The present document specifies the Voice Activity Detector (VAD) used in the Discontinuous Transmission (DTX) of the EVS Codec. Although the main application of the VAD algorithm is the detection of speech or voice signals, the algorithm is more accurately described as a Signal Activity Detection (SAD) algorithm. 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. ACELP AMR-WB CNG DTX EVS FB FEC IP JBM MSB MTSI NB PS PSTN SAD SC-VBR SID SWB VAD WB WMOPS

The function of the Enhanced Voice Services coder VAD algorithm, or more accurately the SAD algorithm, is to indicate whether each 20 ms frame contains signals that should be transmitted, e.g. speech, music or other audio. The output of the SAD algorithm is a Boolean flag (ƒ_SAD) that is set to one for the active signal, which is any useful signal bearing some meaningful information. Otherwise, the flag is set to zero indicating an inactive signal, which has no meaningful information. The inactive signal is mostly a pause or background noise. The procedure of the present document is mandatory for implementation in all network entities and User Equipment (UE)s supporting the EVS coder. The present document does not describe the ANSI-C code of this procedure. 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 the TS 26.442 prevails.

The Enhanced Voice Services codec signal activity detection (SAD) module described in the present document consists of three sub-SAD modules; SAD1, SAD2 and SAD3. SAD1 and SAD2 are combined initially to provide an efficient preliminary activity decision. This preliminary decision is then modified by the third sub-SAD module, SAD3, depending upon the codec mode of operation. The efficient preliminary activity output is used as the final SAD decision for the AMR-WB IO modes, while the activity output with SAD3 is used as the final SAD decision for all other bit-rates. Sub-clause 5.1.12 of TS 26.445 describes the operation of the SAD and the algorithms involved in the three sub-SAD modules in detail.