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Replace audio::feedforward/feedback filters with general linear filters

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This commit is contained in:
Nikita Lisitsa 2026-07-19 01:24:31 +03:00
parent 4de0499d29
commit b39475d3ac
2 changed files with 45 additions and 67 deletions
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9
libs/audio/include/psemek/audio/effect/filter.hpp
View file

@ -1,15 +1,14 @@
#pragma once
#include <psemek/audio/stream.hpp>
#include <psemek/util/array.hpp>
namespace psemek::audio
{
// Turns x[n] into y[n] = a0*x[n] + a1*x[n-1]
stream_ptr feedforward_filter(stream_ptr stream, float a0, float a1);
// Turns x[n] into y[n] = a0*x[n] + b1*y[n-1]
stream_ptr feedback_filter(stream_ptr stream, float a0, float b1);
// Turns x[n] into y[n] = b0*x[n] + b1*x[n-1] + ... - a1*y[n-1] - a2*y[n-2] - ...
// NB: a[0] is implicitly 1 and is omitted
stream_ptr linear_filter(stream_ptr stream, util::array<float> a, util::array<float> b);
stream_ptr low_pass_filter(stream_ptr stream);
stream_ptr high_pass_filter(stream_ptr stream);

103
libs/audio/source/effect/filter.cpp
View file

@ -6,14 +6,19 @@ namespace psemek::audio
namespace
{
struct feedforward_filter_impl
struct linear_filter_impl
: stream
{
feedforward_filter_impl(stream_ptr stream, float a0, float a1)
linear_filter_impl(stream_ptr stream, util::array<float> a, util::array<float> b)
: stream_(std::move(stream))
, a0_(a0)
, a1_(a1)
{}
, a_(std::move(a))
, b_(std::move(b))
{
prev_in_[0].resize(b_.size(), 0.f);
prev_in_[1].resize(b_.size(), 0.f);
prev_out_[0].resize(a_.size(), 0.f);
prev_out_[1].resize(a_.size(), 0.f);
}
std::optional<std::size_t> length() const override
{
@ -24,13 +29,32 @@ namespace psemek::audio
{
std::size_t count = stream_->read(samples);
// TODO: optimize? Maybe split into 3 loops (first N, main part, last N) to
// get rid of shift_right on each iteration
for (std::size_t i = 0; i < count; i += 2)
{
std::swap(prev_[0], samples[i + 0]);
std::swap(prev_[1], samples[i + 1]);
for (std::size_t s : {0, 1})
{
if (!prev_in_[s].empty())
{
std::shift_right(prev_in_[s].begin(), prev_in_[s].end(), 1);
prev_in_[s][0] = samples[i + s];
}
samples[i + 0] = prev_[0] * a0_ + samples[i + 0] * a1_;
samples[i + 1] = prev_[1] * a0_ + samples[i + 1] * a1_;
float value = 0.f;
for (std::size_t j = 0; j < b_.size(); ++j)
value += prev_in_[s][j] * b_[j];
for (std::size_t j = 0; j < a_.size(); ++j)
value -= prev_out_[s][j] * a_[j];
samples[i + s] = value;
if (!prev_out_[s].empty())
{
std::shift_right(prev_out_[s].begin(), prev_out_[s].end(), 1);
prev_out_[s][0] = value;
}
}
}
return count;
@ -43,72 +67,27 @@ namespace psemek::audio
private:
stream_ptr stream_;
float a0_;
float a1_;
float prev_[2]{0.f};
};
struct feedback_filter_impl
: stream
{
feedback_filter_impl(stream_ptr stream, float a0, float b1)
: stream_(std::move(stream))
, a0_(a0)
, b1_(b1)
{}
std::optional<std::size_t> length() const override
{
return stream_->length();
}
std::size_t read(util::span<float> samples) override
{
std::size_t count = stream_->read(samples);
for (std::size_t i = 0; i < count; i += 2)
{
samples[i + 0] = prev_[0] * b1_ + samples[i + 0] * a0_;
samples[i + 1] = prev_[1] * b1_ + samples[i + 1] * a0_;
prev_[0] = samples[i + 0];
prev_[1] = samples[i + 1];
}
return count;
}
std::size_t played() const override
{
return stream_->played();
}
private:
stream_ptr stream_;
float a0_;
float b1_;
float prev_[2]{0.f};
util::array<float> a_;
util::array<float> b_;
util::array<float> prev_in_[2];
util::array<float> prev_out_[2];
};
}
stream_ptr feedforward_filter(stream_ptr stream, float a0, float a1)
stream_ptr linear_filter(stream_ptr stream, util::array<float> a, util::array<float> b)
{
return std::make_shared<feedforward_filter_impl>(std::move(stream), a0, a1);
}
stream_ptr feedback_filter(stream_ptr stream, float a0, float b1)
{
return std::make_shared<feedback_filter_impl>(std::move(stream), a0, b1);
return std::make_shared<linear_filter_impl>(std::move(stream), std::move(a), std::move(b));
}
stream_ptr low_pass_filter(stream_ptr stream)
{
return feedforward_filter(std::move(stream), 1.f, 1.f);
return linear_filter(std::move(stream), {}, {1.f, 1.f});
}
stream_ptr high_pass_filter(stream_ptr stream)
{
return feedforward_filter(std::move(stream), 1.f, -1.f);
return linear_filter(std::move(stream), {}, {1.f, -1.f});
}
}