06.913GPPSubstitution and muting of lost frames for AMR speech traffic channelsTS
The purpose of frame substitution is to conceal the effect of lost frames. The purpose of muting the output in the case of several lost frames is to indicate the breakdown of the channel to the user and to avoid generating possible annoying sounds as a result from the frame substitution procedure.
The RSS indicates lost speech or lost SID frames by setting its Bad Frame Indication flag (BFI) based on CRCs and possibly other error detection mechanisms. The TRAU calculates from the CRCs inserted by the CCU in the TRAU frames one BSI_Abis flag for every sub-block of speech parameters. If either one or both of these flags are set, the speech decoder shall perform parameter substitution to conceal errors.
The RSS also indicates potentially degraded frames using the flag PDFI. This flag is derived from the soft output of the channel decoder. It may be used by the speech decoder selectively depending on the estimated signal type.
The example solutions provided in paragraphs 6 and 7 apply only to bad frame handling on a complete speech frame basis. However some parts could be modified for substitution of only bad sub-blocks.