Introduce tremulant frequnecy modulation.

Author: Archie3d

Source/aeolus/division.cpp

@@ -35,13 +35,15 @@ Division::Division(Engine& engine, const String& name)
     , _midiChannel{0}
     , _tremulantEnabled{false}
     , _tremulantLevel{0.0f}
-    , _tremulantMaxLevel{1.0f}
+    , _tremulantMaxLevel{TREMULANT_TARGET_LEVEL}
     , _tremulantTargetLevel{0.0f}
     , _paramGain{nullptr}
     , _params{Division::NUM_PARAMS}
     , _swellFilterSpec{}
     , _swellFilterStateL{}
     , _swellFilterStateR{}
+    , _tremulantDelayL(TREMULANT_DELAY_LENGTH)
+    , _tremulantDelayR(TREMULANT_DELAY_LENGTH)
     , _stops{}
     , _activeVoices{}
     , _keysState{}
@@ -313,7 +315,7 @@ float Division::getTremulantLevel(bool update)
     const auto level = _tremulantLevel;
 
     if (update)
-        _tremulantLevel += 0.5f * (_tremulantTargetLevel - _tremulantLevel);
+        _tremulantLevel += 0.1f * (_tremulantTargetLevel - _tremulantLevel);
 
     return level;
 }
@@ -473,9 +475,20 @@ void Division::modulate(juce::AudioBuffer<float>& targetBuffer, const juce::Audi
     jassert(outR != nullptr);
 
     for (int i = 0; i < SUB_FRAME_LENGTH; ++i) {
+        _tremulantDelayL.write(outL[i]);
+        _tremulantDelayR.write(outR[i]);
+
         const float g = (1.0f + gain[i] * lvl) * paramGain.nextValue();
-        outL[i] *= g;
-        outR[i] *= g;
+
+        constexpr float freqModCenter = TREMULANT_DELAY_LENGTH * 0.5f;
+        constexpr float freqModAmp = TREMULANT_DELAY_LENGTH * 0.5f * TREMULANT_DELAY_MODULATION_LEVEL;
+
+        const float p = freqModCenter + freqModAmp * (0.5f - gain[i] * lvl);
+        outL[i] = _tremulantDelayL.read(p) * g;
+        outR[i] = _tremulantDelayR.read(p) * g;
+
+        //outL[i] *= g;
+        //outR[i] *= g;
     }
 
     // Apply swell filter

Source/aeolus/division.h

@@ -192,6 +192,10 @@ class Division
     dsp::BiquadFilter::State _swellFilterStateL;
     dsp::BiquadFilter::State _swellFilterStateR;
 
+    /// Delay lines used for tremulant frequency modulation.
+    dsp::DelayLine _tremulantDelayL;
+    dsp::DelayLine _tremulantDelayR;
+
     std::vector<Stop> _stops;   ///< All the stops this division has.
 
     List<Voice> _activeVoices;  ///< Active voices on this division.

Source/aeolus/globals.h

@@ -81,6 +81,9 @@ constexpr static float TREMULANT_PHASE_INCREMENT = juce::MathConstants<float>::t
 
 /// Tremulant OSC wavetable amplitude.
 constexpr static float TREMULANT_LEVEL = 1.0f;
+constexpr static float TREMULANT_TARGET_LEVEL = 0.5f; // Amplitude modulation level.
+constexpr static size_t TREMULANT_DELAY_LENGTH = 32; // Frequency modulation delay line length (in samples).
+constexpr static float TREMULANT_DELAY_MODULATION_LEVEL = 0.9f; // Frequency modulation level.
 
 /// Number of steps in the sequencer.
 constexpr static int SEQUENCER_N_STEPS = 16;

Source/aeolus/voice.cpp

@@ -77,7 +77,7 @@ void Voice::trigger(const Pipewave::State& state)
     _spatialSource.setSampleRate(SAMPLE_RATE_F);
     _spatialSource.setSourcePosition(x, 5.0f);
     _spatialSource.recalculate();
-    _postReleaseCounter = _spatialSource.getPostFxSamplesCount() + int(2*_delay + 0.5f);
+    _postReleaseCounter = _spatialSource.getPostFxSamplesCount() + int(2 * _delay + TREMULANT_DELAY_LENGTH + 0.5f);
 }
 
 void Voice::release()