Emphasis: Add filter to prevent emphasis above 4000 Hz

Remove obsolete filter from fsk.c.

src/common/emphasis.c

@@ -29,6 +29,7 @@
 #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)
 {
@@ -66,8 +67,12 @@ int init_emphasis(emphasis_t *state, int samplerate, double cut_off)
 	state->d.factor = factor;
 	state->d.amp = 1.0;
 
+	/* do not de-emphasis below CUT_OFF_H */
 	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);
+
 	/* calibrate amplification to be neutral at 1000 Hz */
 	gen_sine(test_samples, sizeof(test_samples) / sizeof(test_samples[0]), samplerate, 1000.0);
 	pre_emphasis(state, test_samples, sizeof(test_samples) / sizeof(test_samples[0]));
@@ -84,6 +89,8 @@ void pre_emphasis(emphasis_t *state, sample_t *samples, int num)
 	double x, y, x_last, factor, amp;
 	int i;
 
+	iir_process(&state->p.lp, samples, num);
+
 	x_last = state->p.x_last;
 	factor = state->p.factor;
 	amp = state->p.amp;

src/common/emphasis.h

@@ -1,5 +1,6 @@
 typedef struct emphasis {
 	struct {
+		iir_filter_t lp;
 		double x_last;
 		double factor;
 		double amp;

src/common/fsk.c

@@ -115,9 +115,6 @@ int fsk_init(fsk_t *fsk, void *inst, int (*send_bit)(void *inst), void (*receive
 		fsk->cycles_per_bit65536[1] = waves * 65536.0;
 	}
 
-	/* filter prevents emphasis to overshoot on bit change */
-	iir_lowpass_init(&fsk->tx_filter, 4000.0, samplerate, 2);
-
 	return 0;
 
 error:
@@ -278,8 +275,6 @@ next_bit:
 done:
 	fsk->tx_phase65536 = phase;
 
-	iir_process(&fsk->tx_filter, sample, count);
-
 	return count;
 }
 

src/common/fsk.h

@@ -5,7 +5,6 @@ typedef struct ffsk {
 	int (*send_bit)(void *inst);
 	void (*receive_bit)(void *inst, int bit, double quality, double level);
 	fm_demod_t	demod;
-	iir_filter_t	tx_filter;
 	double		bits_per_sample;	/* fraction of a bit per sample */
 	double		*sin_tab;		/* sine table with correct peak level */
 	double		phaseshift65536[2];	/* how much the phase of fsk synbol changes per sample */