Taking it as given that outstanding deficiencies are mostly due to poor design, starts breaking out the envelope and phase generators.

Components/OPL2/Implementation/EnvelopeGenerator.hpp

@@ -0,0 +1,113 @@
+//
+//  EnvelopeGenerator.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 01/05/2020.
+//  Copyright © 2020 Thomas Harte. All rights reserved.
+//
+
+#ifndef EnvelopeGenerator_h
+#define EnvelopeGenerator_h
+
+#include <optional>
+#include <functional>
+
+namespace Yamaha {
+namespace OPL {
+
+/*!
+	Models an OPL-style envelope generator.
+
+	Damping is optional; if damping is enabled then if there is a transition to key-on while
+	attenuation is less than maximum then attenuation will be quickly transitioned to maximum
+	before the attack phase can begin.
+
+	in real hardware damping is used by the envelope generators associated with
+	carriers, with phases being reset upon the transition from damping to attack.
+
+	This code considers application of tremolo to be a function of the envelope generator;
+	this is largely for logical conformity with the phase generator that necessarily has to
+	apply vibrato.
+*/
+template <int precision> class EnvelopeGenerator {
+	public:
+		/*!
+			Advances the envelope generator a single step, given the current state of the low-frequency oscillator, @c oscillator.
+		*/
+		void update(const LowFrequencyOscillator &oscillator);
+
+		/*!
+			@returns The current attenuation from this envelope generator.
+		*/
+		int attenuation() const;
+
+		/*!
+			Enables or disables damping on this envelope generator. If damping is enabled then this envelope generator will
+			use the damping phase when necessary (i.e. when transitioning to key on if attenuation is not already at maximum)
+			and in any case will call @c will_attack before transitioning from any other state to attack.
+
+			@param will_attack Supply a will_attack callback to enable damping mode; supply nullopt to disable damping mode.
+		*/
+		void set_should_damp(const std::optional<std::function<void(void)>> &will_attack);
+
+		/*!
+			Sets the current state of the key-on input.
+		*/
+		void set_key_on(bool);
+
+		/*!
+			Sets the attack rate, which should be in the range 0–15.
+		*/
+		void set_attack_rate(int);
+
+		/*!
+			Sets the decay rate, which should be in the range 0–15.
+		*/
+		void set_decay_rate(int);
+
+		/*!
+			Sets the release rate, which should be in the range 0–15.
+		*/
+		void set_release_rate(int);
+
+		/*!
+			Sets the sustain level, which should be in the range 0–15.
+		*/
+		void set_sustain_level(int);
+
+		/*!
+			Enables or disables use of the sustain level. If this is disabled, the envelope proceeds
+			directly from decay to release.
+		*/
+		void set_use_sustain_level(bool);
+
+		/*!
+			Enables or disables key-rate scaling.
+		*/
+		void set_key_rate_scaling_enabled(bool enabled);
+
+		/*!
+			Enables or disables application of the low-frequency oscillator's tremolo.
+		*/
+		void set_tremolo_enabled(bool enabled);
+
+		/*!
+			Sets the current period associated with the channel that owns this envelope generator;
+			this is used to select a key scaling rate if key-rate scaling is enabled.
+		*/
+		void set_period(int period, int octave);
+
+	private:
+		enum class ADSRPhase {
+			Attack, Decay, Sustain, Release, Damp
+		} adsr_phase_ = ADSRPhase::Attack;
+		int adsr_attenuation_ = 511;
+
+		bool key_on_ = false;
+		std::optional<std::function<void(void)>> will_attack_;
+};
+
+}
+}
+
+#endif /* EnvelopeGenerator_h */

Components/OPL2/Implementation/PhaseGenerator.hpp

@@ -0,0 +1,108 @@
+//
+//  PhaseGenerator.h
+//  Clock Signal
+//
+//  Created by Thomas Harte on 30/04/2020.
+//  Copyright © 2020 Thomas Harte. All rights reserved.
+//
+
+#ifndef PhaseGenerator_h
+#define PhaseGenerator_h
+
+#include <cassert>
+#include "LowFrequencyOscillator.hpp"
+#include "Tables.hpp"
+
+namespace Yamaha {
+namespace 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) {
+			constexpr int vibrato_shifts[8] = {3, 1, 0, 1, 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.
+			const int vibrato = (top_freq >> vibrato_shifts[oscillator.vibrato]) * vibrato_signs[oscillator.vibrato >> 2] * enable_vibrato_;
+
+			// Apply phase update with vibrato from the low-frequency oscillator.
+			raw_phase_ += multiple_ * (period_ + vibrato) << octave_;
+		}
+
+
+		/*!
+			@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 raw_phase_ >> precision_shift;
+		}
+
+		/*!
+			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);
+		}
+
+		enum class Waveform {
+			Sine, HalfSine, AbsSine, PulseSine
+		};
+
+	private:
+		static constexpr int precision_shift =  1 + precision;
+
+		int raw_phase_ = 0;
+
+		int multiple_ = 0;
+		int period_ = 0;
+		int octave_ = 0;
+		int enable_vibrato_ = 0;
+
+		Waveform waveform_ = Waveform::Sine;
+};
+
+}
+}
+
+#endif /* PhaseGenerator_h */

Components/OPL2/OPL2.cpp

@@ -11,6 +11,9 @@
 #include <cassert>
 #include <cmath>
 
+#include "Implementation/PhaseGenerator.hpp"
+#include "Implementation/EnvelopeGenerator.hpp"
+
 using namespace Yamaha::OPL;
 
 template <typename Child>

OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj

@@ -1151,6 +1151,8 @@
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@@ -3532,6 +3534,8 @@
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 				4B61908E24526E640013F202 /* LowFrequencyOscillator.cpp */,
 				4B61908F24526E640013F202 /* LowFrequencyOscillator.hpp */,
+				4B619092245BC1000013F202 /* PhaseGenerator.hpp */,
+				4B619093245CD63E0013F202 /* EnvelopeGenerator.hpp */,
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