# HG changeset patch
# User Mychaela Falconia <falcon@freecalypso.org>
# Date 1683609391 0
# Node ID f081a6850fb54a4ec3f8babb59f8586579d5380b
# Parent  a48ddaa0f9a749a6f7325fe40dfafdb65c775e13
libgsmfrp: new refined implementation

The previous implementation exhibited the following defects,
which are now fixed:

1) The last received valid SID was cached forever for the purpose of
   handling future invalid SIDs - we could have received some valid
   SID ages ago, then lots of speech or NO_DATA, and if we then get
   an invalid SID, we would resurrect the last valid SID from ancient
   history - a bad design.  In our new design, we handle invalid SID
   based on the current state, much like BFI.

2) GSM 06.11 spec says clearly that after the second lost SID
   (received BFI=1 && TAF=1 in CN state) we need to gradually decrease
   the output level, rather than jump directly to emitting silence
   frames - we previously failed to implement such logic.

3) Per GSM 06.12 section 5.2, Xmaxc should be the same in all 4 subframes
   in a SID frame.  What should we do if we receive an otherwise valid
   SID frame with different Xmaxc?  Our previous approach would
   replicate this Xmaxc oddity in every subsequent generated CN frame,
   which is rather bad.  In our new design, the very first CN frame
   (which can be seen as a transformation of the SID frame itself)
   retains the original 4 distinct Xmaxc, but all subsequent CN frames
   are based on the Xmaxc from the last subframe of the most recent SID.

diff -r a48ddaa0f9a7 -r f081a6850fb5 libgsmfrp/bad_frame.c
--- a/libgsmfrp/bad_frame.c	Tue May 09 03:13:22 2023 +0000
+++ b/libgsmfrp/bad_frame.c	Tue May 09 05:16:31 2023 +0000
@@ -1,5 +1,6 @@
 /*
- * In this module we implement our handling of BFI frame gaps.
+ * In this module we implement our handling of BFI frame gaps
+ * and invalid SID frames.
  */
 
