wiki的解释更加清楚： Discontinuous transmission (DTX) is a method of momentarilypowering-down, or muting, a mobile or portable wireless telephone setwhen there is no voice input to the set. This optimizes the overallefficiency of a wireless voice communications system. In a typicaltwo-way conversation, each individual speaks slightly less than half ofthe time. If the transmitter signal is switched on only during periodsof voice input, the duty cycle of the telephone set can be cut to lessthan 50 percent. This conserves battery power, eases the workload ofthe components in the transmitter amplifiers, and reduces interference.
A common misconception is that DTX improves capacity by freeing upTDMA time slots for use by other conversations. In practice, theunpredictable availability of time slots makes this difficult toimplement. However, reducing interference is a significant component inhow GSM and other TDMA based mobile phone systems make better use ofthe available spectrum compared to older analog systems such as AMPS and NMT.While older network types theoretically allocated two 25–30 kHzchannels per conversation, in practice some radios would causeinterference on neighbouring channels making them unusable, and asingle radio may broadcast too strong a signal to let nearby cellsreuse the same channel.
GSM combines short packet sizes, frequency hopping,redundancy, power control, digital encoding, and DTX to minimizeinterference and the effects of interference on a conversation. In thisrespect, 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 usedfor telephony. In such cases, resources freed up when one user is insilence can be used to serve another user. The increase of the numberof users will contribute to the interference level. Systems that usevoice 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 asAMR. As with the packet radio systems above, when one side of theconversion 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 circuitsof this type operate properly and do not swallow parts of words whichwere 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).
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