the anti- acceptable if appropriate echo control is provided. aliasing filters in PCM coders. Delay planning is essential but needs to be done in association Overall distortion now tends to be less than the values that were with echo control planning. common when the majority of long distance transmission systems used FDM techniques where the channel filters introduced signifi- 30.4.6 Talker echo cant distortion. For this reason it is not common to include overall loss/frequency distortion planning as part of the transmission plan. However techniques for such planning exist and are documented in As its name implies, talker echo is concerned with echo perceived Annex B to Recommendation P.11. by the talker. It occurs when the talker's speech is returned with sufficient delay to worsen customer opinion. Figure 30.4 illustrates a typical talker echo path, from a connection through a digital 30.4.4 Non linear distortion and quantising network, where signals are reflected from the distant 2-wire to distortion 4-wire conversion unit. The subjective effect of the talker echo is a function of the acoustic Non linear distortion results from systems where the output signal to acoustic loss of the echo path and the delay of the echo path, is not exactly proportional to the input signal and harmonics or sometimes called the round-trip delay. The acoustic to acoustic loss intermodulation products are generated. The impairment is intro- of the talker echo path can be expressed in terms similar to those duced by the analogue part of transmission and switching systems used for end to end transmission loss i.e. overall loudness ratings. and needs to be considered in the specification of the performance For the example shown in Figure 30.5 the overall loudness rating of of the individual network components. However the levels of dis- the talker echo path (usually called talker echo loudness rating tortion encountered are such that it is rarely necessary to consider (TELR), can be estimated from the send and receive loudness the di