Fix filter env: modulate relative to sustain so cutoff knob stays audible at any env amount

Source/DSP/Voice.h

@@ -128,8 +128,14 @@ public:
             left *= noteVelocity * envSample;
             right *= noteVelocity * envSample;
 
-            // Filter cutoff with env + keytrack
+            // Filter cutoff with env + keytrack.
-            float modCutoff = filterCutoff * std::pow(2.0f, fEnvSample * filterEnvAmount * 4.0f);
+            // Modulate RELATIVE to the envelope's sustain level so the held
+            // (sustain) cutoff always equals the base cutoff knob regardless of
+            // env amount — the envelope only opens above (attack) and closes
+            // below (release) that centre. This keeps the cutoff knob audible
+            // at any env amount instead of collapsing when env amt is lowered.
+            float modCutoff = filterCutoff * std::pow(2.0f,
+                (fEnvSample - filterEnvSustain) * filterEnvAmount * 4.0f);
             float keyTrackFactor = std::pow(2.0f, (static_cast<float>(currentMidiNote) - 60.0f) / 12.0f * keyTrack);
             modCutoff *= keyTrackFactor;
 
@@ -191,6 +197,7 @@ public:
         filterResonance = res;
         filterType = ft;
         filterEnvAmount = fEnvAmt;
+        filterEnvSustain = fSus;
         keyTrack = kTrack;
         pan = p;
         drive = std::max(drv, 1.001f);
@@ -245,6 +252,7 @@ private:
     float filterResonance = 0.7f;
     FilterType filterType = FilterType::LowPass12;
     float filterEnvAmount = 0.0f;
+    float filterEnvSustain = 0.5f;
     float keyTrack = 0.5f;
     float pan = 0.0f;
     float drive = 1.5f;