Add an optional LFO onepole for FROM_LAST envelope mode (#6699)

* Add an optional LFO onepole for FROM_LAST envelope mode

In envelope mode with high deform in FROM_LAST mode the
signal from the envelope is not continuous. This is basically
unsolvable analytically - the deform plus state is too hard -
and so leave it on by default but add an optional one pole to
smooth it out, which may be useful in some cases.

Expose configurations of that to the rack LFO

Addresses surge-synthesizer/surge-rack#607

src/common/dsp/modulators/LFOModulationSource.cpp

@@ -77,6 +77,9 @@ void LFOModulationSource::assign(SurgeStorage *storage, LFOStorage *lfo, pdata *
     {
         wf_history[i] = 0.f;
     }
+
+    for (int i = 0; i < 3; ++i)
+        onepoleState[i] = 0.f;
 }
 
 float LFOModulationSource::bend1(float x)
@@ -1289,6 +1292,16 @@ void LFOModulationSource::process_block()
             output_multi[1] *= useenvval;
             output_multi[1] += useenv0;
         }
+
+        if (envRetrigMode == FROM_LAST && lfo->deform.deform_type != type_3)
+        {
+            // Now appy the one pole
+            float alpha = onepoleFactor;
+            output_multi[0] = (1.0 - alpha) * output_multi[0] + alpha * onepoleState[0];
+            output_multi[1] = (1.0 - alpha) * output_multi[1] + alpha * onepoleState[1];
+            onepoleState[0] = output_multi[0];
+            onepoleState[1] = output_multi[1];
+        }
     }
 }
 

src/common/dsp/modulators/LFOModulationSource.h

@@ -111,6 +111,8 @@ class LFOModulationSource : public ModulationSource
     inline int getEnvState() { return env_state; }
     inline int getStep() { return step; }
 
+    float onepoleFactor{0};
+
   private:
     pdata *localcopy;
     bool phaseInitialized;
@@ -128,6 +130,8 @@ class LFOModulationSource : public ModulationSource
     int step, shuffle_id;
     int magn, rate, iattack, idecay, idelay, ihold, isustain, irelease, startphase, ideform;
 
+    float onepoleState[3];
+
     std::default_random_engine gen;
     std::uniform_real_distribution<float> distro;
     std::function<float()> urng;