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 definitions contained in this header file originally lived in
 * dtx.c; they are being factored out into a new header file as part of
 * splitting of dtx.c code into common, encoder-only and decoder-only
 * parts.
 */

/* Inverse values of DTX hangover period and DTX hangover period + 1 */

#define INV_DTX_HANGOVER (0x7fff / DTX_HANGOVER)
#define INV_DTX_HANGOVER_P1 (0x7fff / (DTX_HANGOVER+1))

#define NB_PULSE 10 /* Number of pulses in fixed codebook excitation */

/* Constant DTX_ELAPSED_THRESHOLD is used as threshold for allowing
   SID frame updating without hangover period in case when elapsed
   time measured from previous SID update is below 24 */

#define DTX_ELAPSED_THRESHOLD (24 + DTX_HANGOVER - 1)