 significant effect on voice band data services than on achieved by the use of separate devices such as echo cancellers or speech transmission. Recommendation G.122 (CCITT, 1989c) in- echo suppressors. Application rules for such echo control devices cludes information on required signal to listener echo ratios for are included in Recommendation G.131. voiceband data modems operating at a range of speeds In summary, there is no single objective value for end to end talker echo. The procedure is to calculate TELR, round trip delay and to then calculate the resultant probability of encountering objection- able echo. G.131 recommends that this calculated probability should not exceed 1%. Echo Loss Talker Listener Figure 30.5 Talker Echo Loudness Rating (TELR) 30.4.7 Listener echo Whereas talker echo is a result of a single reflection in the network, 10 15 20 25 30 35 40 45 50 ms listener echo is caused by a further reflection of the echo signal. The Mean one-way propagation time listener then hears the wanted signal together with a delayed, atte- nuated version of the original signal, as shown in Figure 30.7. As for talker echo the subjective effect is a function of the level of the listener-echo and, to a lesser extent its delay. From Figure 30.7 it Figure 30.6 Echo tolerance curves. (Source: CCITT can be seen that if listener echo is present then talker echo will also Recommendation 1.131.) Solid lines indicate analogue be encountered and this talker echo is likely to be the most demand- circuits, with indication of number of 4-wire circuits. Dotted ing requirement. Required values for listener echo are given in lines indicate fully digital connections Supplement 3 to CCITT Volume V (CCITT, 1989d). Listener echo End to end requirements 30/7 WANTED SIGNAL LISTENER ECHO Talker Listener Figure 30.7 Signal to listener echo path and ratio 30.4.8 Stability Sidetone Masking Rating (STMR) is used to describe the talking effects of sidetone and Listener Sidetone Rating (LSTR) for the room noise sidetone effect. The signal t