Discontinuous transmission
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Discontinuous transmission (DTX) is a method of momentarily powering-down, or muting, a mobile or portable wireless telephone set when there is no voice input to the set. This optimizes the overall efficiency of a wireless voice communications system. In a typical two-way conversation, each individual speaks slightly less than half of the time. If the transmitter signal is switched on only during periods of voice input, the duty cycle of the telephone set can be cut to less than 50 percent. This conserves battery power, eases the workload of the components in the transmitter amplifiers, and reduces interference.
A common misconception is that DTX improves capacity by freeing up TDMA time slots for use by other conversations. In practice, the unpredictable availability of time slots makes this difficult to implement. However, reducing interference is a significant component in how GSM and other TDMA based mobile phone systems make better use of the available spectrum compared to older analog systems such as AMPS and NMT. While older network types theoretically allocated two 25-30kHz channels per conversation, in practice some radios would cause interference on neighbouring channels making them unusable, and a single radio may broadcast too strong a signal to let nearby cells reuse the same channel.
GSM combines short packet sizes, frequency hopping, redundancy, power control, digital encoding, and DTX to minimize interference and the effects of interference on a conversation. In this respect, DTX indirectly improves the over-all capacity of a network.
In packet radio systems such as GPRS/EDGE, it is possible to combine DTX with capacity increase when VoIP is used for telephony. In such cases, resources freed up when one user is in silence can be used to serve another user. The increase of the number of users will contribute to the interference level. Systems that use voice codecs such as AMR can reduce vocoder rate adaptively to better combat interference.
Systems based upon CDMA air interfaces such as IS-95/CDMA2000, and most forms of UMTS, can use a form of implied DTX by usage of a variable rate codec such as AMR. As with the packet radio systems above, when one side of the conversion is silent, the amount of transmitted data is minimized. Again, the effect is reduce interference.
A DTX circuit operates using voice activity detection (VAD). Sophisticated engineering is necessary to ensure that circuits of this type operate properly and do not swallow parts of words which were mistaken as background noise. Usually comfort noise is used to fill phases of discontinued transmission.
In wireless transmitters, VAD is sometimes called voice-operated transmission (VOX).
