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64 lines
1.9 KiB
C++
64 lines
1.9 KiB
C++
//
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// LowFrequencyOscillator.hpp
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// Clock Signal
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//
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// Created by Thomas Harte on 23/04/2020.
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// Copyright © 2020 Thomas Harte. All rights reserved.
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//
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#pragma once
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#include "../../../Numeric/LFSR.hpp"
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namespace Yamaha::OPL {
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/*!
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Models the output of the OPL low-frequency oscillator, which provides a couple of optional fixed-frequency
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modifications to an operator: tremolo and vibrato. Also exposes a global time counter, which oscillators use
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as part of their ADSR envelope.
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*/
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class LowFrequencyOscillator {
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public:
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/// Current attenuation due to tremolo / amplitude modulation, between 0 and 26.
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int tremolo = 0;
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/// A number between 0 and 7 indicating the current vibrato offset; this should be combined by operators
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/// with their frequency number to get the actual vibrato.
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int vibrato = 0;
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/// A counter of the number of operator update cycles (i.e. input clock / 72) since an arbitrary time.
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int counter = 0;
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/// Describes the current output of the LFSR; will be either 0 or 1.
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int lfsr = 0;
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/// Updates the oscillator outputs. Should be called at the (input clock/72) rate.
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void update() {
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++counter;
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// This produces output of:
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//
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// four instances of 0, four instances of 1... _three_ instances of 26,
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// four instances of 25, four instances of 24... _three_ instances of 0.
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//
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// ... advancing once every 64th update.
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const int tremolo_index = (counter >> 6) % 210;
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const int tremolo_levels[2] = {tremolo_index >> 2, 52 - ((tremolo_index+1) >> 2)};
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tremolo = tremolo_levels[tremolo_index / 107];
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// Vibrato is relatively simple: it's just three bits from the counter.
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vibrato = (counter >> 10) & 7;
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}
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/// Updartes the LFSR output. Should be called at the input clock rate.
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void update_lfsr() {
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lfsr = noise_source_.next();
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}
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private:
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// This is the correct LSFR per forums.submarine.org.uk.
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Numeric::LFSR<int, 0x800302> noise_source_;
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};
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}
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