basic low pass filter app

CMakeLists.txt

@@ -1,6 +1,6 @@
 cmake_minimum_required(VERSION 3.12)
 
-project(pico-radio C)
+project(pico-radio)
 
 set(CMAKE_EXPORT_COMPILE_COMMANDS true)
 
@@ -8,3 +8,4 @@ include_directories(include /opt/picoscope/include third-party/c-flags/single-he
 link_directories(/opt/picoscope/lib)
 
 add_subdirectory(src/recorder)
+add_subdirectory(src/filter)

include/filter/filter/filter.hpp

@@ -0,0 +1,21 @@
+#ifndef FILTER_IFILTER_H
+#define FILTER_IFILTER_H
+
+#include <cstdint>
+#include <vector>
+
+namespace Filter {
+
+class IFilter {
+ protected:
+  std::vector<float> impulseResponse;
+
+ public:
+  IFilter(std::size_t size);
+  virtual ~IFilter(){};
+
+  void ConvolveIQ(const std::vector<float>& input, std::vector<float>& output);
+};
+} // namespace Filter
+
+#endif

include/filter/filter/low_pass_filter.hpp

@@ -0,0 +1,18 @@
+#ifndef FILTER_LOW_PASS_FILTER_H
+#define FILTER_LOW_PASS_FILTER_H
+
+#include <vector>
+
+#include <filter/filter/filter.hpp>
+#include <filter/window/window.hpp>
+
+namespace Filter {
+
+class LowPassFilter : public IFilter {
+ public:
+  LowPassFilter(std::size_t size, std::size_t cutoffHz, std::size_t sampleRateHz, const std::vector<float>& window);
+  ~LowPassFilter() override{};
+};
+} // namespace Filter
+
+#endif

include/filter/window/hamming_window.hpp

@@ -0,0 +1,21 @@
+#ifndef FILTER_HAMMING_WINDOW_H
+#define FILTER_HAMMING_WINDOW_H
+
+#include <cstddef>
+#include <vector>
+#include <filter/window/window.hpp>
+
+namespace Filter {
+class HammingWindow : public IWindow {
+ private:
+  std::vector<float> window;
+
+ public:
+  HammingWindow(std::size_t width);
+
+  ~HammingWindow() override {}
+  const std::vector<float>& getWindow() override { return window; }
+};
+} // namespace Filter
+
+#endif

include/filter/window/window.hpp

@@ -0,0 +1,14 @@
+#ifndef FILTER_IWINDOW_H
+#define FILTER_IWINDOW_H
+
+#include <vector>
+
+namespace Filter {
+class IWindow {
+ public:
+  virtual ~IWindow() {}
+  virtual const std::vector<float>& getWindow() = 0;
+};
+} // namespace Filter
+
+#endif

src/filter/CMakeLists.txt

@@ -0,0 +1,7 @@
+file(GLOB_RECURSE SOURCES "./*.cpp")
+add_executable(filter ${SOURCES})
+target_compile_options(filter PRIVATE -Wall -Wextra)
+set_target_properties(filter PROPERTIES
+    RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}"
+    CXX_STANDARD 20
+)

src/filter/filter/filter.cpp

@@ -0,0 +1,31 @@
+#include <cstddef>
+#include <vector>
+
+#include <filter/filter/filter.hpp>
+
+namespace Filter {
+IFilter::IFilter(std::size_t size) {
+  impulseResponse.reserve(size);
+}
+
+void IFilter::ConvolveIQ(const std::vector<float>& input, std::vector<float>& output) {
+  // TODO: SIMD
+
+  auto itIn = input.begin();
+  auto itOut = output.begin();
+
+  while (itIn != input.end()) {
+    for (float h : impulseResponse) {
+      // I
+      *itOut += *itIn * h;
+      itIn++;
+      itOut++;
+
+      // Q
+      *itOut += *itIn * h;
+      itIn++;
+      itOut++;
+    }
+  }
+}
+} // namespace Filter

src/filter/filter/low_pass_filter.cpp

@@ -0,0 +1,22 @@
+#include <cmath>
+#include <numbers>
+#include <vector>
+
+#include <filter/filter/low_pass_filter.hpp>
+
+namespace Filter {
+LowPassFilter::LowPassFilter(std::size_t size, std::size_t cutoffHz, std::size_t sampleRateHz, const std::vector<float>& window) : IFilter(size) {
+  for (std::size_t i = 0; i < size; i++) {
+    std::size_t shift = std::round(size / 2);
+    float sampleTime = 1.0f / sampleRateHz;
+    if (shift != i) {
+      IFilter::impulseResponse[i] = std::sin(2.0f * std::numbers::pi * sampleTime * (i - shift)) / (std::numbers::pi * sampleTime * (i - shift));
+    } else {
+      IFilter::impulseResponse[i] = 2.0f * cutoffHz;
+    }
+
+    IFilter::impulseResponse[i] *= sampleTime * window[i];
+  }
+}
+
+} // namespace Filter

src/filter/main.cpp

@@ -0,0 +1,54 @@
+#include <cstdio>
+#include <cstring>
+#include <iostream>
+#include <vector>
+
+#include <filter/filter/low_pass_filter.hpp>
+#include <filter/window/hamming_window.hpp>
+
+#include <c-flags.h>
+
+const std::size_t BUFFER_SIZE = 256;
+
+using namespace Filter;
+
+int main(int argc, char** argv) {
+  (void)argc;
+  (void)argv;
+
+  // prepare pipes for binary data
+  std::freopen(nullptr, "rb", stdin);
+  if (std::ferror(stdin)) {
+    std::cerr << "Failed to reopen stdin: " << std::strerror(errno) << std::endl;
+    return errno;
+  }
+
+  std::freopen(nullptr, "wb", stdout);
+  if (std::ferror(stdout)) {
+    std::cerr << "Failed to reopen stdout: " << std::strerror(errno) << std::endl;
+    return errno;
+  }
+
+  // create low pass filter
+  LowPassFilter lpf(BUFFER_SIZE, 1000, 10000, HammingWindow(BUFFER_SIZE).getWindow());
+
+  // read data
+  std::vector<float> inputBuffer;
+  inputBuffer.reserve(BUFFER_SIZE);
+
+  std::vector<float> outputBuffer;
+  outputBuffer.reserve(BUFFER_SIZE);
+
+  std::size_t len;
+
+  while ((len = std::fread(inputBuffer.data(), sizeof(inputBuffer[0]), BUFFER_SIZE, stdin)) > 0) {
+    if (std::ferror(stdin) && !std::feof(stdin)) {
+      std::cerr << "Failed to read from stdin: " << std::strerror(errno) << std::endl;
+      return errno;
+    }
+
+    lpf.ConvolveIQ(inputBuffer, outputBuffer);
+  }
+
+  return 0;
+}

src/filter/window/hamming_window.cpp

@@ -0,0 +1,13 @@
+#include <cmath>
+#include <numbers>
+
+#include <filter/window/hamming_window.hpp>
+
+namespace Filter {
+HammingWindow::HammingWindow(std::size_t width) {
+  window.reserve(width);
+  for (std::size_t i = 0; i < width; i++) {
+    window[i] = (25 / 46.0) - (21 / 46.0) * std::cos(2 * std::numbers::pi * i / width);
+  }
+}
+} // namespace Filter