fix a bug in fm-demo math

- fix a sign error in fm demodulation math
- remove (-Werror) from build options; turn off trace debug output
- reduce sample rate back to 240k and 16khz audio

Author: thenatefisher

builder/options.mk

@@ -9,7 +9,7 @@ CFLAGS  += $(ARCH)
 
 # compile using c11, all warnings, output listing
 C_APP_FLAGS += -std=c11
-C_APP_FLAGS += -Werror -Wall -Wextra
+C_APP_FLAGS += -Wall -Wextra
 C_APP_FLAGS += -Wa,-ashld=$(LST_DIR)/$(@F:.o=.lst)
 
 # ST HAL settings

source/application/audio_player.c

@@ -7,7 +7,7 @@ int8_t audio_init(void) {
     self.volume = 70;
     self.is_playing = 0;
 
-    if (BSP_AUDIO_OUT_Init(OUTPUT_DEVICE_BOTH, self.volume, I2S_AUDIOFREQ_32K) == AUDIO_OK) {
+    if (BSP_AUDIO_OUT_Init(OUTPUT_DEVICE_BOTH, self.volume, I2S_AUDIOFREQ_16K) == AUDIO_OK) {
 
         BSP_AUDIO_OUT_SetAudioFrameSlot(CODEC_AUDIOFRAME_SLOT_02);
 

source/application/fm_radio.c

@@ -77,8 +77,7 @@ void fmradio_process() {
         s2[1] = (int16_t)raw_buf_complete[sample_index + 3] - 127; // Q
 
         // find the phase shift in these two samples (this is the essence of the FM demod)
-        int16_t pcm = fmradio_polar_disc(s2[0], s2[1],
-                                      s1[0], s1[1]);
+        int16_t pcm = fmradio_polar_disc(s2[0], s2[1], s1[0], s1[1]);
 
         pcm = pcm * 8.0f / PI; // scale from radians
 

source/application/fm_radio_math_utils.c

@@ -32,11 +32,13 @@ int32_t fmradio_atan2(int32_t y, int32_t x) {
 // fm demodulation math taken from librtlsdr
 int32_t fmradio_polar_disc(int32_t ar, int32_t aj, int32_t br, int32_t bj) {
 
-    int32_t cr, cj;
-
-    // complex multiply
-    cr = ar * br - aj * bj;
-    cj = aj * br + ar * bj;
+    // complex multiply first sample with
+    // second sample flipped over the real axis
+    // (ar+aj) * (br-bj)
+    int32_t cr = ar * br - aj * -bj;
+    int32_t cj = aj * br + ar * -bj;
 
+    // find angle of resultant
     return fmradio_atan2(cj, cr);
+    
 }

source/application/main.c

@@ -147,7 +147,14 @@ static void SystemClock_Config(void) {
     }
 }
 
-void BSP_AUDIO_OUT_ClockConfig(SAI_HandleTypeDef* hsai, uint32_t AudioFreq, void* Params) {
+void BSP_AUDIO_OUT_ClockConfig(SAI_HandleTypeDef* hsai,
+                            uint32_t AudioFreq,
+                            void* Params)
+{
+
+    UNUSED(Params);
+    UNUSED(hsai);
+
     RCC_PeriphCLKInitTypeDef RCC_ExCLKInitStruct;
 
     HAL_RCCEx_GetPeriphCLKConfig(&RCC_ExCLKInitStruct);

source/application/main.h

@@ -10,13 +10,13 @@
 #include "fm_radio.h"
 #include "audio_player.h"
 
-#define USB_PIPE_NUMBER (0x81)
+#define USB_PIPE_NUMBER 0x81
 #define KILOBYTES 1024
 #define RAW_BUFFER_BYTES (25*KILOBYTES)
 #define SIZEOF_DEMOD_BUF_EL 2
 #define DEMOD_BUFF_BYTES (RAW_BUFFER_BYTES/SIZEOF_DEMOD_BUF_EL)
-#define DOWNSAMPLE 30
-#define RTL_SAMPLERATE 960000
+#define DOWNSAMPLE 15
+#define RTL_SAMPLERATE 240000
 
 volatile uint8_t raw_bufA[RAW_BUFFER_BYTES];
 volatile uint8_t raw_bufB[RAW_BUFFER_BYTES];

source/application/trace.h

@@ -3,7 +3,7 @@
 
 #include "stm32f7xx_hal.h"
 
-#if 1
+#if 0
 #define DEBUG_PRINT(...) printf(__VA_ARGS__)
 #else
 #define DEBUG_PRINT(...)