Adds vibrato.

This would complete melodic output, subject to bug fixes.

Components/OPL2/Implementation/LowFrequencyOscillator.cpp

@@ -7,3 +7,22 @@
 //
 
 #include "LowFrequencyOscillator.hpp"
+
+using namespace Yamaha::OPL;
+
+void LowFrequencyOscillator::update() {
+	++counter;
+
+	// This produces output of:
+	//
+	// four instances of 0, four instances of 1... _three_ instances of 26,
+	// four instances of 25, four instances of 24... _three_ instances of 0.
+	//
+	// ... advancing once every 64th update.
+	const int tremolo_index = (counter >> 6) % 210;
+	const int tremolo_levels[2] = {tremolo_index >> 2, 52 - ((tremolo_index+1) >> 2)};
+	tremolo = tremolo_levels[tremolo_index / 107];
+
+	// Vibrato is relatively simple: it's just three bits from the counter.
+	vibrato = (counter >> 10) & 7;
+}

Components/OPL2/Implementation/LowFrequencyOscillator.hpp

@@ -21,31 +21,14 @@ class LowFrequencyOscillator {
 	public:
 		/// Current attenuation due to tremolo / amplitude modulation, between 0 and 26.
 		int tremolo = 0;
-		/// TODO
+		/// A number between 0 and 7 indicating the current vibrato offset; this should be combined by operators
+		/// with their frequency number to get the actual vibrato.
 		int vibrato = 0;
 		/// A counter of the number of operator update cycles (i.e. input clock / 72) since an arbitrary time.
 		int counter = 0;
 
 		/// Updates the oscillator outputs
-		void update() {
+		void update();
-			++counter;
-
-			// Update tremolo.
-			++tremolo_phase_;
-
-			// This produces output of:
-			//
-			// four instances of 0, four instances of 1... _three_ instances of 26,
-			// four instances of 25, four instances of 24... _three_ instances of 0.
-			//
-			// ... advancing once every 64th update.
-			const int tremolo_index = (tremolo_phase_ >> 6) % 210;
-			const int tremolo_levels[2] = {tremolo_index >> 2, 52 - ((tremolo_index+1) >> 2)};
-			tremolo = tremolo_levels[tremolo_index / 107];
-		}
-
-	private:
-		int tremolo_phase_ = 0;
 };
 
 }

Components/OPL2/Implementation/Operator.cpp

@@ -81,11 +81,16 @@ void Operator::update(
 	constexpr int multipliers[] = {
 		1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 20, 24, 24, 30, 30
 	};
+	const int top_freq = channel_period >> 7;
+	assert(top_freq < 8);
+	constexpr int vibrato_shifts[8] = {3, 1, 0, 1, 3, 1, 0, 1};
+	constexpr int vibrato_signs[2] = {1, -1};
+	const int vibrato = (top_freq >> vibrato_shifts[oscillator.vibrato]) * vibrato_signs[oscillator.vibrato >> 2] * int(apply_vibrato_);
 
 	// Update the raw phase.
-	state.raw_phase_ += multipliers[frequency_multiple_] * channel_period << channel_octave;
+	state.raw_phase_ += multipliers[frequency_multiple_] * (channel_period + vibrato) << channel_octave;
 
-	// Hence calculate phase (TODO: by also taking account of vibrato).
+	// Hence calculate phase.
 	constexpr int waveforms[4][4] = {
 		{1023, 1023, 1023, 1023},	// Sine: don't mask in any quadrant.
 		{511, 511, 0, 0},			// Half sine: keep the first half intact, lock to 0 in the second half.