feat(dsp): add core synthesizer oscillators, wavetable and voice engine

Source/SubOscillator.cpp

@@ -0,0 +1,39 @@
+#include "SubOscillator.h"
+
+namespace serum
+{
+
+void SubOscillator::noteOn (double freqHz, int octaveShift)
+{
+    // octaveShift: -2, -1 or 0 (from the subOct param).
+    const double mult = (octaveShift == -2) ? 0.25 : (octaveShift == -1) ? 0.5 : 1.0;
+    (void) freqHz;
+    (void) mult;
+    phase = 0.0;
+}
+
+void SubOscillator::processAdd (double freqHz, int shape, float level, float& out) noexcept
+{
+    if (level <= 0.0f || freqHz <= 0.0)
+        return;
+
+    // The octave shift is baked into freqHz by the voice; here we just phase-accumulate.
+    const double inc = kTwoPi * freqHz / sr;
+    phase += inc;
+    phase -= std::floor (phase * (1.0 / kTwoPi)) * kTwoPi;
+
+    float s = 0.0f;
+    if (shape == (int) SubShape::Triangle)
+    {
+        const float p = (float) (phase * (1.0 / kTwoPi));
+        s = 1.0f - 4.0f * std::abs (p - 0.5f);
+    }
+    else
+    {
+        s = (float) std::sin (phase);
+    }
+
+    out += s * level;
+}
+
+} // namespace serum