8 Discontinuous transmission (DTX)

26.4413GPPCodec for Enhanced Voice Services (EVS)General overviewRelease 17TS

The discontinuous transmission (DTX) of the Enhanced Voice Services codec is defined in [2] & [10].

During a normal telephone conversation, the participants alternate so that, on the average, each direction of transmission is occupied about 50 % of the time. Source controlled operation is a mode of operation where the speech encoder encodes speech frames containing only background noise with a lower bit-rate than normally used for encoding speech. A network may adapt its transmission scheme to take advantage of the varying bit-rate. This may be done for the following two purposes:

1) In the UE, battery life will be prolonged or a smaller battery could be used for a given operational duration.

2) The average required bit-rate is reduced, leading to a more efficient transmission with decreased load and hence increased capacity.

The following functions are provided by the EVS codec for the source controlled rate operation:

– a Voice Activity Detector (VAD) or more accurately Sound Activity Detector (SAD) on the TX side;

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

– generation of comfort noise on the RX side during periods when no normal speech frames are received.

The transmission of comfort noise information to the RX side is achieved by means of a Silence Descriptor (SID) frame, which is sent at regular intervals.

– a Source-Controlled Variable Bit Rate operational mode (SC-VBR) operating at 5,9 kbit/s is also provided to reduce the operational bit rate still further for NB and WB speech services.