Preemphasis Implementation

README.md

@@ -51,7 +51,7 @@ To test stereophonic audio, you can try the file `stereo_44100.wav` provided.
 The more general syntax for running Pi-FM-RDS is as follows:
 
 ```
-pi_fm_rds [-freq freq] [-audio file] [-ppm ppm_error] [-pi pi_code] [-ps ps_text] [-rt rt_text]
+pi_fm_rds [-freq freq] [-audio file] [-ppm ppm_error] [-pi pi_code] [-ps ps_text] [-rt rt_text] [-cutoff cutoff_freq] [-preemph preemphasis_mode]
 ```
 
 All arguments are optional:
@@ -63,6 +63,8 @@ All arguments are optional:
 * `-rt` specifies the radiotext (RT) to be transmitted. Limit: 64 characters. Example: `-rt 'Hello, world!'`.
 * `-ctl` specifies a named pipe (FIFO) to use as a control channel to change PS and RT at run-time (see below).
 * `-ppm` specifies your Raspberry Pi's oscillator error in parts per million (ppm), see below.
+* `-cutoff` specifies the cutoff frequency (in Hz, 'compliant' for 15,000Hz or 'quality' for 22,050Hz) used by Pi-FM-RDS' internal lowpass filter. Values greater than 15000 are not compliant. Use carefully.
+* `-preemph` specifies which preemph should be used, since it differs from location. For Europe choose 'eu', for the US choose 'us'.
 
 By default the PS changes back and forth between `Pi-FmRds` and a sequence number, starting at `00000000`. The PS changes around one time per second.
 

src/fm_mpx.c

@@ -66,6 +66,10 @@ float fir_buffer_stereo[FIR_SIZE] = {0};
 int fir_index = 0;
 int channels;
 
+float *last_buffer_val;
+float preemphasis_prewarp;
+float preemphasis_coefficient;
+
 SNDFILE *inf;
 
 
@@ -80,7 +84,7 @@ float *alloc_empty_buffer(size_t length) {
 }
 
 
-int fm_mpx_open(char *filename, size_t len) {
+int fm_mpx_open(char *filename, size_t len, float cutoff_freq, float preemphasis_corner_freq) {
     length = len;
 
     if(filename != NULL) {
@@ -114,13 +118,19 @@ int fm_mpx_open(char *filename, size_t len) {
             printf("%d channels, generating stereo multiplex.\n", channels);
         } else {
             printf("1 channel, monophonic operation.\n");
-        }
-
+        } 
+
+        // Create the preemphasis
+        last_buffer_val = (float*) malloc(sizeof(float)*channels);
+        for(int i=0;i<channels;i++) last_buffer_val[i] = 0;
+
+        preemphasis_prewarp = tan(PI*preemphasis_corner_freq/in_samplerate);
+        preemphasis_coefficient = (1.0 + (1.0 - preemphasis_prewarp)/(1.0 + preemphasis_prewarp))/2.0;
+        printf("Created preemphasis with cutoff at %.1f Hz\n", preemphasis_corner_freq);
+
 
         // Create the low-pass FIR filter
-        float cutoff_freq = 15000 * .8;
-        if(in_samplerate/2 < cutoff_freq) cutoff_freq = in_samplerate/2 * .8;
-
+        if(in_samplerate/2 < cutoff_freq) cutoff_freq = in_samplerate/2;
 
 
         low_pass_fir[FIR_HALF_SIZE-1] = 2 * cutoff_freq / 228000 /2;
@@ -181,6 +191,18 @@ int fm_mpx_get_samples(float *mpx_buffer) {
                             return -1;
                         }
                     } else {
+                        //apply preemphasis
+                        int k;
+                        int l;
+                        float tmp;
+                        for(k=0;k<audio_len;k+=channels) {
+                            for(l=0;l<channels;l++) {
+                                tmp = audio_buffer[k+l];
+                                audio_buffer[k+l] = audio_buffer[k+l] - preemphasis_coefficient*last_buffer_val[l];
+                                last_buffer_val[l] = tmp;
+                            }
+                        }
+
                         break;
                     }
                 }

src/fm_mpx.h

@@ -21,6 +21,6 @@
     along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-extern int fm_mpx_open(char *filename, size_t len);
+extern int fm_mpx_open(char *filename, size_t len, float cutoff_freq, float preemphasis_corner_freq);
 extern int fm_mpx_get_samples(float *mpx_buffer);
 extern int fm_mpx_close();

src/pi_fm_rds.c

@@ -191,7 +191,6 @@
 // (broadcast radio) and about 3.5 for NBFM (walkie-talkie style radio)
 #define DEVIATION        25.0
 
