feat(fx): add effect rack and effect units

Source/EffectUnits/Phaser.cpp

@@ -0,0 +1,54 @@
+#include "Phaser.h"
+
+namespace serum
+{
+
+void PhaserUnit::prepare (double sampleRate, int)
+{
+    sr = sampleRate;
+    reset();
+}
+
+void PhaserUnit::reset()
+{
+    lfoPhase = 0.0;
+    for (int i = 0; i < 6; ++i)
+    {
+        xL[i] = xR[i] = yL[i] = yR[i] = 0.0f;
+    }
+}
+
+void PhaserUnit::process (float* l, float* r, int numSamples, const float p[4])
+{
+    const double rate     = 0.05 + p[0] * 2.0;
+    const float  depth    = 0.3f + p[1] * 0.6f;
+    const float  feedback = p[2] * 0.7f;
+    const int    stages   = juce::jlimit (2, 6, 2 + (int) (p[3] * 4.0f + 0.5f));
+
+    for (int i = 0; i < numSamples; ++i)
+    {
+        lfoPhase += 6.28318530717958647692 * rate / sr;
+        if (lfoPhase > 6.28318530717958647692) lfoPhase -= 6.28318530717958647692;
+
+        const float sweep = (float) (0.5 + 0.5 * std::sin (lfoPhase));
+        const float a = 0.3f + depth * sweep;   // allpass coefficient
+
+        float outL = l[i] + yL[5] * feedback;
+        float outR = r[i] + yR[5] * feedback;
+
+        for (int s = 0; s < stages; ++s)
+        {
+            // y = a*x + x_prev - a*y_prev
+            const float inL = outL, inR = outR;
+            outL = a * inL + xL[s] - a * yL[s];
+            outR = a * inR + xR[s] - a * yR[s];
+            xL[s] = inL; xR[s] = inR;
+            yL[s] = outL; yR[s] = outR;
+        }
+
+        l[i] = outL;
+        r[i] = outR;
+    }
+}
+
+} // namespace serum