tener echo connection. A suitable calculation procedure is outlined in Annex A to Recommendation G.103. Where digital systems are used in the To a first approximation, the signal to listener echo ratio is given by connection an additional source of noise (errors) is introduced, the sum of the two echo losses as shown in Figure 30.7 and Equation which is decoded as analogue noise. 30.3. 30/10 Transmission planning (30.3) LSTR = STMR + D ( in dB ) (30.5) Signal Listener Echo Ratio - EL1 + EL2 ( dB) Values of D are determined by the acoustics of the handset and the A more accurate calculation would need to take into account type of microphone used as indicated in Recommendation G.111. multiple transmission around the loop but this is generally not needed as typical signal to listener echo ratios are in the order of 30.6.10 Crosstalk 20dB or better. The second reflection is therefore 40dB lower and can be neglected. Recommendation P.16 gives a detailed description of the factors influencing the intelligibility of any crosstalk. These are: 30.6.8 Stability 1. Loudness ratings of telephone sets. As outlined in Section 30.4.8, the margin against instability is 2. Circuit noise (including telephone noise). indicated by the worst case signal to listener echo ratio i.e. under 3. Room noise. worst case terminating conditions and at the frequency giving the 4. Crosstalk coupling between circuits. lowest ratio. Combinations of open circuit and short circuit are commonly used as test conditions although it is necessary to check what conditions are actually encountered in practice. An additional The crosstalk coupling between circuits in multi-channel equip- factor to be considered is the switching characteristics of the ex- ment e.g. exchanges and line systems, generally forms part of the changes. In some cases the transmission path is not complete at the specification of that equipment and can be suitably controlled. time of application of worst case conditions such as open circuits However crosstalk coupling between 2-wire l