From 1e778230b935453d1183d8a457438221030dd5ee Mon Sep 17 00:00:00 2001
From: Andreas Eversberg <jolly@eversberg.eu>
Date: Fri, 20 Jan 2012 20:28:55 +0100
Subject: [PATCH] Adding handling of bad GSM audio frames

In this case the frame is dropped, but audio of the last frame is repeated
with a reduced level. The level is reduced again an again until a new
valid frame is received. This way there is no silent gap in the audio
stream.
---
 gsm.cpp    | 35 ++++++++++++++++++++++++++++-------
 gsm.h      |  1 +
 gsm_bs.cpp |  3 ++-
 gsm_ms.cpp |  3 ++-
 mncc.h     |  1 +
 5 files changed, 34 insertions(+), 9 deletions(-)

diff --git a/gsm.cpp b/gsm.cpp
index 8e5a2d6..9bfa1a5 100644
--- a/gsm.cpp
+++ b/gsm.cpp
@@ -139,6 +139,7 @@ Pgsm::Pgsm(int type, char *portname, struct port_settings *settings, struct inte
 	p_g_rtp_bridge = 0;
 	if (interface->rtp_bridge)
 		p_g_rtp_bridge = 1;
+	memset(&p_g_samples, 0, sizeof(p_g_samples));
 	SCPY(p_g_interface_name, interface->name); 
 	p_callerinfo.itype = (interface->extension)?INFO_ITYPE_ISDN_EXTENSION:INFO_ITYPE_ISDN;
 	memset(&p_g_delete, 0, sizeof(p_g_delete));
@@ -193,22 +194,42 @@ Pgsm::~Pgsm()
 void Pgsm::frame_receive(void *arg)
 {
 	struct gsm_data_frame *frame = (struct gsm_data_frame *)arg;
-	signed short samples[160];
 	unsigned char data[160];
 	int i;
 
 	if (!p_g_decoder)
 		return;
 
-	if ((frame->data[0]>>4) != 0xd)
-		PERROR("received GSM frame with wrong magig 0x%x\n", frame->data[0]>>4);
+	if (frame->msg_type != GSM_TCHF_BAD_FRAME) {
+		if ((frame->data[0]>>4) != 0xd)
+			PERROR("received GSM frame with wrong magig 0x%x\n", frame->data[0]>>4);
 	
-	/* decode */
-	gsm_audio_decode(p_g_decoder, frame->data, samples);
-	for (i = 0; i < 160; i++) {
-		data[i] = audio_s16_to_law[samples[i] & 0xffff];
+		/* decode */
+		gsm_audio_decode(p_g_decoder, frame->data, p_g_samples);
+		for (i = 0; i < 160; i++) {
+			data[i] = audio_s16_to_law[p_g_samples[i] & 0xffff];
+		}
+	} else if (p_echotest) {
+		/* beep on bad frame */
+		for (i = 0; i < 160; i++) {
+			if ((i & 3) > 2)
+				p_g_samples[i] = 15000;
+			else
+				p_g_samples[i] = -15000;
+			data[i] = audio_s16_to_law[p_g_samples[i] & 0xffff];
+		}
+	} else {
+		/* repeat on bad frame */
+		for (i = 0; i < 160; i++) {
+			p_g_samples[i] = (p_g_samples[i] * 14) >> 4;
+			data[i] = audio_s16_to_law[p_g_samples[i] & 0xffff];
+		}
 	}
 
+	/* local echo */
+	if (p_echotest)
+		bridge_rx(data, 160);
+
 	/* send to remote*/
 	bridge_tx(data, 160);
 }
diff --git a/gsm.h b/gsm.h
index 724a4c2..0ccac2b 100644
--- a/gsm.h
+++ b/gsm.h
@@ -35,6 +35,7 @@ class Pgsm : public Port
 	~Pgsm();
 
 	char p_g_interface_name[64];
+	signed short p_g_samples[160]; /* last received audi packet */
 	int p_g_tones; /* set, if tones are to be generated */
 	int p_g_earlyb; /* set, if patterns are available */
 	struct lcr_gsm *p_g_lcr_gsm; /* pointer to network/ms instance */
diff --git a/gsm_bs.cpp b/gsm_bs.cpp
index 7748f7a..d062e3d 100644
--- a/gsm_bs.cpp
+++ b/gsm_bs.cpp
@@ -396,7 +396,8 @@ int message_bsc(struct lcr_gsm *lcr_gsm, int msg_type, void *arg)
 		port = port->next;
 	}
 
-	if (msg_type == GSM_TCHF_FRAME) {
+	if (msg_type == GSM_TCHF_FRAME
+	 || msg_type == GSM_TCHF_BAD_FRAME) {
 		if (port) {
 			/* inject DTMF, if enabled */
 			if (pgsm_bs->p_g_dtmf) {
diff --git a/gsm_ms.cpp b/gsm_ms.cpp
index cfb1bf6..bb2ad67 100644
--- a/gsm_ms.cpp
+++ b/gsm_ms.cpp
@@ -345,7 +345,8 @@ int message_ms(struct lcr_gsm *gsm_ms, int msg_type, void *arg)
 		port = port->next;
 	}
 
-	if (msg_type == GSM_TCHF_FRAME) {
+	if (msg_type == GSM_TCHF_FRAME
+	 || msg_type == GSM_TCHF_BAD_FRAME) {
 		if (port)
 			pgsm_ms->frame_receive(arg);
 		return 0;
diff --git a/mncc.h b/mncc.h
index 04b3241..4028b67 100644
--- a/mncc.h
+++ b/mncc.h
@@ -58,6 +58,7 @@
 
 #define GSM_TCHF_FRAME		0x0300
 #define GSM_TCHF_FRAME_EFR	0x0301
+#define GSM_TCHF_BAD_FRAME	0x03ff
 
 #define GSM_MAX_FACILITY	128
 #define GSM_MAX_SSVERSION	128
-- 
2.13.6