/*
 * 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 */
}