-
 typedef struct {
     uint32_t info, src, dst, length,
          stride, next, pad[2];
@@ -311,7 +310,7 @@ map_peripheral(uint32_t base, uint32_t len)
 #define DATA_SIZE 5000
 
 
-int tx(uint32_t carrier_freq, char *audio_file, uint16_t pi, char *ps, char *rt, float ppm, char *control_pipe) {
+int tx(uint32_t carrier_freq, char *audio_file, uint16_t pi, char *ps, char *rt, float ppm, char *control_pipe, float cutoff, float preemphasis_cutoff) {
     // Catch all signals possible - it is vital we kill the DMA engine
     // on process exit!
     for (int i = 0; i < 64; i++) {
@@ -446,7 +445,7 @@ int tx(uint32_t carrier_freq, char *audio_file, uint16_t pi, char *ps, char *rt,
     int data_index = 0;
 
     // Initialize the baseband generator
-    if(fm_mpx_open(audio_file, DATA_SIZE) < 0) return 1;
+    if(fm_mpx_open(audio_file, DATA_SIZE, cutoff, preemphasis_cutoff) < 0) return 1;
 
     // Initialize the RDS modulator
     char myps[9] = {0};
@@ -537,6 +536,11 @@ int tx(uint32_t carrier_freq, char *audio_file, uint16_t pi, char *ps, char *rt,
     return 0;
 }
 
+#define PREEMPHASIS_EU 3185
+#define PREEMPHASIS_US 2120
+
+#define CUTOFF_COMPLIANT 15000
+#define CUTOFF_QUALITY 22050
 
 int main(int argc, char **argv) {
     char *audio_file = NULL;
@@ -546,6 +550,8 @@ int main(int argc, char **argv) {
     char *rt = "PiFmRds: live FM-RDS transmission from the RaspberryPi";
     uint16_t pi = 0x1234;
     float ppm = 0;
+	float cutoff = CUTOFF_COMPLIANT;
+	float preemphasis_cutoff = PREEMPHASIS_US;
 
 
     // Parse command-line arguments
@@ -578,14 +584,34 @@ int main(int argc, char **argv) {
         } else if(strcmp("-ctl", arg)==0 && param != NULL) {
             i++;
             control_pipe = param;
+		} else if(strcmp("-preemph", arg)==0 && param != NULL) {
+			i++;
+			if(strcmp("eu", param)==0) {
+				preemphasis_cutoff = PREEMPHASIS_EU;
+			} else if(strcmp("us", param)==0) {
+				preemphasis_cutoff = PREEMPHASIS_US;
+			}
+			else {
+				preemphasis_cutoff = atof(param);
+			}
+		} else if(strcmp("-cutoff", arg)==0 && param != NULL) {
+			i++;
+			if(strcmp("compliant", param)==0) {
+				cutoff = CUTOFF_COMPLIANT;
+			} else if(strcmp("quality", param)==0) {
+				cutoff = CUTOFF_QUALITY;
+			}
+			else {
+				cutoff = atof(param);
+			}
         } else {
             fatal("Unrecognised argument: %s.\n"
             "Syntax: pi_fm_rds [-freq freq] [-audio file] [-ppm ppm_error] [-pi pi_code]\n"
             "                  [-ps ps_text] [-rt rt_text] [-ctl control_pipe]\n", arg);
         }
     }
 
-    int errcode = tx(carrier_freq, audio_file, pi, ps, rt, ppm, control_pipe);
+    int errcode = tx(carrier_freq, audio_file, pi, ps, rt, ppm, control_pipe, cutoff, preemphasis_cutoff);
 
     terminate(errcode);
 }