common: Add pre and de emphasis, if the radio does not support it

Connect to the oscillator of the transmitter - good girl!
You need to enable pre emphasis in this case.

Connect to the discriminator of the receiver - good boy!
You need to enable de emphasis in this case.

src/test/Makefile.am

@@ -1,7 +1,8 @@
 AM_CPPFLAGS = -Wall -g $(all_includes)
 
 noinst_PROGRAMS = \
-	test_compander
+	test_compander \
+	test_emphasis
 
 test_compander_SOURCES = test_compander.c
 
@@ -10,3 +11,10 @@ test_compander_LDADD = \
 	$(top_builddir)/src/common/libcommon.a \
 	-lm
 
+test_emphasis_SOURCES = test_emphasis.c
+
+test_emphasis_LDADD = \
+	$(COMMON_LA) \
+	$(top_builddir)/src/common/libcommon.a \
+	-lm
+

src/test/test_emphasis.c

@@ -0,0 +1,75 @@
+#include <stdio.h>
+#include <stdint.h>
+#include <math.h>
+#include <string.h>
+#include "../common/emphasis.h"
+
+#define level2db(level)		(20 * log10(level))
+#define db2level(db)		pow(10, (double)db / 20.0)
+
+#define SAMPLERATE	48000
+#define DEVIATION	8000.0
+
+static void check_level(int16_t *samples, const char *desc)
+{
+	int i;
+	int last = 0, envelope = 0;
+	int up = 0;
+	int freq;
+
+	for (i = 0; i < SAMPLERATE; i++) {
+		if (last < samples[i]) {
+			up = 1;
+		} else if (last > samples[i]) {
+			if (up) {
+				envelope = last;
+			}
+			up = 0;
+		}
+		if ((i % (SAMPLERATE/40)) == 0) {
+			freq = 500 + 500 * (i / (SAMPLERATE / 8));
+			printf("%s: f = %d envelop = %.4f\n", desc, freq, level2db((double)envelope / DEVIATION));
+		}
+		last = samples[i];
+	}
+}
+
+static void gen_samples(int16_t *samples)
+{
+	int i;
+	double value;
+	int freq;
+
+	for (i = 0; i < SAMPLERATE; i++) {
+		freq = 500 + 500 * (i / (SAMPLERATE / 8));
+		value = sin(2.0 * M_PI * (double)freq / (double)SAMPLERATE * (double)i);
+		samples[i] = value * DEVIATION;
+	}
+}
+
+int main(void)
+{
+	emphasis_t estate;
+	int16_t samples[SAMPLERATE];
+
+	/* generate sweep 0..4khz */
+	gen_samples(samples);
+
+	init_emphasis(&estate, SAMPLERATE);
+
+//	check_level(samples, "unchanged");
+
+	pre_emphasis(&estate, samples, SAMPLERATE);
+
+	check_level(samples, "pre-emphasis");
+
+	/* generate sweep 0..4khz */
+	gen_samples(samples);
+
+	de_emphasis(&estate, samples, SAMPLERATE);
+
+	check_level(samples, "de-emphasis");
+
+	return 0;
+}
+