Add dial tone detection to disable echo suppressor

src/libecho/echo_suppress.c

@@ -60,6 +60,7 @@
 #include <osmocom/core/talloc.h>
 
 #include <stdio.h>
+#include <stdbool.h>
 #include <math.h>
 
 #include "echo_suppress.h"
@@ -70,15 +71,19 @@
 #define SUPPRESS_THRESHOLD	-31.0
 #define RELEASE_SUPPRESS	-31.0
 #define INSERTION_LOSS		6.0
+#define DT_MIN_LEVEL_DB		-20
+#define DT_MAX_OFFSET_DB	0.2
 
 #define	SUPPRESS_OPERATE	0.000
 #define	SUPPRESS_HANGOVER	0.050
 #define	BREAK_IN_OPERATE	0.030
 #define	BREAK_IN_HANGOVER	0.050
+#define	DT_INTERVAL		0.250
 
 #define	LP_FREQUENCY		3400.0
 #define	HP_FREQUENCY		500.0
 #define	ENV_FREQUENCY		500.0
+#define	DT_ENV_FREQUENCY	10.0
 
 //#define DEBUG_ES
 
@@ -181,6 +186,9 @@ echo_sup_state_t *echo_sup_create(void *ctx, int samplerate)
 	iir_highpass_init(&es->hp_receive, HP_FREQUENCY, samplerate);
 	iir_lowpass_init(&es->env_send, ENV_FREQUENCY, samplerate);
 	iir_lowpass_init(&es->env_receive, ENV_FREQUENCY, samplerate);
+	iir_lowpass_init(&es->dt_receive, DT_ENV_FREQUENCY, samplerate);
+
+	es->dt_interval = floor(DT_INTERVAL * (double)samplerate);
 
 	return es;
 }
@@ -194,7 +202,7 @@ void echo_sup_free(echo_sup_state_t *es)
 /* tx is what we sent to user interface, aka 'receive' */
 int16_t echo_sup_update(echo_sup_state_t *es, int16_t tx, int16_t rx)
 {
-	float send, receive;
+	float send, receive, dt, offset;
 
 	/* convert from integer samples to float (@ 0 dBm scale) */
 	receive = (float)tx / 23196.0; /* towards user interface */
@@ -208,11 +216,44 @@ int16_t echo_sup_update(echo_sup_state_t *es, int16_t tx, int16_t rx)
 
 	/* get absolute value (rectifying with 3 dB gain) */
 	send = fabsf(send) * 1.4142136;
-	receive = fabsf(receive) * 1.4142136;
+	receive = dt = fabsf(receive) * 1.4142136;
 
 	/* filter envelope */
 	iir_process(&es->env_send, &send, 1);
 	iir_process(&es->env_receive, &receive, 1);
+	iir_process(&es->dt_receive, &dt, 1);
+
+	/* detect dial tone and do not suppress while it is detected */
+	if (++es->dt_timer == es->dt_interval) {
+#ifdef DEBUG_ES
+		printf("dt_level=%.3f dt_offset=%.3f*\n", level2db(es->dt_level), level2db(es->dt_offset));
+#endif
+		if (level2db(es->dt_level) > DT_MIN_LEVEL_DB && level2db(es->dt_offset) < DT_MAX_OFFSET_DB) {
+			if (!es->dt_disable) {
+#ifdef DEBUG_ES
+				printf("Detected dial tone, disabling suppressor\n");
+#endif
+				es->dt_disable = true;
+			}
+		} else {
+			if (es->dt_disable) {
+#ifdef DEBUG_ES
+				printf("dial tone ceases, enabling suppressor\n");
+#endif
+				es->dt_disable = false;
+				es->state = SUP_STATE_SILENCE;
+				es->timer = 0;
+			}
+		}
+		es->dt_timer = 0;
+		es->dt_level = dt;
+		es->dt_offset = 0.0;
+	}
+	offset = fabsf(es->dt_level / dt);
+	if (offset > es->dt_offset)
+		es->dt_offset = offset;
+	if (es->dt_disable)
+		return rx;
 
 #ifdef DEBUG_ES
 	static int debug_interval = 0;

src/libecho/echo_suppress.h

@@ -23,6 +23,10 @@ typedef struct echo_sup_state {
 	iir_filter_t	lp_send, lp_receive;	/* filter to cut off high frequencies */
 	iir_filter_t	hp_send, hp_receive;	/* filter to cut off low frequencies */
 	iir_filter_t	env_send, env_receive;	/* filter to get the envelope after rectifying */
+	iir_filter_t	dt_receive;		/* filter to detect steady tone (dial tone) */
+	int		dt_timer, dt_interval;	/* timer of dial tone detection interval */
+	float		dt_level, dt_offset;	/* initial level and max offset during interval */
+	bool		dt_disable;		/* true if suppressor is disabled, due to dial tone */
 } echo_sup_state_t;
 
 echo_sup_state_t *echo_sup_create(void *ctx, int samplerate);