fm demod

.gitignore

@@ -1 +1,2 @@
 */build/
+*/.cache/

fm/.clang-format

@@ -0,0 +1,50 @@
+---
+BasedOnStyle: Microsoft
+AlignAfterOpenBracket: Align
+AlignConsecutiveMacros: 'true'
+AlignConsecutiveAssignments: 'false'
+AlignEscapedNewlines: Left
+AlignOperands: 'true'
+AlignTrailingComments: 'true'
+AllowAllArgumentsOnNextLine: 'true'
+AllowShortBlocksOnASingleLine: 'false'
+AllowShortCaseLabelsOnASingleLine: 'true'
+AllowShortIfStatementsOnASingleLine: Never
+BinPackArguments: 'false'
+BinPackParameters: 'false'
+IncludeBlocks: Regroup
+BraceWrapping:
+  AfterClass:      true
+  AfterControlStatement: true
+  AfterEnum:       true # <-- You need this
+  AfterFunction:   true
+  AfterNamespace:  true
+  AfterObjCDeclaration: true
+  AfterStruct:     true
+  AfterUnion:      true
+  AfterExternBlock: false # <-- And this
+  BeforeCatch:     true
+  BeforeElse:      true
+  IndentBraces:    false
+  SplitEmptyFunction: false
+  SplitEmptyRecord: false
+  SplitEmptyNamespace: false
+BreakBeforeBraces: Custom # <-- And this
+BreakBeforeBinaryOperators: All
+ColumnLimit: 0
+IndentCaseLabels: 'false'
+Language: Cpp
+MaxEmptyLinesToKeep: '1'
+PointerAlignment: Left
+ReflowComments: 'true'
+SortIncludes: 'true'
+SortUsingDeclarations: 'true'
+SpaceAfterLogicalNot: 'false'
+SpaceBeforeAssignmentOperators: 'true'
+SpaceBeforeCpp11BracedList: 'true'
+SpaceBeforeInheritanceColon: 'true'
+TabWidth: '4'
+UseTab: Never
+AccessModifierOffset: '-4'
+
+...

fm/CMakeLists.txt

@@ -0,0 +1,21 @@
+cmake_minimum_required(VERSION 3.10)
+project(fm_demodulate)
+
+file(GLOB SRC_FILES src/*.cpp)
+
+add_executable(fm_receiver ${SRC_FILES})
+
+target_include_directories(fm_receiver PRIVATE src)
+
+find_package(ALSA REQUIRED)
+target_link_libraries(fm_receiver PRIVATE ALSA::ALSA)
+find_library(RTLSDR_LIB rtlsdr)
+find_path(RTLSDR_INCLUDE rtl-sdr.h)
+
+if(NOT RTLSDR_LIB OR NOT RTLSDR_INCLUDE)
+    message(FATAL_ERROR "librtlsdr not found")
+endif()
+
+target_include_directories(fm_receiver PRIVATE ${RTLSDR_INCLUDE})
+target_link_libraries(fm_receiver PRIVATE ${RTLSDR_LIB})
+

fm/src/main.cpp

@@ -0,0 +1,109 @@
+#include <algorithm>
+#include <alsa/asoundlib.h>
+#include <cmath>
+#include <complex>
+#include <cstring>
+#include <iostream>
+#include <rtl-sdr.h>
+#include <signal.h>
+#include <vector>
+
+#define DR_WAV_IMPLEMENTATION
+#include "dr_wav.h"
+
+#define DEFAULT_SAMPLE_RATE  2400000 // 2.4 MS/s
+#define AUDIO_SAMPLE_RATE    48000
+#define RTL_FM_DEMOD_BUF_LEN 16384 // 16kB
+
+#include <stdio.h>
+#include <stdlib.h>
+
+volatile bool stop_flag = 0;
+
+void handle_sigint(int sig)
+{
+    (void)sig;
+    stop_flag = 1;
+}
+
+int main()
+{
+    signal(SIGINT, handle_sigint);
+
+    rtlsdr_dev_t* dev = NULL;
+    rtlsdr_open(&dev, 0);
+    rtlsdr_set_center_freq(dev, 96700000);
+    rtlsdr_set_sample_rate(dev, DEFAULT_SAMPLE_RATE);
+    rtlsdr_set_tuner_gain_mode(dev, 1);
+    rtlsdr_set_tuner_gain(dev, 400); // 40dB
+    rtlsdr_reset_buffer(dev);
+
+    snd_pcm_t* pcm_handle;
+    snd_pcm_open(&pcm_handle, "default", SND_PCM_STREAM_PLAYBACK, 0);
+    snd_pcm_set_params(pcm_handle, SND_PCM_FORMAT_S16_LE, SND_PCM_ACCESS_RW_INTERLEAVED, 1, AUDIO_SAMPLE_RATE, 1, 500000);
+
+    drwav wav;
+    drwav_data_format format = {
+        .container = drwav_container_riff,
+        .format = DR_WAVE_FORMAT_PCM,
+        .channels = 1,
+        .sampleRate = 48000,
+        .bitsPerSample = 16};
+
+    // Open file for writing
+    drwav_init_file_write(&wav, "output.wav", &format, NULL);
+
+    uint8_t rtl_buf[RTL_FM_DEMOD_BUF_LEN];
+    float iq_buf[RTL_FM_DEMOD_BUF_LEN / 2];
+    float demod_buf[RTL_FM_DEMOD_BUF_LEN / 2];
+    int16_t audio_buf[8192];
+
+    while (!stop_flag)
+    {
+        int n_read;
+        rtlsdr_read_sync(dev, rtl_buf, RTL_FM_DEMOD_BUF_LEN, &n_read);
+        int n_samples = n_read / 2;
+
+        // i/q around 0
+        for (int i = 0; i < n_samples; i++)
+        {
+            int8_t i_sample = rtl_buf[2 * i] - 127;
+            int8_t q_sample = rtl_buf[2 * i + 1] - 127;
+            iq_buf[i] = atan2(q_sample, i_sample);
+        }
+
+        // differential phase
+        static float prev_phase = 0;
+        for (int i = 0; i < n_samples; i++)
+        {
+            float dphi = iq_buf[i] - prev_phase;
+            // keep dphi within -pi and pi (as phase is circular)
+            if (dphi > M_PI)
+                dphi -= 2 * M_PI;
+            if (dphi < -M_PI)
+                dphi += 2 * M_PI;
+            prev_phase = iq_buf[i];
+
+            // low pass
+            static float y = 0;
+            float x = dphi;
+            y = y + (x - y) * 0.294f; // alpha 50us 48kHz
+            iq_buf[i] = y;
+        }
+
+        // then decimate
+        int out_index = 0;
+        for (int i = 0; i < n_samples; i += 50)
+        {
+            if (out_index < 8192)
+            {
+                audio_buf[out_index++] = (int16_t)(iq_buf[i] * 32767.0f);
+            }
+        }
+
+        snd_pcm_writei(pcm_handle, audio_buf, out_index);
+        drwav_write_pcm_frames(&wav, out_index, audio_buf);
+    }
+    drwav_uninit(&wav);
+    rtlsdr_close(dev);
+}