Add post hoc filter, attempt to juice precision.

Components/SID/SID.cpp

@@ -10,7 +10,13 @@
 
 using namespace MOS::SID;
 
-SID::SID(Concurrency::AsyncTaskQueue<false> &audio_queue) : audio_queue_(audio_queue) {}
+SID::SID(Concurrency::AsyncTaskQueue<false> &audio_queue) :
+	audio_queue_(audio_queue),
+	output_filter_(
+		SignalProcessing::BiquadFilter::Type::LowPass,
+		1000000.0f,
+		15000.0f
+	) {}
 
 void SID::write(const Numeric::SizedInt<5> address, const uint8_t value) {
 	last_write_ = value;
@@ -158,7 +164,7 @@ void SID::apply_samples(const std::size_t number_of_samples, Outputs::Speaker::M
 			);
 
 		// Sum, apply volume and output.
-		const auto sample =
+		const auto sample = output_filter_.apply(int16_t(
 			(
 				volume_ * (
 					direct_sample +
@@ -166,7 +172,8 @@ void SID::apply_samples(const std::size_t number_of_samples, Outputs::Speaker::M
 						- 227	// DC offset.
 				)
 				- 88732
-			) / 3;
+			) / 3
+		));
 		// Maximum range of above: 15 * (4095 * 3 - 227) = [-3405, 180870]
 		// So subtracting 88732 will move to the centre of the range, and 3 is the smallest
 		// integer that avoids clipping.

Components/SID/SID.hpp

@@ -299,6 +299,8 @@ private:
 	Numeric::SizedInt<4> filter_channels_;
 	Numeric::SizedInt<3> filter_mode_;
 	void update_filter();
+
+	SignalProcessing::BiquadFilter output_filter_;
 };
 
 }

SignalProcessing/BiquadFilter.hpp

@@ -8,6 +8,7 @@
 
 #pragma once
 
+#include <cassert>
 #include <cmath>
 #include <numbers>
 
@@ -31,9 +32,9 @@ public:
 		const Type type,
 		const float sample_rate,
 		const float frequency,
-		const float resonance,
-		const float gain,
-		const bool normalise
+		const float resonance = 0.707f,
+		const float gain = 8,
+		const bool normalise = true
 	) {
 		const float w0 = 2.0f * std::numbers::pi_v<float> * frequency / sample_rate;
 		const float alpha = std::sin(w0) / (2.0f * resonance);
@@ -133,7 +134,8 @@ public:
 			}
 		}
 		for(int c = 0; c < 5; c++) {
-			coefficients_[c] = int16_t(coefficients[c] * 4096.0f);
+			assert(std::abs(coefficients[c]) < 32768.0f / 16384.0f);
+			coefficients_[c] = int16_t(coefficients[c] * 16384.0f);
 		}
 	}
 
@@ -144,7 +146,7 @@ public:
 			coefficients_[2] * inputs_[1] -
 			coefficients_[3] * outputs_[0] -
 			coefficients_[4] * outputs_[1]
-		) >> 12;
+		) >> 14;
 
 		inputs_[1] = inputs_[0];
 		inputs_[0] = input;