ABSTRArnThis paper describes the Voice Activity Detector standardised by CE€T for use in the Pan-European Digital Cellular Mobile Telephone Service. The difficulties of detecting speech in a noisy environment are discussed and the performance tests carried out to validate the design are described.

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An important mode of operation for mobile radio systems is known as discontinuous transmission (DTX). In this mode the transmitter is switched on when a user begins to talk and switched off again when he stops.There are two reasons for using DTX: to increase the number of radio channels (by 100% or more) which can be accommodated in a given bandwidth and to reduce the power drain in hand portable equipment.Since system operators rely on the DTX working efficiently (to avoid excessive co-channel interference) it is also necessary to find ways of specifying and measuring the performance.This paper describes the DTX system which has been standardized by the CEPT for the Pan-European digital mobile radio system.

British Telecom International is to introduce an aeronautical telephone service to be known as the Skyphone. This service will use 9.6 kbit/s speech codecs because of the limited satellite power available. The codec to be used will be chosen from a number of codecs developed in several different countries. This paper gives a brief description of the Skyphone and the factors which influence the speech codec design. It then describes the codec proposed by British Telecom Research Laboratories as a candidate for the service. Subjective test results show this codec to be better than the other candidates declared to date.

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