The Transmission Characteristics of Toll Telephone Cables at Carrier Frequencies

N T H E design of a new telephone transmission system a knowledge of the characteristics of the medium over which the waves are to pass is, of course, a prerequisite. What painstaking experimentation is necessary to accumulate such knowledge, however, what voluminous data are involved, what minutiae of detail, and what extremes of accuracy, are things far less obvious. Recent papers have described a new 12-channel carrier telephone system for operation over cable pairs.1 For this system a knowledge of the maximum cable losses is needed in order to determine the necessary repeater gains. Accurate data on the insertion loss slope versus frequency are required so that compensating equalizers can be designed to give uniform transmission over the frequency band. In order to design a regulating system to compensate for the variations in attenuation which result from changes in cable temperature, precise knowledge of these variations as a function of frequency is essential. It is necessary to know the impedance of the cable pairs in order that the amplifier impedance may be matched to it, thereby avoiding reflections which would aggravate cross-talk effects. For various purposes, e.g., testing the cables, designing the coils to balance out crosstalk, etc., it is also necessary to know the fundamental parameters (resistance, inductance, capacitance and conductance) or so-called primary constants of the pairs. The velocity of transmission also plays a part in determining the characteristics of the channels.