 #include <stdint.h>
@@ -38,6 +39,17 @@
 	}
 }
 
+static int reduce_xmaxc_sid(struct gsmfr_preproc_state *st)
+{
+	if (st->sid_xmaxc > 4) {
+		st->sid_xmaxc -= 4;
+		return 0;
+	} else {
+		st->sid_xmaxc = 0;
+		return 1;
+	}
+}
+
 void gsmfr_preproc_bfi(struct gsmfr_preproc_state *st, int taf, gsm_byte *frame)
 {
 	int mute;
@@ -58,14 +70,65 @@
 			st->rx_state = NO_DATA;
 		return;
 	case COMFORT_NOISE:
-		gsmfr_preproc_gen_cn(st, frame);
 		if (taf)
 			st->rx_state = LOST_SID;
+		gsmfr_preproc_gen_cn(st, frame);
 		return;
 	case LOST_SID:
+		if (taf) {
+			st->rx_state = CN_MUTING;
+			reduce_xmaxc_sid(st);
+		}
 		gsmfr_preproc_gen_cn(st, frame);
-		if (taf)
+		return;
+	case CN_MUTING:
+		if (reduce_xmaxc_sid(st)) {
 			st->rx_state = NO_DATA;
+			memcpy(frame, &gsmfr_preproc_silence_frame,
+				sizeof(gsm_frame));
+		} else
+			gsmfr_preproc_gen_cn(st, frame);
 		return;
 	}
 }
+
+void gsmfr_preproc_invalid_sid(struct gsmfr_preproc_state *st, gsm_byte *frame)
+{
+	switch (st->rx_state) {
+	case NO_DATA:
+		memcpy(frame, &gsmfr_preproc_silence_frame, sizeof(gsm_frame));
+		return;
+	case SPEECH:
+		/*
+		 * Make CN out of the last good speech frame, following the
+		 * "NOTE" at the end of section 6.1.2 in TS 46.031.
+		 */
+		st->rx_state = COMFORT_NOISE;
+		memcpy(st->sid_prefix, &st->speech_frame, 5);
+		st->sid_xmaxc = ((st->speech_frame[27] & 0x1F) << 1) |
+				(st->speech_frame[28] >> 7);
+		gsmfr_preproc_gen_cn(st, frame);
+		return;
+	case SPEECH_MUTING:
+		/* same as above, but set the state to CN_MUTING */
+		st->rx_state = CN_MUTING;
+		memcpy(st->sid_prefix, &st->speech_frame, 5);
+		st->sid_xmaxc = ((st->speech_frame[27] & 0x1F) << 1) |
+				(st->speech_frame[28] >> 7);
+		gsmfr_preproc_gen_cn(st, frame);
+		return;
+	case COMFORT_NOISE:
+	case LOST_SID:
+		st->rx_state = COMFORT_NOISE;
+		gsmfr_preproc_gen_cn(st, frame);
+		return;
+	case CN_MUTING:
+		if (reduce_xmaxc_sid(st)) {
+			st->rx_state = NO_DATA;
+			memcpy(frame, &gsmfr_preproc_silence_frame,
+				sizeof(gsm_frame));
+		} else
+			gsmfr_preproc_gen_cn(st, frame);
+		return;
+	}
+}
diff -r a48ddaa0f9a7 -r f081a6850fb5 libgsmfrp/comfort_noise.c
--- a/libgsmfrp/comfort_noise.c	Tue May 09 03:13:22 2023 +0000
+++ b/libgsmfrp/comfort_noise.c	Tue May 09 05:16:31 2023 +0000
@@ -48,8 +48,8 @@
 		       | ((bc[sub] >> 1) & 0x1);
 		*c++ =   ((bc[sub] & 0x1) << 7)
 		       | ((Mc & 0x3) << 5)
-		       | ((st->sid_xmaxc[sub] >> 1) & 0x1F);
-		*c++ =   ((st->sid_xmaxc[sub] & 0x1) << 7)
+		       | ((st->sid_xmaxc >> 1) & 0x1F);
+		*c++ =   ((st->sid_xmaxc & 0x1) << 7)
 		       | ((xmc[0] & 0x7) << 4)
 		       | ((xmc[1] & 0x7) << 1)
 		       | ((xmc[2] >> 2) & 0x1);
@@ -68,3 +68,44 @@
 		       | (xmc[12] & 0x7);
 	}
 }
+
+void gsmfr_preproc_sid2cn(struct gsmfr_preproc_state *st, gsm_byte *frame)
+{
+	unsigned sub, pulse;
+	uint8_t Mc, xmc[13];
+	gsm_byte *c;
+
+	/* save LARc and Xmaxc from the last subframe for subsequent CN gen */
+	memcpy(st->sid_prefix, frame, 5);
+	st->sid_xmaxc = ((frame[27] & 0x1F) << 1) | (frame[28] >> 7);
+	/* ... and turn *this* frame into very first CN output */
