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.

/*
 * This header file is the external public interface to libgsmfrp;
 * it should be installed in the same system include directory
 * as <gsm.h> from libgsm, on which we depend.
 */

#ifndef	__GSM_FR_PREPROC_H
#define	__GSM_FR_PREPROC_H

#include <gsm.h>

struct gsmfr_preproc_state;	/* opaque to external users */

extern struct gsmfr_preproc_state *gsmfr_preproc_create(void);
/* use standard free() call to free it afterward */

/* reset state to initial */
extern void gsmfr_preproc_reset(struct gsmfr_preproc_state *state);

/* main processing functions */
extern void gsmfr_preproc_good_frame(struct gsmfr_preproc_state *state,
				     gsm_byte *frame);
extern void gsmfr_preproc_bfi(struct gsmfr_preproc_state *state, int taf,
			      gsm_byte *frame_out);

/* utility function */
extern int gsmfr_preproc_sid_classify(const gsm_byte *frame);

/* utility datum */
extern const gsm_frame gsmfr_preproc_silence_frame;

#endif	/* include guard */