MicroDexed/lfo.cpp

/*
   Copyright 2013 Google Inc.

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

        http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.
*/

// Low frequency oscillator, compatible with DX7

#include "synth.h"

#include "sin.h"
#include "lfo.h"

uint32_t Lfo::unit_;

void Lfo::init(FRAC_NUM sample_rate) {
  // constant is 1 << 32 / 15.5s / 11
  Lfo::unit_ = (int32_t)(_N_ * 25190424 / sample_rate + 0.5);
}

void Lfo::reset(const uint8_t params[6]) {
  int rate = params[0];  // 0..99
  int sr = rate == 0 ? 1 : (165 * rate) >> 6;
  sr *= sr < 160 ? 11 : (11 + ((sr - 160) >> 4));
  delta_ = unit_ * sr;
  int a = 99 - params[1];  // LFO delay
  if (a == 99) {
    delayinc_ = ~0u;
    delayinc2_ = ~0u;
  } else {
    a = (16 + (a & 15)) << (1 + (a >> 4));
    delayinc_ = unit_ * a;
    a &= 0xff80;
    a = max(0x80, a);
    delayinc2_ = unit_ * a;
  }
  waveform_ = params[5];
  sync_ = params[4] != 0;
}

int32_t Lfo::getsample() {
  phase_ += delta_;
  int32_t x;
  switch (waveform_) {
    case 0:  // triangle
      x = phase_ >> 7;
      x ^= -(phase_ >> 31);
      x &= (1 << 24) - 1;
      return x;
    case 1:  // sawtooth down
      return (~phase_ ^ (1U << 31)) >> 8;
    case 2:  // sawtooth up
      return (phase_ ^ (1U << 31)) >> 8;
    case 3:  // square
      return ((~phase_) >> 7) & (1 << 24);
    case 4:  // sine
      return (1 << 23) + (Sin::lookup(phase_ >> 8) >> 1);
    case 5:  // s&h
      if (phase_ < delta_) {
        randstate_ = (randstate_ * 179 + 17) & 0xff;
      }
      x = randstate_ ^ 0x80;
      return (x + 1) << 16;
  }
  return 1 << 23;
}

int32_t Lfo::getdelay() {
  uint32_t delta = delaystate_ < (1U << 31) ? delayinc_ : delayinc2_;
  uint64_t d = ((uint64_t)delaystate_) + delta;
  if (d > ~0u) {
    return 1 << 24;
  }
  delaystate_ = d;
  if (d < (1U << 31)) {
    return 0;
  } else {
    return (d >> 7) & ((1 << 24) - 1);
  }
}

void Lfo::keydown() {
  if (sync_) {
    phase_ = (1U << 31) - 1;
  }
  delaystate_ = 0;
}