Improved emphasis

src/amps/amps.c

@@ -587,7 +587,8 @@ int amps_create(int channel, enum amps_chan_type chan_type, const char *audiodev
 
 	amps->pre_emphasis = pre_emphasis;
 	amps->de_emphasis = de_emphasis;
-	rc = init_emphasis(&amps->estate, samplerate, CUT_OFF_EMPHASIS_DEFAULT);
+	/* the AMPS uses a frequency rage of 300..3000 Hz, but we still use the default low pass filter, wich is not too far above */
+	rc = init_emphasis(&amps->estate, samplerate, CUT_OFF_EMPHASIS_DEFAULT, CUT_OFF_HIGHPASS_DEFAULT, CUT_OFF_LOWPASS_DEFAULT);
 	if (rc < 0)
 		goto error;
 

src/cnetz/cnetz.c

@@ -358,7 +358,7 @@ int cnetz_create(int kanal, enum cnetz_chan_type chan_type, const char *audiodev
 
 	cnetz->pre_emphasis = pre_emphasis;
 	cnetz->de_emphasis = de_emphasis;
-	rc = init_emphasis(&cnetz->estate, samplerate, CUT_OFF_EMPHASIS_CNETZ);
+	rc = init_emphasis(&cnetz->estate, samplerate, CUT_OFF_EMPHASIS_CNETZ, CUT_OFF_HIGHPASS_DEFAULT, CUT_OFF_LOWPASS_DEFAULT);
 	if (rc < 0)
 		goto error;
 

src/libemphasis/emphasis.c

@@ -27,9 +27,6 @@
 
 #define PI		M_PI
 
-#define CUT_OFF_H	100.0	/* cut-off frequency for high-pass filter */
-#define CUT_OFF_L	4000.0	/* cut-off frequency for low-pass filter */
-
 static void gen_sine(sample_t *samples, int num, int samplerate, double freq)
 {
 	int i;
@@ -50,7 +47,13 @@ static double get_level(sample_t *samples, int num)
 	return envelope;
 }
 
-int init_emphasis(emphasis_t *state, int samplerate, double cut_off)
+/* calculate cut off from time constant in uS */
+double timeconstant2cutoff(double time_constant_us)
+{
+	return 1.0 / (2.0 * PI * time_constant_us / 1e6);
+}
+
+int init_emphasis(emphasis_t *state, int samplerate, double cut_off, double cut_off_h, double cut_off_l)
 {
 	double factor;
 	sample_t test_samples[samplerate / 10];
@@ -67,10 +70,12 @@ int init_emphasis(emphasis_t *state, int samplerate, double cut_off)
 	state->d.amp = 1.0;
 
 	/* do not de-emphasis below CUT_OFF_H */
-	iir_highpass_init(&state->d.hp, CUT_OFF_H, samplerate, 1);
+	iir_highpass_init(&state->d.hp, cut_off_h, samplerate, 1);
 
-	/* do not pre-emphasis above CUT_OFF_L */
-	iir_lowpass_init(&state->p.lp, CUT_OFF_L, samplerate, 1);
+	/* do not pre-emphasis above CUT_OFF_L
+	 * Mobile network specifications want -18 dB per octave.
+	 * With two interations we have 24 dB, - 6 dB (from emphasis). */
+	iir_lowpass_init(&state->p.lp, cut_off_l, samplerate, 2);
 
 	/* calibrate amplification to be neutral at 1000 Hz */
 	gen_sine(test_samples, sizeof(test_samples) / sizeof(test_samples[0]), samplerate, 1000.0);

src/libemphasis/emphasis.h

@@ -15,9 +15,13 @@ typedef struct emphasis {
 	} d;
 } emphasis_t;
 
+/* refers to NMT specs, cnetz uses different emphasis cutoff */
 #define CUT_OFF_EMPHASIS_DEFAULT 300.0
+#define CUT_OFF_HIGHPASS_DEFAULT 300.0
+#define CUT_OFF_LOWPASS_DEFAULT 3400.0
 
-int init_emphasis(emphasis_t *state, int samplerate, double cut_off);
+double timeconstant2cutoff(double time_constant_us);
+int init_emphasis(emphasis_t *state, int samplerate, double cut_off, double cut_off_h, double cut_off_l);
 void pre_emphasis(emphasis_t *state, sample_t *samples, int num);
 void de_emphasis(emphasis_t *state, sample_t *samples, int num);
 void dc_filter(emphasis_t *state, sample_t *samples, int num);

src/libmobile/sender.c

@@ -142,7 +142,7 @@ int sender_create(sender_t *sender, int kanal, double sendefrequenz, double empf
 		goto error;
 	}
 
-	rc = init_emphasis(&sender->estate, samplerate, CUT_OFF_EMPHASIS_DEFAULT);
+	rc = init_emphasis(&sender->estate, samplerate, CUT_OFF_EMPHASIS_DEFAULT, CUT_OFF_HIGHPASS_DEFAULT, CUT_OFF_LOWPASS_DEFAULT);
 	if (rc < 0)
 		goto error;
 

src/r2000/r2000.c

@@ -464,7 +464,8 @@ int r2000_create(int band, int channel, enum r2000_chan_type chan_type, const ch
 
 	r2000->pre_emphasis = pre_emphasis;
 	r2000->de_emphasis = de_emphasis;
-	rc = init_emphasis(&r2000->estate, samplerate, CUT_OFF_EMPHASIS_R2000);
+	/* we don't know anything about frequency response, so we use NMT defaults */
+	rc = init_emphasis(&r2000->estate, samplerate, CUT_OFF_EMPHASIS_R2000, CUT_OFF_HIGHPASS_DEFAULT, CUT_OFF_LOWPASS_DEFAULT);
 	if (rc < 0)
 		goto error;
 

src/test/test_emphasis.c

@@ -45,7 +45,7 @@ int main(void)
 	double level;
 	double i;
 
-	init_emphasis(&estate, SAMPLERATE, cut_off);
+	init_emphasis(&estate, SAMPLERATE, cut_off, CUT_OFF_HIGHPASS_DEFAULT, CUT_OFF_LOWPASS_DEFAULT);
 
 	printf("testing pre-emphasis filter with cut-off frequency %.1f\n", cut_off);