From 4782f1ff322d66c825e035bc2ced8aa7643de8fc Mon Sep 17 00:00:00 2001
From: Geraint <luffgd@gmail.com>
Date: Sun, 6 Jul 2025 09:15:00 +0100
Subject: [PATCH] Chorus pans shared delay taps instead of per-channel offset

---
 chorus.h | 68 +++++++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 48 insertions(+), 20 deletions(-)

diff --git a/chorus.h b/chorus.h
index 02a496e..5b9bca9 100644
--- a/chorus.h
+++ b/chorus.h
@@ -40,6 +40,7 @@ struct ChorusSTFX : public BaseEffect {
 	ParamRange mix{0.6};
 	ParamRange depthMs{15};
 	ParamRange detune{5};
+	ParamRange stereo{1};
 	
 	template<class Storage>
 	void state(Storage &storage) {
@@ -59,6 +60,10 @@ struct ChorusSTFX : public BaseEffect {
 			.info("detune", "detuning depth")
 			.range(1, 8, 50)
 			.unit(" s.t.", 0);
+
+		stfx::units::rangePercent(storage.range("stereo", stereo)
+			.info("stereo", "")
+			.range(0, 0.5, 1.5));
 	}
 
 	template<class Config>
@@ -74,12 +79,16 @@ struct ChorusSTFX : public BaseEffect {
 	void reset() {
 		delay.reset();
 		phase = 0;
+		stereoPhase = 0;
 	}
 	
 	template<class Io, class Config, class Block>
 	void processSTFX(Io &io, Config &config, Block &block) {
 		Sample detuneHz = detune*0.45f/depthMs; // 0.45ms oscillation at 1Hz is about 1 semitone
 		Sample phaseStep = detuneHz/config.sampleRate;
+		Sample stereoPhaseStep = phaseStep/Sample(M_PI);
+		Complex stereoComplexPerOutput = std::polar(Sample(1), Sample(2*M_PI)/config.outputChannels);
+		Complex stereoComplexPerInternal = std::polar(Sample(1), Sample(1));
 		
 		bool fading = depthMs.from() != depthMs.to();
 		Sample depthSamples = depthMs.to()*0.001*config.sampleRate;
@@ -90,6 +99,8 @@ struct ChorusSTFX : public BaseEffect {
 		
 		auto dry = block.smooth(mix, [](double m){return 1 - m*m;});
 		auto wet = block.smooth(mix, [](double m){return m*(2 - m)/std::sqrt(Sample(6));});
+		bool notMono = (config.outputChannels > 1);
+		auto width = block.smooth(stereo);
 		
 		for (size_t i = 0; i < block.length; ++i) {
 			for (size_t c = 0; c < 6; ++c) {
@@ -98,34 +109,50 @@ struct ChorusSTFX : public BaseEffect {
 			}
 			delay.write(multiIn);
 			
-			for (size_t oc = 0; oc < config.outputChannels; ++oc) {
-				Complex phaseComplex = std::polar(Sample(1), phase*Sample(2*M_PI) + oc*Sample(2.632));
+			Complex phaseComplex = std::polar(Sample(1), phase*Sample(2*M_PI));
+			for (size_t c = 0; c < 6; ++c) {
+				Sample osc = (phaseComplex*oscillatorOffsets[c]).real();
+				delaySamples[c] = depthSamples*Sample(0.5)*(1 + osc);
+			}
+			delay.readMulti(delaySamples, multiOut);
+
+			if (fading) {
+				// read a second set of delay times, and fade between them
+				std::array<Sample, 6> multiOutFrom;
 				for (size_t c = 0; c < 6; ++c) {
 					Sample osc = (phaseComplex*oscillatorOffsets[c]).real();
-					delaySamples[c] = depthSamples*Sample(0.5)*(1 + osc);
+					delaySamples[c] = depthSamplesFrom*Sample(0.5)*(1 + osc);
 				}
-				delay.readMulti(delaySamples, multiOut);
-
-				if (fading) {
-					// read a second set of delay times, and fade between them
-					std::array<Sample, 6> multiOutFrom;
-					for (size_t c = 0; c < 6; ++c) {
-						Sample osc = (phaseComplex*oscillatorOffsets[c]).real();
-						delaySamples[c] = depthSamplesFrom*Sample(0.5)*(1 + osc);
-					}
-					delay.readMulti(delaySamples, multiOutFrom);
-					for (size_t c = 0; c < 6; ++c) {
-						multiOut[c] = block.fade(i, multiOutFrom[c], multiOut[c]);
-					}
+				delay.readMulti(delaySamples, multiOutFrom);
+				for (size_t c = 0; c < 6; ++c) {
+					multiOut[c] = block.fade(i, multiOutFrom[c], multiOut[c]);
 				}
-
-				// Arbitrarily chosen gains, 4 positive, 2 negative
-				Sample sum = multiOut[0] - multiOut[1] + multiOut[2] + multiOut[3] - multiOut[4] + multiOut[5];
-				io.output[oc][i] = multiIn[oc]*dry.at(i) + sum*wet.at(i);
 			}
+
+			// Arbitrarily chosen gains, 4 positive, 2 negative
+			Sample sum = multiOut[0] - multiOut[1] + multiOut[2] + multiOut[3] - multiOut[4] + multiOut[5];
+			if (notMono) {
+				Complex stereoComplex = std::polar(Sample(width.at(i)), stereoPhase*Sample(2*M_PI));
+				for (size_t oc = 0; oc < config.outputChannels; ++oc) {
+					Sample sumC = sum;
+					Complex rot = stereoComplex;
+					for (size_t i = 0; i < 6; ++i) {
+						sumC += rot.real()*multiOut[i];
+						rot *= stereoComplexPerInternal;
+					}
+
+					io.output[oc][i] = multiIn[oc]*dry.at(i) + sumC*wet.at(i);
+					stereoComplex *= stereoComplexPerOutput;
+				}
+				stereoPhase += stereoPhaseStep;
+			} else {
+				io.output[0][i] = multiIn[0]*dry.at(i) + sum*wet.at(i);
+			}
+
 			phase += phaseStep;
 		}
 		phase -= std::floor(phase);
+		stereoPhase -= std::floor(stereoPhase);
 	}
 	
 	template<class Storage>
@@ -136,6 +163,7 @@ private:
 
 	signalsmith::delay::MultiDelay<Sample, signalsmith::delay::InterpolatorLagrange7> delay;
 	Sample phase = 0;
+	Sample stereoPhase = 0;
 };
 
 }} // namespace