backyard/sound/limiter.js

// port of https://github.com/wareya/LimiterTest/blob/main/Main.gd

class Limiter extends AudioWorkletProcessor {
  constructor(options) {
    super();

    this.srate = sampleRate;

    this.pre_gain = 1.0;
    this.post_gain = 1.0;
    this.limit = 0.8912509381337456; // -1 decibels

    if (false) {
      this.attack  = 0.0010;
      this.sustain = 0.0400;
      this.release = 0.0400;
    } else { // ultrafast
      this.attack  = 0.0006;
      this.sustain = 0.0060;
      this.release = 0.0300;
    }

    this.release_memory = 0.0;

    this.box_blur = 0;
    // this.amp = 1.0; // not stateful
    this.ref_amp = 1.0;

    this.memory_cursor = 0;
    this.sustained_amp = null;
    this.sample_memory = null;

    this.amp_memory_cursor = 0;
    this.amp_memory = null;
    this.amp_min_buckets = null;
    this.amp_bucket_size = 0;

    this.peaks = null;
    this.delays = [];

    const toSamples = (t) => Math.max(Math.round(this.srate * t), 1);

    let memory_count = toSamples(this.attack);
    this.sample_memory = new Float32Array(memory_count);
    this.sustained_amp = new Float32Array(memory_count);

    let amp_count = toSamples(this.attack + this.sustain);
    this.amp_bucket_size = Math.sqrt(amp_count) | 0;
    this.amp_memory = new Float32Array(amp_count);
    this.amp_memory.fill(1.0);

    let amp_bucket_count = amp_count / this.amp_bucket_size | 0;
    this.amp_min_buckets = new Float32Array(amp_bucket_count);
    this.amp_min_buckets.fill(1.0);

    this.peaks = new Float32Array(128); // guess the buffer size
    this.delays = [this.sample_memory];
  }

  process(inputs, outputs, parameters) {
    const memory_count = this.sample_memory.length; // this shows up a lot

    const input = inputs[0];
    const output = outputs[0];

    if (input.length === 0) {
      return false;
    }

    while (this.delays.length < input.length) {
      this.delays.push(new Float32Array(memory_count));
    }

    const blockSize = input[0].length;
    if (blockSize > 0 && this.peaks.length !== blockSize) {
      this.peaks = new Float32Array(blockSize);
    }
    this.peaks.fill(0.0);

    for (let c = 0; c < input.length; c++) {
      const source = input[c];
      for (let i = 0; i < blockSize; i++) {
        this.peaks[i] = Math.max(this.peaks[i], Math.abs(source[i]));
      }
    }

    const delay_cursor = this.memory_cursor;

    for (let i = 0; i < blockSize; i++) {
      const sample = this.peaks[i] * this.pre_gain;

      // do the release curve
      this.release_memory = Math.max(0.0, this.release_memory - 1.0 / this.srate);
      let amp = 1.0;
      if (this.release > 0.0) {
        let t = 1.0 - this.release_memory / this.release;
        amp = (1.0 - t) * this.ref_amp + t;
      }

      // reset sustain if we exceed the limit
      let ref_val = Math.abs(amp * sample);
      if (ref_val >= this.limit) {
        let amount = this.limit / ref_val;
        amp *= amount;
        ref_val *= amount;
        this.ref_amp = amp;
        this.release_memory = this.release;
      }

      this.amp_memory[this.amp_memory_cursor] = amp;

      // update affected bucket
      let bucket = this.amp_memory_cursor / this.amp_bucket_size | 0;
      this.amp_min_buckets[bucket] = 1.0;
      let end = Math.min((bucket + 1) * this.amp_bucket_size, this.amp_memory.length);
      for (let j = bucket * this.amp_bucket_size; j < end; j++) {
        this.amp_min_buckets[bucket] = Math.min(this.amp_min_buckets[bucket], this.amp_memory[j]);
      }

      // now get the current sustain value
      let min_amp = 1.0;
      for (let j = 0; j < this.amp_min_buckets.length; j++) {
        let past_amp = this.amp_min_buckets[j];
        min_amp = Math.min(min_amp, past_amp);
      }

      this.amp_memory_cursor += 1;
      this.amp_memory_cursor %= this.amp_memory.length;

      // update the sustain buffer
      this.box_blur -= this.sustained_amp[this.memory_cursor] * 32767 | 0;
      this.sustained_amp[this.memory_cursor] = min_amp;
      this.box_blur += this.sustained_amp[this.memory_cursor] * 32767 | 0;

      // update the sample memory buffer
      // NOTE(notwa): we instead do this later for each channel.
      //let ret_sample = this.sustain + this.attack > 0.0 ? this.sample_memory[this.memory_cursor] : sample;
      //this.sample_memory[this.memory_cursor] = sample;

      this.memory_cursor += 1;
      this.memory_cursor %= memory_count;

      this.peaks[i] = this.box_blur / 32767 / memory_count;
    }

    const final_gain = this.pre_gain * this.post_gain;
    for (let c = 0; c < input.length; c++) {
      const source = input[c];
      const target = output[c];
      if (this.sustain + this.attack <= 0.0) {
        for (let i = 0; i < blockSize; i++) {
          // return the limited sample
          target[i] = source[i] * this.peaks[i] * final_gain;
        }
      } else {
        const delay = this.delays[c];
        let local_cursor = delay_cursor;
        for (let i = 0; i < blockSize; i++) {
          const ret_sample = delay[local_cursor];
          delay[local_cursor] = source[i];

          local_cursor += 1;
          local_cursor %= memory_count;

          // return the limited sample
          target[i] = ret_sample * this.peaks[i] * final_gain;
        }
      }
    }

    return false; // docs say to return false for processors with no reverb tails
  }
}

registerProcessor("limiter", Limiter);