--- a/.hgignore	Sun Nov 20 16:56:43 2022 +0000
+++ b/.hgignore	Sun Nov 20 18:08:41 2022 +0000
@@ -5,6 +5,7 @@
 ^dev/gsm0611-silence-fr$
 ^dev/gsm0611-silence-fr\.out$
 
+^frtest/cn-debug$
 ^frtest/gsmfr-cvt-dlcap$
 ^frtest/gsmfr-decode$
 ^frtest/gsmfr-encode$

--- a/frtest/Makefile	Sun Nov 20 16:56:43 2022 +0000
+++ b/frtest/Makefile	Sun Nov 20 18:08:41 2022 +0000
@@ -1,6 +1,7 @@
 CC=	gcc
 CFLAGS=	-O2
 PROGS=	gsmfr-cvt-dlcap gsmfr-decode gsmfr-encode gsmfr-preproc
+NOINST=	cn-debug
 LIBPP=	../libgsmfrp/libgsmfrp.a
 LIBTEST=../libtest/libtest.a
 LIBDEC=	${LIBTEST} ${LIBPP}
@@ -8,7 +9,10 @@
 
 CVT_OBJS=cvt-dlcap.o tidsp.o
 
-all:	${PROGS}
+all:	${PROGS} ${NOINST}
+
+cn-debug:	cn-debug.o ${LIBDEC}
+	${CC} ${CFLAGS} -o $@ cn-debug.o ${LIBDEC} -lgsm
 
 gsmfr-cvt-dlcap:	${CVT_OBJS}
 	${CC} ${CFLAGS} -o $@ ${CVT_OBJS}
@@ -27,4 +31,4 @@
 	install -c ${PROGS} ${INSTBIN}
 
 clean:
-	rm -f *.o *.out ${PROGS}
+	rm -f *.o *.out ${PROGS} ${NOINST}

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/frtest/cn-debug.c	Sun Nov 20 18:08:41 2022 +0000
@@ -0,0 +1,99 @@
+/*
+ * This program has been written as part of the debug investigation,
+ * seeking to understand how and why our FR comfort noise generator
+ * is currently broken.
+ */
+
+#include <stdio.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <gsm.h>
+#include "../libgsmfrp/gsm_fr_preproc.h"
+#include "../libtest/binreader.h"
+
+main(argc, argv)
+	char **argv;
+{
+	FILE *binf;
+	gsm dec_state;
+	struct gsmfr_preproc_state *pp_state;
+	unsigned frame_index;
+	uint8_t frame[BINFILE_MAX_FRAME];
+	int16_t pcm[160];
+	gsm_signal fr_params[76];
+	int rc, bfi, taf;
+	int i, j, n;
+	unsigned samp_abs, samp_max;
+
+	if (argc != 2) {
+		fprintf(stderr, "usage: %s bin-stream-file\n", argv[0]);
+		exit(1);
+	}
+	binf = fopen(argv[1], "r");
+	if (!binf) {
+		perror(argv[1]);
+		exit(1);
+	}
+	dec_state = gsm_create();
+	if (!dec_state) {
+		fprintf(stderr, "gsm_create() failed!\n");
+		exit(1);
+	}
+	pp_state = gsmfr_preproc_create();
+	if (!pp_state) {
+		fprintf(stderr, "gsmfr_preproc_create() failed!\n");
+		exit(1);
+	}
+	for (frame_index = 0; ; frame_index++) {
+		rc = binfile_read_frame(binf, frame);
+		if (rc < 0) {
+			fprintf(stderr, "error: garbage in %s\n", argv[1]);
+			exit(1);
+		}
+		if (!rc)
+			break;
+		printf("#%u: ", frame_index);
+		switch (frame[0] & 0xF0) {
+		case 0xB0:
+			bfi = 1;
+			taf = frame[1] & 1;
+			printf("BFI TAF=%u\n", taf);
+			break;
+		case 0xC0:
+			fprintf(stderr, "error: %s is not in FR codec format\n",
+				argv[1]);
+			exit(1);
+		case 0xD0:
+			bfi = 0;
+			fputs("FR", stdout);
+			gsm_explode(dec_state, frame, fr_params);
+			n = 0;
+			for (i = 0; i < 8; i++)
+				printf(" %u", fr_params[n++]);
+			putchar('\n');
+			for (i = 0; i < 4; i++) {
+				putchar(' ');
+				for (j = 0; j < 17; j++)
+					printf(" %u", fr_params[n++]);
+				putchar('\n');
+			}
+			break;
+		}
+		if (bfi)
+			gsmfr_preproc_bfi(pp_state, taf, frame);
+		else
+			gsmfr_preproc_good_frame(pp_state, frame);
+		gsm_decode(dec_state, frame, pcm);
+		samp_max = 0;
+		for (i = 0; i < 160; i++) {
+			if (pcm[i] >= 0)
+				samp_abs = pcm[i];
+			else
+				samp_abs = -pcm[i];
+			if (samp_abs > samp_max)
+				samp_max = samp_abs;
+		}
+		printf("  MAX=%u\n", samp_max);
+	}
+	exit(0);
+}