Add BiquadTDF2

CMakeLists.txt

@@ -63,6 +63,8 @@ if(NOT CMAKE_CROSSCOMPILING)
 
             "lib/grit/audio/envelope/envelope_follower_test.cpp"
 
+            "lib/grit/audio/filter/biquad_test.cpp"
+
             "lib/grit/audio/music/note_test.cpp"
 
             "lib/grit/audio/noise/dither_test.cpp"

lib/CMakeLists.txt

@@ -39,6 +39,7 @@ target_sources(gritwave-grit INTERFACE
         "grit/audio/envelope/envelope_follower.hpp"
 
         "grit/audio/filter.hpp"
+        "grit/audio/filter/biquad.hpp"
         "grit/audio/filter/dynamic_smoothing.hpp"
 
         "grit/audio/mix.hpp"

lib/grit/audio/filter.hpp

@@ -3,4 +3,5 @@
 /// \defgroup grit-audio-filter Filter
 /// \ingroup grit-audio
 
+#include <grit/audio/filter/biquad.hpp>
 #include <grit/audio/filter/dynamic_smoothing.hpp>

lib/grit/audio/filter/biquad.hpp

@@ -0,0 +1,77 @@
+#pragma once
+
+#include <etl/algorithm.hpp>
+#include <etl/array.hpp>
+#include <etl/concepts.hpp>
+#include <etl/span.hpp>
+
+namespace grit {
+
+/// Calculates coefficents for 2nd order biquad filters
+/// \see BiquadTDF2
+/// \ingroup grit-audio-filter
+template<etl::floating_point Float>
+struct BiquadCoefficients
+{
+    using value_type = Float;
+    using SampleType = Float;
+
+    [[nodiscard]] static constexpr auto makeBypass() -> etl::array<Float, 6>
+    {
+        return {Float(1), Float(0), Float(0), Float(1), Float(0), Float(0)};
+    }
+};
+
+/// \brief 2nd order IIR filter using the transpose direct form 2 structure.
+/// \ingroup grit-audio-filter
+template<etl::floating_point Float>
+struct BiquadTDF2
+{
+    using value_type   = Float;
+    using SampleType   = Float;
+    using Coefficients = BiquadCoefficients<Float>;
+
+    constexpr BiquadTDF2() = default;
+
+    constexpr auto setCoefficients(etl::span<Float const, 6> coefficients) -> void
+    {
+        etl::copy(coefficients.begin(), coefficients.end(), _coefficients.begin());
+    }
+
+    constexpr auto reset() -> void
+    {
+        _z[0] = Float(0);
+        _z[1] = Float(0);
+    }
+
+    [[nodiscard]] constexpr auto operator()(Float x) -> Float
+    {
+        auto const b0 = _coefficients[Index::B0];
+        auto const b1 = _coefficients[Index::B1];
+        auto const b2 = _coefficients[Index::B2];
+        auto const a1 = _coefficients[Index::A1];
+        auto const a2 = _coefficients[Index::A2];
+
+        auto const y = b0 * x + _z[0];
+        _z[0]        = b1 * x - a1 * y + _z[1];
+        _z[1]        = b2 * x - a2 * y;
+        return y;
+    }
+
+private:
+    enum Index
+    {
+        B0,
+        B1,
+        B2,
+        A0,
+        A1,
+        A2,
+        NumCoefficients,
+    };
+
+    etl::array<Float, NumCoefficients> _coefficients{Coefficients::makeBypass()};
+    etl::array<Float, 2> _z{};
+};
+
+}  // namespace grit

lib/grit/audio/filter/biquad_test.cpp

@@ -0,0 +1,53 @@
+#include "biquad.hpp"
+
+#include <etl/random.hpp>
+
+#include <catch2/catch_get_random_seed.hpp>
+#include <catch2/catch_template_test_macros.hpp>
+#include <catch2/matchers/catch_matchers_floating_point.hpp>
+
+TEMPLATE_TEST_CASE("audio/filter: BiquadCoefficients::makeBypass", "", float, double)
+{
+    using Float        = TestType;
+    using Coefficients = grit::BiquadCoefficients<Float>;
+
+    static constexpr auto bypass = Coefficients::makeBypass();
+    STATIC_REQUIRE(bypass.size() == 6U);
+
+    REQUIRE_THAT(bypass[0], Catch::Matchers::WithinAbs(1.0, 1e-6));
+    REQUIRE_THAT(bypass[1], Catch::Matchers::WithinAbs(0.0, 1e-6));
+    REQUIRE_THAT(bypass[2], Catch::Matchers::WithinAbs(0.0, 1e-6));
+    REQUIRE_THAT(bypass[3], Catch::Matchers::WithinAbs(1.0, 1e-6));
+    REQUIRE_THAT(bypass[4], Catch::Matchers::WithinAbs(0.0, 1e-6));
+    REQUIRE_THAT(bypass[5], Catch::Matchers::WithinAbs(0.0, 1e-6));
+}
+
+TEMPLATE_TEST_CASE("audio/filter: BiquadTDF2", "", float, double)
+{
+    using Float  = TestType;
+    using Filter = grit::BiquadTDF2<Float>;
+
+    static constexpr auto iterations = 1'000;
+
+    auto rng  = etl::xoshiro128plusplus{Catch::getSeed()};
+    auto dist = etl::uniform_real_distribution<Float>{Float(-1), Float(+1)};
+
+    SECTION("default coefficents are bypass")
+    {
+        auto filter = Filter{};
+        for (auto i{0}; i < iterations; ++i) {
+            auto const x = dist(rng);
+            auto const y = filter(x);
+            REQUIRE_THAT(y, Catch::Matchers::WithinAbs(x, 1e-6));
+        }
+
+        filter.setCoefficients(Filter::Coefficients::makeBypass());
+        filter.reset();
+
+        for (auto i{0}; i < iterations; ++i) {
+            auto const x = dist(rng);
+            auto const y = filter(x);
+            REQUIRE_THAT(y, Catch::Matchers::WithinAbs(x, 1e-6));
+        }
+    }
+}