 on which subscriber is talking at the moment. The block of reflected energy is carried out by simply inserting a high loss in the return four-wire path. Figure 8.18 shows the echo path on a four-wire circuit. An echo canceler generates an echo-canceling signal. 17 13 Remember, for any number divided by zero, the result is infinity. 14 Called echo return loss (ERL) in North America, but with a slightly different definition. 15 Recognize this as the CCITT definition of the standard analog voice channel. 16 From the point of view of stability- this discussion, it may be called from the point of view of singing. 17 Echo canceler, as defined by CCITT, is a voice-operated device placed in the four-wire portion of a circuit and used for reducing near-end echo present on the send path by subtracting an estimation of that echo from the near-end echo (Ref. 7). LOCAL AND LONG-DISTANCE NETWORKS Two-wire to Two-wire to talker listener Talker's speech path Four-wire terminating set Talker's echo path Listener's echo path Compromise network Figure 8.18 Echo paths in a four-wire circuit. 8.5.5 Transmission Design to Control Echo and Singing As stated previously, echo is an annoyance to the subscriber. Figure 8.19 relates echo path delay to echo path loss. The curve in Figure 8.19 traces a group of points at which the average subscriber will tolerate echo as a function of its delay. Remember that the longer the return signal is delayed, the more annoying it is to the telephone talker (i.e., the more the signal has to be attenuated). For example, if the echo delay on a particular circuit is 20 msec, an 11-dB loss must be inserted to make the echo tolerable to the talker. Be careful here. The reader should note that the 11 dB designed into the circuit to control echo will increase the end-to-end loudness loss (see Section 3.2.2.4) an equal amount, which is quite undesirable. The effect of loss design on loudness ratings and the tradeoffs available are discussed in the paragraphs that follow. If singing is to be controlled, all fou