ocal circuits in the hence no loop path is present. For example when the calling cus- same cable is more complex. Achieving adequate coupling requires that the balance about earth of both the cable and the termination is tomer replaces his telephone handset the digital exchange detects the on-hook condition and breaks the 4-wire switch path thus well controlled. For example poor termination balance about earth removing any chance of instability. can result in crosstalk between the pairs of a perfectly balanced cable ITU (1976). 30.6.9 Sidetone 30.6.11 Digital impairments The sidetone masking rating (STMR) is a function of the telephone set characteristics and the impedance presented by the local connec- In Section 30.5.2. it was shown that end to end performance was a tion as shown in Figure 30.9. In Recommendation G. 111 expression function of: given in Equation 30.4 is developed, where Am is a weighted sidetone balance return loss. 1. Network Structure and routeing rules. 2. Performance of individual systems. STMR = SLR set + RLR set ( in dB ) (30.4) 3. System interface requirements. From Figure 30.9 the impedance presented to the telephone is a This approach is especially useful for the complex situation of function of the characteristic impedance of the local cable and the digital impairments. impedance presented by the 2-wire interface of the digital exchange. For example end to end error performance is a function of the error Again, the use of a transmission calculation programme simplifies contribution of the individual line systems and exchanges in the the evaluation of STMR for any given connection. connection. However a straightforward addition of the contribution- The Listener Sidetone LSTR is given by Equation 30.5, where D S may not be accurate as errors may be generated by interface is a constant which reflects the sensitivity of the telephone set to problems. The jitter and wander requirements at interfaces are room noise. particularly important. If the input port of one equipment is unable Local D