The present document defines a frame loss concealment procedure, also termed frame substitution and muting procedure, which is executed by the Enhanced Voice Services (EVS) decoder when one or more frames (speech or audio or SID frames) are unavailable for decoding due to e.g. packet loss, corruption of a packet or late arrival of a packet.

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The purpose of the frame loss concealment procedure is to conceal the effect of any unavailable EVS frame (speech or audio or SID) for decoding. The concealment of erased frames covers both the reconstruction of missing frames and the techniques to ensure smooth and rapid recovery of normal synthesis after erased segments. The frame loss concealment procedure also covers proper strategies including muting or fading to background noise for the case of multiple frame losses in a row. The purpose of muting the output or fading to background noise in the case of several lost frames in a row is to indicate the breakdown of the channel to the user and to avoid generating possible annoying sounds as a result from the frame loss concealment procedure. Unless stated differently, fading operations (described in subclause 5.3.4 and subclause 5.4.6) start already with the first lost frame. Given the architecture and features of the EVS codec (details in [EVS Codec Detailed Algorithmic Description]) the frame loss concealment procedure comprises concealment methods for the various major codec modules, such as signal classification, spectral envelope (LPC), ACELP, MDCT and Bandwidth Extension. A particular feature of the EVS codec is 'guided' frame loss concealment for which the encoder provides specific supplementary data guiding the concealment during erased frames and enhancing the convergence and recovery of the decoder after erased frames. The description in this specification is limited on how to apply the 'guided' frame loss concealment data; the corresponding encoding operations are described as part of the EVS codec algorithmic description TS 26.445. The procedure of the present document is mandatory for implementation in all network entities and User Equipment (UE)s supporting the EVS decoder. The present document does not describe the ANSI C code of this procedure. For a description of the two reference fixed-point ANSI C code implementations, using different sets of basic operators, see TS 26.442 and TS 26.452 respectively; for a description of the reference floating-point ANSI C code implementation see TS 26.443. In the case of discrepancy between the procedure described in the present document and its ANSI-C code specifications contained in TS 26.442 the procedure defined by TS 26.442 prevails. In the case of discrepancy between the procedure described in the present document and its ANSI-C code specifications contained in TS 26.443 the procedure defined by TS 26.443 prevails. In the case of discrepancy between the procedure described in the present document and its ANSI-C code specifications contained in TS 26.452 the procedure defined by TS 26.452 prevails.