03.503GPPTransmission planning aspects of the speech service in the GSM Public Land Mobile Network (PLMN) systemTS
The loss of the start or the end of a speech burst is known as clipping, the main cause of which is voice switching controlled by voice activity detection. Voice switching occurs in devices within the network or within terminal devices. The following devices employ voice switching:
1) echo suppressers. These are generally located at an ISC at either end of a long international connection or connections using satellites;
2) echo cancellers with centre clippers. These are located as for the echo suppressers above. In addition, it is recommended that they be used in the MSC at the interface with the PSTN. Clipping in these devices arises from the action of the centre clipper only;
3) digital speech interpolators (DSI). These devices are used in circuit multiplication equipment’s which are often employed on international connections;
4) discontinuous transmission (DTX) devices. These are located in the PLMN;
5) loudspeaking telephones. These are used in the PSTN and in the PLMN. It should be noted that regulations in certain countries prohibit the use of handheld MSs by drivers of moving vehicles.
3.5.2 Properties of voice switches in the PLMN
GSM 06.32 specifies the requirements for the voice activity detector used for DTX and the total clipping allowed in the MS. Any voice switching used for acoustic echo control should not exceed these limits. Information on recommended characteristics of handsfree telephones is given in clause 5 of ITU‑T Recommendation P.34.
3.5.3 Problems of tandem voice switching
The effect of tandem voice switches which are not under one common control will be an increase in clipping. Moreover, under conditions of high or rapidly changing ambient noise, false detection of speech is likely to occur in the voice activity detectors in DSI equipment or network echo control devices. These devices are generally designed for constant and low levels of noise.
In order to minimize clipping, the following action should be taken:
1) intermediate tandem voice switching devices in the network should be either disabled by signalling means or avoided by routeing means;
2) the voice switching for the MS for acoustic control and for DTX should be under one common control.
However, it should be noted that, in many cases, it will not be possible to exclude DSI equipment or loudspeaking telephones from the connection.