3 files changed, 145 insertions, 2 deletions
diff --git a/src/audio_core/CMakeLists.txt b/src/audio_core/CMakeLists.txt
index ec71524a3..92322f59b 100644
--- a/src/audio_core/CMakeLists.txt
+++ b/src/audio_core/CMakeLists.txt
@@ -1,4 +1,6 @@
 add_library(audio_core STATIC
+    algorithm/filter.cpp
+    algorithm/filter.h
     audio_out.cpp
     audio_out.h
     audio_renderer.cpp
@@ -7,12 +9,12 @@ add_library(audio_core STATIC
     codec.cpp
     codec.h
     null_sink.h
-    stream.cpp
-    stream.h
     sink.h
     sink_details.cpp
     sink_details.h
     sink_stream.h
+    stream.cpp
+    stream.h
 
     $<$<BOOL:${ENABLE_CUBEB}>:cubeb_sink.cpp cubeb_sink.h>
 )
diff --git a/src/audio_core/algorithm/filter.cpp b/src/audio_core/algorithm/filter.cpp
new file mode 100644
index 000000000..403b8503f
--- /dev/null
+++ b/src/audio_core/algorithm/filter.cpp
@@ -0,0 +1,79 @@
+// Copyright 2018 yuzu Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#define _USE_MATH_DEFINES
+
+#include <algorithm>
+#include <array>
+#include <cmath>
+#include <vector>
+#include "audio_core/algorithm/filter.h"
+#include "common/common_types.h"
+
+namespace AudioCore {
+
+Filter Filter::LowPass(double cutoff, double Q) {
+    const double w0 = 2.0 * M_PI * cutoff;
+    const double sin_w0 = std::sin(w0);
+    const double cos_w0 = std::cos(w0);
+    const double alpha = sin_w0 / (2 * Q);
+
+    const double a0 = 1 + alpha;
+    const double a1 = -2.0 * cos_w0;
+    const double a2 = 1 - alpha;
+    const double b0 = 0.5 * (1 - cos_w0);
+    const double b1 = 1.0 * (1 - cos_w0);
+    const double b2 = 0.5 * (1 - cos_w0);
+
+    return {a0, a1, a2, b0, b1, b2};
+}
+
+Filter::Filter() : Filter(1.0, 0.0, 0.0, 1.0, 0.0, 0.0) {}
+
+Filter::Filter(double a0, double a1, double a2, double b0, double b1, double b2)
+    : a1(a1 / a0), a2(a2 / a0), b0(b0 / a0), b1(b1 / a0), b2(b2 / a0) {}
+
+void Filter::Process(std::vector<s16>& signal) {
+    const size_t num_frames = signal.size() / 2;
+    for (size_t i = 0; i < num_frames; i++) {
+        std::rotate(in.begin(), in.end() - 1, in.end());
+        std::rotate(out.begin(), out.end() - 1, out.end());
+
+        for (size_t ch = 0; ch < channel_count; ch++) {
+            in[0][ch] = signal[i * channel_count + ch];
+
+            out[0][ch] = b0 * in[0][ch] + b1 * in[1][ch] + b2 * in[2][ch] - a1 * out[1][ch] -
+                         a2 * out[2][ch];
+
+            signal[i * 2 + ch] = std::clamp(out[0][ch], -32768.0, 32767.0);
+        }
+    }
+}
+
+/// Calculates the appropriate Q for each biquad in a cascading filter.
+/// @param total_count The total number of biquads to be cascaded.
+/// @param index 0-index of the biquad to calculate the Q value for.
+static double CascadingBiquadQ(size_t total_count, size_t index) {
+    const double pole = M_PI * (2 * index + 1) / (4.0 * total_count);
+    return 1.0 / (2.0 * std::cos(pole));
+}
+
+CascadingFilter CascadingFilter::LowPass(double cutoff, size_t cascade_size) {
+    std::vector<Filter> cascade(cascade_size);
+    for (size_t i = 0; i < cascade_size; i++) {
+        cascade[i] = Filter::LowPass(cutoff, CascadingBiquadQ(cascade_size, i));
+    }
+    return CascadingFilter{std::move(cascade)};
+}
+
+CascadingFilter::CascadingFilter() = default;
+CascadingFilter::CascadingFilter(std::vector<Filter> filters) : filters(std::move(filters)) {}
+
+void CascadingFilter::Process(std::vector<s16>& signal) {
+    for (auto& filter : filters) {
+        filter.Process(signal);
+    }
+}
+
+} // namespace AudioCore
diff --git a/src/audio_core/algorithm/filter.h b/src/audio_core/algorithm/filter.h
new file mode 100644
index 000000000..a41beef98
--- /dev/null
+++ b/src/audio_core/algorithm/filter.h
@@ -0,0 +1,62 @@
+// Copyright 2018 yuzu Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <array>
+#include <vector>
+#include "common/common_types.h"
+
+namespace AudioCore {
+
+/// Digital biquad filter:
+///
+///          b0 + b1 z^-1 + b2 z^-2
+///  H(z) = ------------------------
+///          a0 + a1 z^-1 + b2 z^-2
+class Filter {
+public:
+    /// Creates a low-pass filter.
+    /// @param cutoff Determines the cutoff frequency. A value from 0.0 to 1.0.
+    /// @param Q Determines the quality factor of this filter.
+    static Filter LowPass(double cutoff, double Q = 0.7071);
+
+    /// Passthrough filter.
+    Filter();
+
+    Filter(double a0, double a1, double a2, double b0, double b1, double b2);
+
+    void Process(std::vector<s16>& signal);
+
+private:
+    static constexpr size_t channel_count = 2;
+
+    /// Coefficients are in normalized form (a0 = 1.0).
+    double a1, a2, b0, b1, b2;
+    /// Input History
+    std::array<std::array<double, channel_count>, 3> in;
+    /// Output History
+    std::array<std::array<double, channel_count>, 3> out;
+};
+
+/// Cascade filters to build up higher-order filters from lower-order ones.
+class CascadingFilter {
+public:
+    /// Creates a cascading low-pass filter.
+    /// @param cutoff Determines the cutoff frequency. A value from 0.0 to 1.0.
+    /// @param cascade_size Number of biquads in cascade.
+    static CascadingFilter LowPass(double cutoff, size_t cascade_size);
+
+    /// Passthrough.
+    CascadingFilter();
+
+    explicit CascadingFilter(std::vector<Filter> filters);
+
+    void Process(std::vector<s16>& signal);
+
+private:
+    std::vector<Filter> filters;
+};
+
+} // namespace AudioCore