o listener echo ratio referred to in Section 30.4.7 is Both STMR and LSTR are defined in Recommendations P.76 and commonly expressed in terms of a weighted power average across the frequency band of interest. The minimum value of this ratio at P.79 (CCITT, 1989b). any one frequency gives the margin against instability or oscillation. Subjective tests have shown that the preferred range for STMR From Figure 30.7 it can be seen that a signal to listener echo ratio values is 7dB to 12dB (Recommendations G.122). Values of less approaching zero dB at some frequency will result in oscillation of than 7dB (i.e. louder sidetone) will result in a reduction in talker the 4 wire circuit at that frequency. Stability is of most importance level and values significantly greater than 12dB will give the im- during the set up and clear down phases of a call when there is the pression of the telephone being 'dead' or inactive. From the above it can be seen that sidetone has some similarities likelihood of the 2-wire to 4-wire converters being terminated with virtual open circuit or short circuit impedances, thus minimising the to talker echo. In practice the only significant difference is the delay trans-hybrid loss. of the reflected signal For delays in the order of up to two or three milliseconds the effect is considered as sidetone; for longer delays Stability is commonly specified under worst case terminating when it may be possible to perceive a distinct echo, the reflection is conditions and Recommendation G.122 provides the international considered as talker-echo. planning guidance in this area. 30.4.10 Crosstalk 30.4.9 Sidetone Customers expect telephony systems to provide some security in Sidetone, i.e. the transmission of sound from the microphone to terms of freedom from overhearing. Overhearing other conversa- receiver of the same telephone is not strictly an end to end impair- tions clearly undermines a listener's confidence that his conversa- ment as it is primarily determined by the telephone set and local line tion