From d2e771d3218e9a074af1429430b16558ec262b2f Mon Sep 17 00:00:00 2001
From: Tortuga Veloz <danielcampora@outlook.es>
Date: Mon, 9 Feb 2026 12:01:30 +0100
Subject: [PATCH] Surround.

---
 include/sound_matrix_decoder.h | 116 +++++++++++++++++++++++++++++++++
 1 file changed, 116 insertions(+)
 create mode 100644 include/sound_matrix_decoder.h

diff --git a/include/sound_matrix_decoder.h b/include/sound_matrix_decoder.h
new file mode 100644
index 0000000..b800015
--- /dev/null
+++ b/include/sound_matrix_decoder.h
@@ -0,0 +1,116 @@
+#pragma once
+
+#include <cstdint>
+#include <cmath>
+#include <array>
+#include <vector>
+#include <tuple>
+
+#ifndef M_PI
+#define M_PI 3.14159265358979323846
+#endif
+
+/**
+ * Passive matrix decoder for stereo to 5.1 surround upmixing.
+ * Implements standard audio matrix decoding techniques using:
+ * - Linkwitz-Riley crossover filters for frequency band separation
+ * - All-pass filters for phase manipulation
+ * - Delay lines for surround channel timing
+ */
+class SoundMatrixDecoder {
+  public:
+    /**
+     * Construct and initialize the decoder with a specific sample rate.
+     * @param sampleRate The audio sample rate in Hz
+     */
+    SoundMatrixDecoder(int32_t sampleRate);
+    ~SoundMatrixDecoder() = default;
+
+    /**
+     * Reset filter states without recomputing coefficients.
+     * Useful when audio is interrupted to prevent clicks.
+     */
+    void ResetState();
+
+    /**
+     * Decode stereo to 5.1 surround
+     * @param stereoInput Interleaved stereo samples [L0, R0, L1, R1, ...]
+     * @param samplePairs Number of stereo sample pairs to process
+     * @return Pointer to internal buffer with interleaved 5.1 samples [FL, FR, C, LFE, SL, SR, ...]
+     */
+    std::tuple<const float*, size_t> Process(const float* stereoInput, size_t samplePairs);
+
+  private:
+    // 4th-order IIR filter (Linkwitz-Riley) for 24dB/octave slopes
+    struct BiquadCascade {
+        double X[4] = {}; // Input history
+        double Y[4] = {}; // Output history
+    };
+
+    struct FilterCoefficients {
+        double A[5]; // Feedforward (numerator)
+        double B[4]; // Feedback (denominator, excluding b0=1)
+    };
+
+    // Sweeping all-pass for phase decorrelation
+    struct AllPassChain {
+        double Freq = 0;
+        double FreqMin = 0;
+        double FreqMax = 0;
+        double SweepRate = 0;
+        double XHist[4] = {};
+        double YHist[4] = {};
+        bool Ready = false;
+    };
+
+    // Circular delay buffer
+    static constexpr int gMaxDelay = 1024;
+    struct CircularDelay {
+        std::array<float, gMaxDelay> Data = {};
+        int Head = 0;
+        int Length = 0;
+    };
+
+    // Filter design
+    FilterCoefficients DesignLowPass(double frequency, int32_t sampleRate);
+    FilterCoefficients DesignHighPass(double frequency, int32_t sampleRate);
+
+    // Signal processing
+    float ProcessFilter(float sample, BiquadCascade& state, const FilterCoefficients& coef);
+    void PrepareAllPass(AllPassChain& chain, int32_t sampleRate);
+    float ProcessAllPass(float sample, AllPassChain& chain, bool negate);
+    float ProcessDelay(float sample, CircularDelay& buffer);
+
+    static float Saturate(float value);
+
+    int32_t mDelayLength = 0;
+    double mAllPassBaseRate = 1.0; // Precomputed for ProcessAllPass
+
+    // Filter coefficients (computed once per sample rate)
+    FilterCoefficients mCoefCenterHP;
+    FilterCoefficients mCoefCenterLP;
+    FilterCoefficients mCoefSurroundHP;
+    FilterCoefficients mCoefSubLP;
+
+    // Per-channel filter states
+    BiquadCascade mCenterHighPass;
+    BiquadCascade mCenterLowPass;
+    BiquadCascade mSurrLeftMainHP;
+    BiquadCascade mSurrLeftCrossHP;
+    BiquadCascade mSurrRightMainHP;
+    BiquadCascade mSurrRightCrossHP;
+    BiquadCascade mSubLowPass;
+
+    // Phase processing
+    AllPassChain mPhaseLeftMain;
+    AllPassChain mPhaseLeftCross;
+    AllPassChain mPhaseRightMain;
+    AllPassChain mPhaseRightCross;
+
+    // Timing
+    CircularDelay mDelaySurrLeft;
+    CircularDelay mDelaySurrRight;
+
+    // Output buffer
+    std::vector<float> mSurroundBuffer;
+};