Introduces concept of 'average peak volume' in order better to normalise audio sources like the OPLL.

Components/AY38910/AY38910.cpp

@@ -234,7 +234,7 @@ template <bool is_stereo> void AY38910<is_stereo>::evaluate_output_volume() {
 	}
 }
 
-template <bool is_stereo> bool AY38910<is_stereo>::is_zero_level() {
+template <bool is_stereo> bool AY38910<is_stereo>::is_zero_level() const {
 	// Confirm that the AY is trivially at the zero level if all three volume controls are set to fixed zero.
 	return output_registers_[0x8] == 0 && output_registers_[0x9] == 0 && output_registers_[0xa] == 0;
 }

Components/AY38910/AY38910.hpp

@@ -107,7 +107,7 @@ template <bool is_stereo> class AY38910: public ::Outputs::Speaker::SampleSource
 
 		// to satisfy ::Outputs::Speaker (included via ::Outputs::Filter.
 		void get_samples(std::size_t number_of_samples, int16_t *target);
-		bool is_zero_level();
+		bool is_zero_level() const;
 		void set_sample_volume_range(std::int16_t range);
 		static constexpr bool get_is_stereo() { return is_stereo; }
 

Components/KonamiSCC/KonamiSCC.cpp

@@ -15,7 +15,7 @@ using namespace Konami;
 SCC::SCC(Concurrency::DeferringAsyncTaskQueue &task_queue) :
 	task_queue_(task_queue) {}
 
-bool SCC::is_zero_level() {
+bool SCC::is_zero_level() const {
 	return !(channel_enable_ & 0x1f);
 }
 

Components/KonamiSCC/KonamiSCC.hpp

@@ -27,7 +27,7 @@ class SCC: public ::Outputs::Speaker::SampleSource {
 		SCC(Concurrency::DeferringAsyncTaskQueue &task_queue);
 
 		/// As per ::SampleSource; provides a broadphase test for silence.
-		bool is_zero_level();
+		bool is_zero_level() const;
 
 		/// As per ::SampleSource; provides audio output.
 		void get_samples(std::size_t number_of_samples, std::int16_t *target);

Components/OPL2/OPLL.hpp

@@ -30,6 +30,10 @@ class OPLL: public OPLBase<OPLL> {
 		void get_samples(std::size_t number_of_samples, std::int16_t *target);
 		void set_sample_volume_range(std::int16_t range);
 
+		// The OPLL is generally 'half' as loud as it's told to be. This won't strictly be true in
+		// rhythm mode, but it's correct for melodic output.
+		double get_average_output_peak() const { return 0.5; }
+
 		/// Reads from the OPL.
 		uint8_t read(uint16_t address);
 

Components/SN76489/SN76489.cpp

@@ -86,7 +86,7 @@ void SN76489::write(uint8_t value) {
 	});
 }
 
-bool SN76489::is_zero_level() {
+bool SN76489::is_zero_level() const {
 	return channels_[0].volume == 0xf && channels_[1].volume == 0xf && channels_[2].volume == 0xf && channels_[3].volume == 0xf;
 }
 

Components/SN76489/SN76489.hpp

@@ -30,7 +30,7 @@ class SN76489: public Outputs::Speaker::SampleSource {
 
 		// As per SampleSource.
 		void get_samples(std::size_t number_of_samples, std::int16_t *target);
-		bool is_zero_level();
+		bool is_zero_level() const;
 		void set_sample_volume_range(std::int16_t range);
 		static constexpr bool get_is_stereo() { return false; }