induction of unwanted currents from services (rail traction, grid the regulating pilot frequency) the loss at other frequencies can be systems etc.) that may run parallel with the transmission is propor- deduced. tional to the square of the effective coupled path (CCITT, 1985, The change of the cable loss with temperature obeys the same law K17). and thus correction for temperature is applied with a root f shape Under traction or grid faults where very large currents may flow across the line frequency spectrum with the greatest correction at for a short period in the service cable, excessive voltages may be the highest frequency. coupled into the transmission cable and can damage the repeaters This equalisation which is controlled by the regulating pilot is not and cause an insulation breakdown within the cable. exact and small errors accumulate in the gain frequency response at Even under normal conditions a long coupled path with a.c. each repeater equalisation point. The error systematically adds traction or similar installations can cause the transmission power along the repeatered cable and to avoid excessive systematic error feed current to be modulated with the traction frequency, or its 20/22 Frequency division multiplexing REPEATERED COAXIAL LINE Winter excursion Nominal Summer excursion Pilot Level excursion Transmission Level -4dB in Summer corrected at regulated +4dB in Winter repeater output Winter excursion Nominal Summer excursion Pilot Level excursion -2dB in Summer +2dB in Winter Pre-regulation applied to the output of a regulated repeater Figure 20.23 Repeatered line regulation: (a) pilot levels using conventional regulation; (b) pilot levels using pre-regulation harmonics. This will create unwanted modulation products within Holbrook, B.D. and Dixon, J.T. (1939) Load rating theory for the required multichannel traffic load. multichannel amplifiers, Bell Systems Technical Journal, (18), October. 20.15 References Ribeyre, M. et. al. (1982) Exploration of transmultiplexers in tele- communication n