Discontinuous Transmission (DTX) is a method which switches off the transmitter during a silent period in a speech conversation. The decision if there is speech or a silent period is made by a voice activity detection (VAD) module. In a normal conversation, each person talks, on average, less than 50% of the time. DTX is mainly used in mobile network to reduce radio interference and to extend the battery lifetime of the mobile stations.
If the transmitter is switched off during silent periods, the background acoustic noise abruptly disappears at the receiving end. This can be very annoying for the receiving party. To overcome this problems 'comfort noise' is generated at the receiving end whenever the transmission is switched off. The comfort noise is generated by a Comfort Noise Generator (CNG). To generate 'realistic' acoustic background noise the DTX module sends Silence Insertion Descriptor (SID) frames periodically to the CNG module at the receiving end.