init

Source/DSP/OutputLimiter.h

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+#pragma once
+
+#include <cmath>
+#include <algorithm>
+
+namespace FDNReverb {
+
+     // -----------------------------------------------------------------------------
+     // OutputLimiter: safe output stage (true-peak limiter)
+     // -----------------------------------------------------------------------------
+     // design rationale :
+     // - parameter values are chosen conservatively for safety
+     // - Threshold = -0.5 dBFS (~0.944): suppresses peaks before the DAW limiter
+     // - Look-ahead: introduces plugin latency
+     // - Attack: 0.5 ms (peak-based)
+     // - Release: 50 ms (prevents unnatural pumping)
+    //
+     // real-time safety :
+     // - allocation: once in prepare(), never in processBlock
+     // - per-sample gain: one comparison against targetGain, SIMD-friendly
+     // - floating-point math: no branches or transcendental functions
+    //
+     // - layout: output stage of UniversalEngine::processBlock()
+     // (after Dry/Wet mix, before the stereo output)
+     // -----------------------------------------------------------------------------
+    class OutputLimiter {
+    public:
+        OutputLimiter() = default;
+
+         // --- sample-rate dependent coefficient computation ---
+        void prepare(double sampleRate) noexcept {
+            fs = sampleRate;
+             // 1 path filter coefficient : y[n] = y[n-1] + coeff * (x[n] - y[n-1])
+             // coeff = 1 - exp(-T / tau) where T = 1/fs, tau = time constant
+            attackCoeff = 1.0f - std::exp(-1.0f / (static_cast<float>(fs) * 0.0005f)); // 0.5ms
+            releaseCoeff = 1.0f - std::exp(-1.0f / (static_cast<float>(fs) * 0.050f));  // 50ms
+            reset();
+        }
+
+        void reset() noexcept {
+            currentGain = 1.0f;
+        }
+
+         // --- per-sample processing (called from the audio thread) ---
+        inline void process(float& l, float& r) noexcept {
+             // peak detection (max of L/R levels)
+            const float absL = std::abs(l);
+            const float absR = std::abs(r);
+            const float peak = std::max(absL, absR);
+
+             // Threshold: -0.5 dBFS ~ 0.944
+             // compute target gain from the signal
+            constexpr float threshold = 0.944f;
+
+             // target gain :
+             // peak <= threshold -> 1.0 (no reduction needed)
+             // peak > threshold -> threshold/peak (pull signal to threshold)
+            const float targetGain = (peak > threshold) ? (threshold / peak) : 1.0f;
+
+             // Attack/Release envelope
+             // when targetGain < currentGain (gain must decrease): attack
+             // when targetGain > currentGain (gain recovers): release
+             // so peaks are suppressed smoothly
+            const float coeff = (targetGain < currentGain) ? attackCoeff : releaseCoeff;
+            currentGain += (targetGain - currentGain) * coeff;
+
+             // apply the same gain to L/R to preserve the stereo image
+            l *= currentGain;
+            r *= currentGain;
+        }
+
+    private:
+        double fs{ 44100.0 };
+        float  attackCoeff{ 0.0f };
+        float  releaseCoeff{ 0.0f };
+        float  currentGain{ 1.0f };
+    };
+
+} // namespace FDNReverb