Add chorus effect with multi-voice modulated delays

Makefile

@@ -5,16 +5,17 @@ LDLIBS = -lm
 PYTHON = python3
 PLAY = ffplay -v fatal -nodisp -autoexit -f s32le -ar 48000 -ch_layout mono -i pipe:0
 
-effects = flanger echo fm phaser discont am distortion
+effects = flanger echo fm phaser discont am distortion chorus
 flanger_defaults = 0.6 0.6 0.6 0.6
 echo_defaults = 0.3 0.3 0.3 0.3
 fm_defaults = 0.25 0.25 0.5 0.5
 am_defaults = 0.5 0.5 0.5 0.5
 phaser_defaults = 0.3 0.3 0.5 0.5
 discont_defaults = 0.8 0.1 0.2 0.2
 distortion_defaults = 0.5 0.6 0.8 0.0
+chorus_defaults = 0.3 0.4 0.5 0.7
 
-HEADERS = biquad.h discont.h distortion.h echo.h effect.h flanger.h  fm.h  gensin.h lfo.h  phaser.h  util.h process.h
+HEADERS = biquad.h chorus.h discont.h distortion.h echo.h effect.h flanger.h fm.h gensin.h lfo.h phaser.h process.h util.h
 
 default:
 	@echo "Pick one of" $(effects)

chorus.h

@@ -0,0 +1,67 @@
+//
+// Chorus effect - multiple voices with modulated delays
+//
+// Creates a thicker sound by mixing the original signal with
+// multiple slightly detuned copies using LFO-modulated delays.
+//
+static struct {
+	struct lfo_state lfo1, lfo2, lfo3;
+	float delay_ms;
+	float depth;
+	float mix;
+} chorus;
+
+static inline void chorus_init(float pot1, float pot2, float pot3, float pot4)
+{
+	// pot1: LFO rate (0.1 - 5 Hz)
+	float rate = 0.1 + pot1 * 4.9;
+
+	// Slightly offset rates for each voice to avoid phase lock
+	set_lfo_freq(&chorus.lfo1, rate);
+	set_lfo_freq(&chorus.lfo2, rate * 1.1);
+	set_lfo_freq(&chorus.lfo3, rate * 0.9);
+
+	// pot2: base delay (5 - 30 ms)
+	chorus.delay_ms = 5 + pot2 * 25;
+
+	// pot3: depth/modulation amount (0 - 100%)
+	chorus.depth = pot3;
+
+	// pot4: wet/dry mix (0 = dry, 1 = full wet)
+	chorus.mix = pot4;
+
+	fprintf(stderr, "chorus:");
+	fprintf(stderr, " rate=%g Hz", rate);
+	fprintf(stderr, " delay=%g ms", chorus.delay_ms);
+	fprintf(stderr, " depth=%g", pot3);
+	fprintf(stderr, " mix=%g\n", pot4);
+}
+
+static inline float chorus_step(float in)
+{
+	// Store input in delay buffer
+	sample_array_write(in);
+
+	// Get three modulated delay values
+	float lfo1 = lfo_step(&chorus.lfo1, lfo_sinewave);
+	float lfo2 = lfo_step(&chorus.lfo2, lfo_sinewave);
+	float lfo3 = lfo_step(&chorus.lfo3, lfo_sinewave);
+
+	float base_samples = chorus.delay_ms * SAMPLES_PER_MSEC;
+	float mod_range = base_samples * chorus.depth * 0.5;
+
+	float d1 = base_samples + lfo1 * mod_range;
+	float d2 = base_samples + lfo2 * mod_range;
+	float d3 = base_samples + lfo3 * mod_range;
+
+	// Read delayed samples
+	float v1 = sample_array_read(d1);
+	float v2 = sample_array_read(d2);
+	float v3 = sample_array_read(d3);
+
+	// Mix voices (average of three delayed signals)
+	float wet = (v1 + v2 + v3) / 3.0f;
+
+	// Blend dry and wet
+	return in * (1.0f - chorus.mix) + wet * chorus.mix;
+}

convert.c

@@ -21,6 +21,7 @@
 #include "phaser.h"
 #include "discont.h"
 #include "distortion.h"
+#include "chorus.h"
 
 static void magnitude_init(float pot1, float pot2, float pot3, float pot4) {}
 static float magnitude_step(float in) { return u32_to_fraction(magnitude); }
@@ -36,6 +37,7 @@ struct effect {
 	EFF(echo),
 	EFF(flanger),
 	EFF(phaser),
+	EFF(chorus),
 
 	/* "Helper" effects */
 	EFF(am),