Negative feedback does the squaring thing (#70)

just like surge Closes #68
baconpaul · Jan 3, 2025 · 3d687d7 · 3d687d7
1 parent 0973135
commit 3d687d7
Show file tree

Hide file tree

Showing 2 changed files with 9 additions and 1 deletion.
diff --git a/src/dsp/op_source.h b/src/dsp/op_source.h
@@ -17,6 +17,7 @@
 #define BACONPAUL_SIX_SINES_DSP_OP_SOURCE_H
 
 #include <cstdint>
+#include <cmath>
 
 #include "configuration.h"
 
@@ -178,6 +179,13 @@ struct alignas(16) OpSource : public EnvelopeSupport<Patch::SourceNode>,
 
             phase += dPhase;
             auto fb = 0.5 * (fbVal[0] + fbVal[1]);
+            auto sb = std::signbit(feedbackLevel[i]);
+            // fb = sb ? fb * fb : fb. Ugh a branch. but bool = 0/1, so
+            // (1-sb) * fb + sb * fb * fb - 3 mul, 2 add
+            // fb - sb * fb + sb * fb * fb - 3 nul 2 add
+            // fb * ( 1 - sb * ( 1 - fb)) - 2 mul 2 add
+            fb = fb * (1 - sb * (1 - fb));
+
             auto ph = phase + phaseInput[i] + (int32_t)(feedbackLevel[i] * fb);
             auto out = st.at(ph);
 

diff --git a/src/synth/patch.h b/src/synth/patch.h
@@ -527,7 +527,7 @@ struct Patch
         static constexpr uint32_t idBase{10000}, idStride{100};
         SelfNode(size_t idx)
             : fbLevel(floatMd()
-                          .asPercent()
+                          .asPercentBipolar()
                           .withName(name(idx) + " Level")
                           .withGroupName(name(idx))
                           .withDefault(0.0)