--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libgsmfr2/tfo_dtxd.c	Thu Sep 19 21:00:18 2024 +0000
@@ -0,0 +1,32 @@
+/*
+ * The function implemented in this module is a post-processor to be invoked
+ * after Rx DTX preproc functions if the Rx DTX handler block is used as a
+ * TFO transform with DTXd enabled.  It selectively transforms certain
+ * preproc output frames (CN and silence frames specifically) into SID,
+ * thereby allowing the network operator to reap the benefits of DTX on
+ * the radio downlink leg.
+ */
+
+#include <stdint.h>
+#include "tw_gsmfr.h"
+#include "pp_state.h"
+
+int gsmfr_tfo_xfrm_dtxd(struct gsmfr_preproc_state *st, uint8_t *frame)
+{
+	uint8_t *p, sub;
+
+	if (!st->dtxd_sid_flag)
+		return 1;	/* SP=1: output is a speech frame */
+	/* turn the output frame into SID */
+	p = frame + 5;		/* skip magic+LARc */
+	for (sub = 0; sub < 4; sub++) {
+		*p++ = 0;
+		*p++ &= 0x1F;	/* upper 5 bits of Xmaxc field */
+		*p++ &= 0x80;	/* and the lsb spilling into the next byte */
+		*p++ = 0;
+		*p++ = 0;
+		*p++ = 0;
+		*p++ = 0;
+	}
+	return 0;		/* SP=0: output is SID */
+}