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.

/*
 * This header file is internal to libgsmfr2;
 * here we define our state structure for GSM 06.10 encoder & decoder component.
 */

struct gsmfr_0610_state {
	word		dp0[ 280 ];
	word		e[ 50 ];	/* code.c 			*/

	word		z1;		/* preprocessing.c, Offset_com. */
	longword	L_z2;		/*                  Offset_com. */
	int		mp;		/*                  Preemphasis	*/

	word		u[8];		/* short_term_aly_filter.c	*/
	word		LARpp[2][8]; 	/*                              */
	word		j;		/*                              */

	word		nrp; /* 40 */	/* long_term.c, synthesis	*/
	word		v[9];		/* short_term.c, synthesis	*/
	word		msr;		/* decoder.c,	Postprocessing	*/
};