se of the selectivity of the filter Fb, the selected band of there is no advantage. During talking, therefore, the conversation is frequencies contain some of the adjacent channels from N+1 and subject to the full effect of the system noise. The effect of this noise produces an impairment to speech intelligibility that detracts from the companding advantage by an estimated 5dB. Normally these are removed by subsequent filtering and demodu- Overall advantage is therefore given by Equation 20.39. lation to voice level. If presented for re-translation however the adjacent channels must be removed to prevent crosstalk. Sup- pression of all such possible crosstalk paths is less than 80dBmO. Noise advantage = 25 - W - 5 dB (20.39) Such filters are large and complex and are usually separate equip- ments. 20.11.4 Attack and decay time The compression and expansion process is speech power activated and it therefore takes time to recognise the increase (attack time) or 20.13 Transmultiplexers decrease (recovery time) in speech power. Attack time is normally set at 5ms and recovery time 22.3ms. (CCITT, 1985, Recommenda- A transmultiplexer provides a link between FDM and TDM hierar- tion G.162.) chies without having to translate to baseband voice frequencies. The drive for transmultiplexer development came from analogue 20.11.5 Usage of companders network operators updating to digital switches. With a digital switch the interface to the trunk network is at a 2.048kbit/s CEPT defined Companders are recommended for use with speech transmission if digital bit stream (or 1.554Mbit/s T1 for the USA digital hierarchy) the mean noise power of a circuit in any hour is greater than i.e. there is no point that is suitable for direct connection into an 40000pWOp (-44dBmOp). (CCITT, 1985, Recommendation FDM hierarchy without the cost and attendant distortions of trans- G.143.) The comparable limit for telegraphy is 80000pWOp ( lation to speech band. 41dBmOp). (CCITT, 1985, Recommendation H.21.) As a prime function therefore transmultipl