A.4 General

26.1933GPPAdaptive Multi-Rate - Wideband (AMR-WB) speech codecRelease 17Source controlled rate operationSpeech codec speech processing functionsTS

Discontinuous Transmission (DTX) is a mechanism, which allows the radio transmitter to be switched off most of the time during speech pauses for the following two purposes:

– to save power in the Mobile Station (MS);

– to reduce the overall interference level over the air interface.

DTX in uplink shall be in operation within the GSM MS, if commanded so by the network, see 3GPP TS 24.008. The MS shall handle DTX in downlink at any time, regardless, whether DTX in uplink is commanded or not.

A.4.1 General organisation

The overall DTX mechanism described in the present document requires the following functions:

– a Voice Activity Detector (VAD) on the transmit (TX) side;

– evaluation of the background acoustic noise on the transmit (TX) side, in order to transmit characteristic parameters to the receive (RX) side;

– generation on the receive (RX) side of a similar noise, called comfort noise, during periods where the radio transmission is switched off.

The Voice Activity Detector (VAD) is defined in 3GPP TS 26.194 and the comfort noise functions in 3GPP TS 26.192. Both are based partly on the speech transcoder and its internal variables, defined in 3GPP TS 26.190.

In addition to these functions, if the parameters arriving at the RX side are detected to be seriously corrupted by errors, the speech or comfort noise must be generated from substituted data in order to avoid seriously annoying effects for the listener. This function is defined in 3GPP TS 26.191.

An overall description of the speech processing parts can be found in 3GPP TS 26.171.

The description for Tandem Free Operation is given in 3GPP TS 28.062.