The present document specifies the AMR-WB backward compatible functions of the EVS codec. The present document is a high level overview of the functionality with reference to the Codec Detailed Algorithmic Description where the functionality is specified in detail.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

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For the purposes of the present document, the abbreviations given in TR 21.905 and the following apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905. AMR-WB CNG DTX EVS FB IO JBM NB SID SWB VAD WB

The EVS coder provides enhanced AMR-WB backward compatible operation over all nine source bit rates from 6.6 kbit/s to 23,85 kbit/s, including AMR-WB backward compatible DTX operation. This backward compatible operation is referred to AMR-WB interoperable (AMR-WB IO). Frames generated by an EVS AMR-WB IO encoder can be decoded by an AMR-WB decoder, without the need for transcoding. Correspondingly, frames generated by an AMR-WB encoder can be decoded by an EVS AMR-WB IO decoder, without the need for transcoding. In addition to encoder and decoder enhancements, the AMR-WB backward compatible functions in the EVS coder provide the following extra functionalities compared to the AMR-WB coder specified in [11] to [17] and [20]:

• Support of input and output sampling frequencies other than 16 kHz (i.e. 8, 32, 48 kHz)
• Inclusion of jitter buffer management for AMR-WB backward compatible functions
• Switching between AMR-WB IO bit rates and primary EVS bit rates at any 20 ms speech frame boundary

The present document is mandatory for implementation in all network entities and UEs supporting the EVS codec. In the case of discrepancy between the EVS codec backward compatible functions described in the present document and its ANSI-C code specification contained in [3] the procedure defined by [3] prevails. In the case of discrepancy between the procedure described in the present document and its ANSI-C code specification contained in [4] the procedure defined by [4] prevails. In the case of discrepancy between the procedure described in the present document and its ANSI-C code specification contained in [22] the procedure defined by [22] prevails.

The AMR-WB backward compatible transcoding functions of the EVS coder are described in [2].

The ANSI C-code of the EVS codec, including backward compatible functions, is described in [3] and [22] for fixed point arithmetic operation, using different sets of basic operators, and is described in [4] for floating point arithmetic operation.

The EVS coder AMR-WB backward compatible error concealment of erroneous or lost frames is described in [2], [6] and [7].

The AMR-WB backward compatible comfort noise aspects of EVS coder are described in [2] and [8].

The EVS coder AMR-WB backward compatible source controlled rate operation of the Enhanced Voice Services codec is defined in [2] and [9]. This DTX operation is compliant with [16].

The AMR-WB backward compatible VAD function of EVS coder is described in [2] and [10]. The VAD algorithm description is detailed in [2], and the corresponding C-code is defined in [3], [4] and [22].

The EVS coder AMR-WB backward compatible part of the EVS coder supports the formats (IF1, IF2, MIME, RFC 4867 [21]) defined in [18] and [19].