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CLK/Components/OPx/Implementation/PhaseGenerator.hpp

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//
// PhaseGenerator.h
// Clock Signal
//
// Created by Thomas Harte on 30/04/2020.
// Copyright © 2020 Thomas Harte. All rights reserved.
//
#pragma once
#include <cassert>
#include "LowFrequencyOscillator.hpp"
#include "Tables.hpp"
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namespace Yamaha::OPL {
/*!
Models an OPL-style phase generator of templated precision; having been told its period ('f-num'), octave ('block') and
multiple, and whether to apply vibrato, this will then appropriately update and return phase.
*/
template <int precision> class PhaseGenerator {
public:
/*!
Advances the phase generator a single step, given the current state of the low-frequency oscillator, @c oscillator.
*/
void update(const LowFrequencyOscillator &oscillator) {
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constexpr int vibrato_shifts[4] = {3, 1, 0, 1};
constexpr int vibrato_signs[2] = {1, -1};
// Get just the top three bits of the period_.
const int top_freq = period_ >> (precision - 3);
// Cacluaute applicable vibrato as a function of (i) the top three bits of the
// oscillator period; (ii) the current low-frequency oscillator vibrato output; and
// (iii) whether vibrato is enabled.
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const int vibrato = (top_freq >> vibrato_shifts[oscillator.vibrato & 3]) * vibrato_signs[oscillator.vibrato >> 2] * enable_vibrato_;
// Apply phase update with vibrato from the low-frequency oscillator.
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phase_ += (multiple_ * ((period_ << 1) + vibrato) << octave_) >> 1;
}
/*!
@returns Current phase; real hardware provides only the low ten bits of this result.
*/
int phase() const {
// My table if multipliers is multiplied by two, so shift by one more
// than the stated precision.
return phase_ >> precision_shift;
}
/*!
@returns Current phase, scaled up by (1 << precision).
*/
int scaled_phase() const {
return phase_ >> 1;
}
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/*!
Applies feedback based on two historic samples of a total output level,
plus the degree of feedback to apply
*/
void apply_feedback(LogSign first, LogSign second, int level) {
constexpr int masks[] = {0, ~0, ~0, ~0, ~0, ~0, ~0, ~0};
phase_ += ((second.level(precision) + first.level(precision)) >> (8 - level)) & masks[level];
}
/*!
Sets the multiple for this phase generator, in the same terms as an OPL programmer,
i.e. a 4-bit number that is used as a lookup into the internal multiples table.
*/
void set_multiple(int multiple) {
// This encodes the MUL -> multiple table given on page 12,
// multiplied by two.
constexpr int multipliers[] = {
1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 20, 24, 24, 30, 30
};
assert(multiple < 16);
multiple_ = multipliers[multiple];
}
/*!
Sets the period of this generator, along with its current octave.
Yamaha tends to refer to the period as the 'f-number', and used both 'octave' and 'block' for octave.
*/
void set_period(int period, int octave) {
period_ = period;
octave_ = octave;
assert(octave_ < 8);
assert(period_ < (1 << precision));
}
/*!
Enables or disables vibrato.
*/
void set_vibrato_enabled(bool enabled) {
enable_vibrato_ = int(enabled);
}
/*!
Resets the current phase.
*/
void reset() {
phase_ = 0;
}
private:
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static constexpr int precision_shift = 1 + precision;
int phase_ = 0;
int multiple_ = 0;
int period_ = 0;
int octave_ = 0;
int enable_vibrato_ = 0;
};
}