 is not being overheard. The intelligibility of the crosstalk is the characteristics at each end of the connection. The two customers key factor and this is influenced by talker levels, acuity of hearing, may therefore see quite different sidetone conditions at their respec- noise on the line and the degree of coupling between connections. tive ends. Recommendation P.16 gives details of crosstalk intelligibility Sidetone has a number of effects on the telephone users. High thresholds and methods for calculating the probability of inter- levels of returned signal may be disturbing to the talker who will ference. automatically reduce his talking level hence worsening the perfor- mance as seen by the distant customer. Room noise will also be 30.4.11 Digital impairments picked up by the microphone and returned to the receiver via the sidetone path. If this sidetone path has a low loss then the room noise The major digital impairment is error performance i.e. the received at the receiver will tend to mask the incoming speech and the bits not being the same as the transmitted bits. Errors may be customers may complain of faint transmission. The problem how- contributed by any digital component in a connection hence there is ever is not one of low levels of incoming speech but high levels of a need for end to end transmission planning of error performance. noise. The difficulty is that, unlike the previous parameters where it is Sidetone is quantified using an approach similar to that used for generally possible to predict the performance on a connection, error loudness rating i.e. a weighted value of the acoustic to acoustic loss. performance is impossible to predict on a short term basis. Errors 30/8 Transmission planning can be caused by a variety of mechanisms even on well designed have been adequately covered and their impact on, for example equipment. network structure, already taken into account. Lightning, man-made electrostatic discharges, climatic effects on radio systems all may generate errors. Systems tend 