The present document specifies the Voice Activity Detector (VAD) to be used in the Discontinuous Transmission (DTX) as described in GSM 06.31. It also specifies the test methods to be used to verify that a VAD complies with the technical specification.
The requirements are mandatory on any VAD to be used either in the GSM Mobile Stations (MS)s or Base Station Systems (BSS)s.
The following documents contain provisions which, through reference in this text, constitute provisions of the present document.
References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
For a specific reference, subsequent revisions do not apply.
For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
GSM 01.04: "Digital cellular telecommunications system (Phase 2+); Abbreviations and acronyms".
GSM 06.10: "Digital cellular telecommunications system(Phase 2+); Full rate speech; Transcoding".
GSM 06.12: "Digital cellular telecommunications system(Phase 2+); Full rate speech; Comfort noise aspect for full rate speech traffic channels".
GSM 06.31: "Digital cellular telecommunications system(Phase 2+); Full rate speech; Discontinuous Transmission (DTX) for full rate speech traffic channels".
Abbreviations used in the present document are listed in GSM 01.04 
The function of the VAD is to indicate whether each 20 ms frame produced by the speech encoder contains speech or not. The output is a binary flag which is used by the TX DTX handler defined in GSM 06.31 
The ETS is organized as follows.
describes the principles of operation of the VAD.
In clause 3
, the computational details necessary for the fixed point implementation of the VAD algorithm are given. This clause uses the same notation as used for computational details in GSM 06.10.
The verification of the VAD is based on the use of digital test sequences. Clause 4
defines the input and output signals and the test configuration, whereas the detailed description of the test sequences is contained in clause A.2
The performance of the VAD algorithm is characterized by the amount of audible speech clipping it introduces and the percentage activity it indicates. These characteristics for the VAD defined in the present document have been established by extensive testing under a wide range of operating conditions. The results are summarized in clause A.3