+	c = frame + 5;
+	for (sub = 0; sub < 4; sub++) {
+		Mc = gsmfr_preproc_prng(st, 2);
+		for (pulse = 0; pulse < 13; pulse++)
+			xmc[pulse] = random_1to6(st);
+		/* keep each of Xmaxc and replace the rest with CN */
+		*c++ =   ((Nc[sub] & 0x7F) << 1)
+		       | ((bc[sub] >> 1) & 0x1);
+		*c &= 0x1F;
+		*c++ |=  ((bc[sub] & 0x1) << 7)
+		       | ((Mc & 0x3) << 5);
+		*c &= 0x80;
+		*c++ |=  ((xmc[0] & 0x7) << 4)
+		       | ((xmc[1] & 0x7) << 1)
+		       | ((xmc[2] >> 2) & 0x1);
+		*c++ =   ((xmc[2] & 0x3) << 6)
+		       | ((xmc[3] & 0x7) << 3)
+		       | (xmc[4] & 0x7);
+		*c++ =   ((xmc[5] & 0x7) << 5)
+		       | ((xmc[6] & 0x7) << 2)
+		       | ((xmc[7] >> 1) & 0x3);
+		*c++ =   ((xmc[7] & 0x1) << 7)
+		       | ((xmc[8] & 0x7) << 4)
+		       | ((xmc[9] & 0x7) << 1)
+		       | ((xmc[10] >> 2) & 0x1);
+		*c++ =   ((xmc[10] & 0x3) << 6)
+		       | ((xmc[11] & 0x7) << 3)
+		       | (xmc[12] & 0x7);
+	}
+}
diff -r a48ddaa0f9a7 -r f081a6850fb5 libgsmfrp/good_frame.c
--- a/libgsmfrp/good_frame.c	Tue May 09 03:13:22 2023 +0000
+++ b/libgsmfrp/good_frame.c	Tue May 09 05:16:31 2023 +0000
@@ -17,28 +17,16 @@
 	sid = gsmfr_preproc_sid_classify(frame);
 	switch (sid) {
 	case 0:		/* good speech frame */
+		st->rx_state = SPEECH;
 		memcpy(&st->speech_frame, frame, sizeof(gsm_frame));
-		st->rx_state = SPEECH;
 		return;
 	case 1:		/* invalid SID frame */
-		if (st->got_valid_sid) {
-			st->rx_state = COMFORT_NOISE;
-			gsmfr_preproc_gen_cn(st, frame);
-		} else {
-			st->rx_state = NO_DATA;
-			memcpy(frame, &gsmfr_preproc_silence_frame,
-				sizeof(gsm_frame));
-		}
+		/* state-dependent handling, similar to BFI */
+		gsmfr_preproc_invalid_sid(st, frame);
 		return;
 	case 2:		/* valid SID frame */
-		st->got_valid_sid = 1;
-		memcpy(st->sid_prefix, frame, 5);
-		st->sid_xmaxc[0] = ((frame[6] & 0x1F) << 1) | (frame[7] >> 7);
-		st->sid_xmaxc[1] = ((frame[13] & 0x1F) << 1) | (frame[14] >> 7);
-		st->sid_xmaxc[2] = ((frame[20] & 0x1F) << 1) | (frame[21] >> 7);
-		st->sid_xmaxc[3] = ((frame[27] & 0x1F) << 1) | (frame[28] >> 7);
 		st->rx_state = COMFORT_NOISE;
-		gsmfr_preproc_gen_cn(st, frame);
+		gsmfr_preproc_sid2cn(st, frame);
 		return;
 	}
 }
diff -r a48ddaa0f9a7 -r f081a6850fb5 libgsmfrp/internal.h
--- a/libgsmfrp/internal.h	Tue May 09 03:13:22 2023 +0000
+++ b/libgsmfrp/internal.h	Tue May 09 05:16:31 2023 +0000
@@ -9,14 +9,14 @@
 	SPEECH_MUTING,
 	COMFORT_NOISE,
 	LOST_SID,
+	CN_MUTING,
 };
 
 struct gsmfr_preproc_state {
 	enum rx_state	rx_state;
-	int		got_valid_sid;
 	gsm_frame	speech_frame;
 	gsm_byte	sid_prefix[5];
-	uint8_t		sid_xmaxc[4];
+	uint8_t		sid_xmaxc;
 	uint32_t	cn_random_lfsr;
 	unsigned	cn_random_6fold;
 };
@@ -27,5 +27,9 @@
 /* internal functions */
 extern void gsmfr_preproc_gen_cn(struct gsmfr_preproc_state *state,
 				 gsm_byte *frame);
+extern void gsmfr_preproc_sid2cn(struct gsmfr_preproc_state *state,
+				 gsm_byte *frame);
+extern void gsmfr_preproc_invalid_sid(struct gsmfr_preproc_state *state,
+				      gsm_byte *frame);
 extern uint16_t gsmfr_preproc_prng(struct gsmfr_preproc_state *state,
 				   uint16_t no_bits);