libgsmfrp: new refined implementation The previous implementation exhibited the following defects, which are now fixed: 1) The last received valid SID was cached forever for the purpose of handling future invalid SIDs - we could have received some valid SID ages ago, then lots of speech or NO_DATA, and if we then get an invalid SID, we would resurrect the last valid SID from ancient history - a bad design. In our new design, we handle invalid SID based on the current state, much like BFI. 2) GSM 06.11 spec says clearly that after the second lost SID (received BFI=1 && TAF=1 in CN state) we need to gradually decrease the output level, rather than jump directly to emitting silence frames - we previously failed to implement such logic. 3) Per GSM 06.12 section 5.2, Xmaxc should be the same in all 4 subframes in a SID frame. What should we do if we receive an otherwise valid SID frame with different Xmaxc? Our previous approach would replicate this Xmaxc oddity in every subsequent generated CN frame, which is rather bad. In our new design, the very first CN frame (which can be seen as a transformation of the SID frame itself) retains the original 4 distinct Xmaxc, but all subsequent CN frames are based on the Xmaxc from the last subframe of the most recent SID.

/*
 * In this module we implement preprocessing of received good frames.
 */

#include <stdint.h>
#include <string.h>
#include "gsm_fr_preproc.h"
#include "internal.h"

void gsmfr_preproc_good_frame(struct gsmfr_preproc_state *st, gsm_byte *frame)
{
	int sid;

	/* always set correct magic */
	frame[0] = 0xD0 | frame[0] & 0x0F;
	/* now classify by SID */
	sid = gsmfr_preproc_sid_classify(frame);
	switch (sid) {
	case 0:		/* good speech frame */
		st->rx_state = SPEECH;
		memcpy(&st->speech_frame, frame, sizeof(gsm_frame));
		return;
	case 1:		/* invalid SID frame */
		/* state-dependent handling, similar to BFI */
		gsmfr_preproc_invalid_sid(st, frame);
		return;
	case 2:		/* valid SID frame */
		st->rx_state = COMFORT_NOISE;
		gsmfr_preproc_sid2cn(st, frame);
		return;
	}
}