From 1ea95de1204d70bb98f117bd5c71bff7b8376c72 Mon Sep 17 00:00:00 2001
From: Andreas Eversberg <jolly@eversberg.eu>
Date: Sun, 23 Oct 2016 08:46:05 +0200
Subject: [PATCH] C-Netz: Use emphasis with 200 uS time constant

---
 src/amps/amps.c          |  2 +-
 src/cnetz/cnetz.c        |  4 +++-
 src/common/emphasis.c    | 13 +++++++------
 src/common/emphasis.h    |  4 +++-
 src/common/sender.c      |  2 +-
 src/test/test_emphasis.c |  4 ++--
 6 files changed, 17 insertions(+), 12 deletions(-)

diff --git a/src/amps/amps.c b/src/amps/amps.c
index b526237..88cb95d 100644
--- a/src/amps/amps.c
+++ b/src/amps/amps.c
@@ -422,7 +422,7 @@ int amps_create(int channel, enum amps_chan_type chan_type, const char *sounddev
 
 	amps->pre_emphasis = pre_emphasis;
 	amps->de_emphasis = de_emphasis;
-	rc = init_emphasis(&amps->estate, samplerate);
+	rc = init_emphasis(&amps->estate, samplerate, CUT_OFF_EMPHASIS_DEFAULT);
 	if (rc < 0)
 		goto error;
 
diff --git a/src/cnetz/cnetz.c b/src/cnetz/cnetz.c
index 339362b..49e9430 100644
--- a/src/cnetz/cnetz.c
+++ b/src/cnetz/cnetz.c
@@ -105,6 +105,8 @@
 /* uncomment this to do echo debugging (-L) on Speech Channel */
 //#define DEBUG_SPK
 
+#define CUT_OFF_EMPHASIS_CNETZ	796 /* 200 uS time constant */
+
 /* Call reference for calls from mobile station to network
    This offset of 0x400000000 is required for MNCC interface. */
 static int new_callref = 0x40000000;
@@ -286,7 +288,7 @@ int cnetz_create(int kanal, enum cnetz_chan_type chan_type, const char *sounddev
 
 	cnetz->pre_emphasis = pre_emphasis;
 	cnetz->de_emphasis = de_emphasis;
-	rc = init_emphasis(&cnetz->estate, samplerate);
+	rc = init_emphasis(&cnetz->estate, samplerate, CUT_OFF_EMPHASIS_CNETZ);
 	if (rc < 0)
 		goto error;
 
diff --git a/src/common/emphasis.c b/src/common/emphasis.c
index 3e753e7..c4a8242 100644
--- a/src/common/emphasis.c
+++ b/src/common/emphasis.c
@@ -26,10 +26,9 @@
 
 #define PI		M_PI
 
-#define CUT_OFF_E	300.0	/* cut-off frequency for emphasis filters */
-#define CUT_OFF_H	300.0	/* cut-off frequency for high-pass filters */
+#define CUT_OFF_H	200.0	/* cut-off frequency for high-pass filters */
 
-int init_emphasis(emphasis_t *state, int samplerate)
+int init_emphasis(emphasis_t *state, int samplerate, double cut_off)
 {
 	double factor, rc, dt;
 
@@ -40,13 +39,15 @@ int init_emphasis(emphasis_t *state, int samplerate)
 	}
 
 	/* exp (-2 * PI * CUT_OFF * delta_t) */
-	factor = exp(-2.0 * PI * CUT_OFF_E / samplerate); /* 1/samplerate == delta_t */
+	factor = exp(-2.0 * PI * cut_off / samplerate); /* 1/samplerate == delta_t */
 	PDEBUG(DDSP, DEBUG_DEBUG, "Emphasis factor = %.3f\n", factor);
 	state->p.factor = factor;
-	state->p.amp = samplerate / 6350.0;
+	state->p.amp = samplerate / 6400.0;
 	state->d.d_factor = factor;
-	state->d.amp = 1.0 / (samplerate / 5550.0);
+	state->d.amp = 1.0 / (samplerate / 5750.0);
 
+	/* high-pass filter prevents low frequency noise and dc level
+	 * from being amplified by de-emphasis */
 	rc = 1.0 / (CUT_OFF_H * 2.0 *3.14);
 	dt = 1.0 / samplerate;
 	state->d.h_factor = rc / (rc + dt);
diff --git a/src/common/emphasis.h b/src/common/emphasis.h
index 119693c..8a631e4 100644
--- a/src/common/emphasis.h
+++ b/src/common/emphasis.h
@@ -13,7 +13,9 @@ typedef struct emphasis {
 	} d;
 } emphasis_t;
 
-int init_emphasis(emphasis_t *state, int samplerate);
+#define CUT_OFF_EMPHASIS_DEFAULT	300.0
+
+int init_emphasis(emphasis_t *state, int samplerate, double cut_off);
 void pre_emphasis(emphasis_t *state, int16_t *samples, int num);
 void de_emphasis(emphasis_t *state, int16_t *samples, int num);
 
diff --git a/src/common/sender.c b/src/common/sender.c
index caa2689..054e787 100644
--- a/src/common/sender.c
+++ b/src/common/sender.c
@@ -130,7 +130,7 @@ int sender_create(sender_t *sender, int kanal, const char *sounddev, int sampler
 		}
 	}
 
-	rc = init_emphasis(&sender->estate, samplerate);
+	rc = init_emphasis(&sender->estate, samplerate, CUT_OFF_EMPHASIS_DEFAULT);
 	if (rc < 0)
 		goto error;
 
diff --git a/src/test/test_emphasis.c b/src/test/test_emphasis.c
index 5e4720e..036a45e 100644
--- a/src/test/test_emphasis.c
+++ b/src/test/test_emphasis.c
@@ -11,7 +11,7 @@
 #define SAMPLERATE	48000
 #define DEVIATION	8000.0
 
-static double test_freq[] = { 25, 50, 100, 150, 200, 250, 300, 350, 400, 500, 1000, 2000, 4000, 0 };
+static double test_freq[] = { 25, 50, 100, 200, 250, 300, 400, 500, 1000, 2000, 4000, 0 };
 
 static void check_level(int16_t *samples, double freq, const char *desc)
 {
@@ -55,7 +55,7 @@ int main(void)
 
 	printf("1000 Hz shall be close to 0 dB, that is no significant change in volume.\n\n");
 
-	init_emphasis(&estate, SAMPLERATE);
+	init_emphasis(&estate, SAMPLERATE, CUT_OFF_EMPHASIS_DEFAULT);
 
 	for (i = 0; test_freq[i]; i++) {
 		gen_samples(samples, test_freq[i]);