MicroDexed is a compatible 6-operator-FM-synth based on the Teensy(-3.6/-4.x) Microcontroller. https://www.parasitstudio.de
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/*
Copyright 2012 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.
*/
// Resolve frequency signal (1.0 in Q24 format = 1 octave) to phase delta.
// The LUT is just a global, and we'll need the init function to be called before
// use.
#include <stdint.h>
#include <math.h>
#include "freqlut.h"
#define LG_N_SAMPLES 10
#define N_SAMPLES (1 << LG_N_SAMPLES)
#define SAMPLE_SHIFT (24 - LG_N_SAMPLES)
#define MAX_LOGFREQ_INT 20
int32_t lut[N_SAMPLES + 1];
void Freqlut::init(FRAC_NUM sample_rate) {
FRAC_NUM y = (1LL << (24 + MAX_LOGFREQ_INT)) / sample_rate;
FRAC_NUM inc = pow(2, 1.0 / N_SAMPLES);
for (int i = 0; i < N_SAMPLES + 1; i++) {
lut[i] = (int32_t)floor(y + 0.5);
y *= inc;
}
}
// Note: if logfreq is more than 20.0, the results will be inaccurate. However,
// that will be many times the Nyquist rate.
int32_t Freqlut::lookup(int32_t logfreq) {
int ix = (logfreq & 0xffffff) >> SAMPLE_SHIFT;
int32_t y0 = lut[ix];
int32_t y1 = lut[ix + 1];
int lowbits = logfreq & ((1 << SAMPLE_SHIFT) - 1);
int32_t y = y0 + ((((int64_t)(y1 - y0) * (int64_t)lowbits)) >> SAMPLE_SHIFT);
int hibits = logfreq >> 24;
return y >> (MAX_LOGFREQ_INT - hibits);
}