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