libgsmhr1 RxFE: store CN R0+LPC separately from speech In the original GSM 06.06 code the ECU for speech mode is entirely separate from the CN generator, maintaining separate state. (The main intertie between them is the speech vs CN state variable, distinguishing between speech and CN BFIs, in addition to the CN-specific function of distinguishing between initial and update SIDs.) In the present RxFE implementation I initially thought that we could use the same saved_frame buffer for both ECU and CN, overwriting just the first 4 params (R0 and LPC) when a valid SID comes in. However, I now realize it was a bad idea: the original code has a corner case (long sequence of speech-mode BFIs to put the ECU in state 6, then SID and CN-mode BFIs, then a good speech frame) that would be broken by that buffer reuse approach. We could eliminate this corner case by resetting the ECU state when passing through a CN insertion period, but doing so would needlessly increase the behavioral diffs between GSM 06.06 and our version. Solution: use a separate CN-specific buffer for CN R0+LPC parameters, and match the behavior of GSM 06.06 code in this regard.

/*
 * 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 */
}