
// https://en.m.wikipedia.org/wiki/Phase-shift_keying#Binary_phase-shift_keying_.28BPSK.29

const binaryPhaseShiftKeyingOscillator = (
  t,
  a,
  carrierFrequency,
  bit) => a * Math.cos(2*Math.PI*carrierFrequency*t+Math.PI*(1-bit));

const prepareBinaryPhaseShiftKeyingOscillator = (
  energyPerBit,
  symbolDuration,
  carrierFrequency) => {
    const a = Math.sqrt(2*energyPerBit/symbolDuration);
    return  (t,bit) => {
      return binaryPhaseShiftKeyingOscillator(
        t,
        a,
        carrierFrequency,
        bit);
    }
};


(()=>{
  // According to https://en.m.wikipedia.org/wiki/PSK31#, 8000hz is a common sample rate. It happens to
  // work well with PSK31, but feel free to adjust it, it shouldn't break much.
  // UPDATE: That didn't work because the web audio api requires sample rate to be a power of 2
  // const BPSK31_SAMPLE_RATE = 8000;
  // switching to 44100
  const BPSK31_SAMPLE_RATE = 44100;
  const baudRate = 31.25;
  // in samples
  const bitDuration = BPSK31_SAMPLE_RATE/baudRate;
  const asciiBinaryPayload = Array.prototype.map.call(PAYLOAD,
                                                      getAsciiArrayFromChar)
                                            .reduce((acc,character) => acc.concat(character));
  const messageDuration = bitDuration * asciiBinaryPayload.length;

  const bpsk31 = prepareBinaryPhaseShiftKeyingOscillator(ENERGY_PER_BIT, bitDuration, CARRIER_FREQUENCY);

  const ctx = new AudioContext(1,messageDuration,BPSK31_SAMPLE_RATE);
  // ScriptProcessorNodes are deprecated, but their replacement hasn't been implemented anywhere yet.
  const spn = ctx.createScriptProcessor(16384,1,1);
  spn.onaudioprocess = function(audioProcessingEvent) {
    // We have to write into this reference. It sucks.
    var outputData = audioProcessingEvent.outputBuffer.getChannelData(0);
    for (var sample = 0; sample < outputData.length; sample++) {
      const t = audioProcessingEvent.playbackTime+(sample/ctx.sampleRate);
      const i = asciiBinaryPayload[Math.floor(t/messageDuration)]
      const result = bpsk31(
        t,
        i);
      outputData[sample] = result;
    }
  }

  spn.connect(ctx.destination);

  // ctx.oncomplete = (offlineAudioCompletionEvent) => {
  //   const bpsk31EncodedPayloadAudioBuffer = offlineAudioCompletionEvent.renderedBuffer;
  //   console.log(bpsk31EncodedPayloadAudioBuffer.getChannelData(0).filter((a)=>a!=0));

  //   const onlineCtx = new AudioContext();
  //   const source = onlineCtx.createBufferSource();
  //   source.buffer = bpsk31EncodedPayloadAudioBuffer;
  //   source.connect(onlineCtx.destination);
  //   source.start();
  // }

  // ctx.startRendering();
})();