Oversample the limiter by 2x

src/sfizz/effects/Limiter.cpp

@@ -12,9 +12,12 @@
 
 #include "Limiter.h"
 #include "Opcode.h"
-#include "gen/limiter.cpp"
+#include "AudioSpan.h"
 #include "absl/memory/memory.h"
 
+static constexpr int _oversampling = 2;
+#include "gen/limiter.cpp"
+
 namespace sfz {
 namespace fx {
 
@@ -32,12 +35,20 @@ namespace fx {
     {
         _limiter->classInit(sampleRate);
         _limiter->instanceConstants(sampleRate);
+
+        static constexpr double coefs2x[12] = { 0.036681502163648017, 0.13654762463195794, 0.27463175937945444, 0.42313861743656711, 0.56109869787919531, 0.67754004997416184, 0.76974183386322703, 0.83988962484963892, 0.89226081800387902, 0.9315419599631839, 0.96209454837808417, 0.98781637073289585 };
+
+        for (unsigned c = 0; c < EffectChannels; ++c) {
+            _downsampler2x[c].set_coefs(coefs2x);
+            _upsampler2x[c].set_coefs(coefs2x);
+        }
+
         clear();
     }
 
     void Limiter::setSamplesPerBlock(int samplesPerBlock)
     {
-        (void)samplesPerBlock;
+        _tempBuffer2x.resize(2 * samplesPerBlock);
     }
 
     void Limiter::clear()
@@ -47,7 +58,15 @@ namespace fx {
 
     void Limiter::process(const float* const inputs[], float* const outputs[], unsigned nframes)
     {
-        _limiter->compute(nframes, inputs, outputs);
+        auto inOut2x = AudioSpan<float>( _tempBuffer2x).first(2 * nframes);
+
+        for (unsigned c = 0; c < EffectChannels; ++c)
+            _upsampler2x[c].process_block(inOut2x.getSpan(c).data(), inputs[c], nframes);
+
+        _limiter->compute(2 * nframes, inOut2x, inOut2x);
+
+        for (unsigned c = 0; c < EffectChannels; ++c)
+            _downsampler2x[c].process_block(outputs[c], inOut2x.getSpan(c).data(), nframes);
     }
 
     std::unique_ptr<Effect> Limiter::makeInstance(absl::Span<const Opcode> members)

src/sfizz/effects/Limiter.h

@@ -6,6 +6,8 @@
 
 #pragma once
 #include "Effects.h"
+#include "hiir/Downsampler2xFpu.h"
+#include "hiir/Upsampler2xFpu.h"
 class faustLimiter;
 
 namespace sfz {
@@ -47,6 +49,9 @@ namespace fx {
 
     private:
         std::unique_ptr<faustLimiter> _limiter;
+        AudioBuffer<float, 2> _tempBuffer2x { 2, 2 * config::defaultSamplesPerBlock };
+        hiir::Downsampler2xFpu<12> _downsampler2x[EffectChannels];
+        hiir::Upsampler2xFpu<12> _upsampler2x[EffectChannels];
     };
 
 } // namespace fx

src/sfizz/effects/dsp/limiter.dsp

@@ -1,8 +1,9 @@
 import("stdfaust.lib");
 
 limiter(x) = gain*x with {
-  att = 0.0008;
+  att = 0.0008 * over;
-  rel = 0.5;
+  rel = 0.5 * over;
+  over = fconstant(int _oversampling, <math.h>);
   peak = x : an.amp_follower_ud(att, rel);
   gain = ba.if(peak>1.0, 1.0/peak, 1.0) : si.smooth(ba.tau2pole(0.5*att));
 };

src/sfizz/effects/gen/limiter.cpp

@@ -51,7 +51,7 @@ public:
     void instanceConstants(int sample_rate)
     {
         fSampleRate = sample_rate;
-        fConst0 = std::min<float>(192000.0f, std::max<float>(1.0f, float(fSampleRate)));
+        fConst0 = (std::min<float>(192000.0f, std::max<float>(1.0f, float(fSampleRate))) * float(_oversampling));
         fConst1 = std::exp((0.0f - (2500.0f / fConst0)));
         fConst2 = (1.0f - fConst1);
         fConst3 = std::exp((0.0f - (1250.0f / fConst0)));