Phase Distortion in Telephone Apparatus*

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Abstract
This paper shows that if, over its transmitting range, the phase shift, B, in radians, of a four-terminal network may be written B = a0 + a1ω (ω = 2πx frequency in cycles per second), there is no phase distortion if a0 = N, N being any integer. However, there is a delay, for any signal, given by dB/dω = a1 (seconds). If N is not an integer, there is a delay, a1, and in addition a distortion, which distortion, generally for speech and music, may be neglected. Typical phase characteristics for lines, filters and all-pass networks are shown. In general over their transmitting range, such phase characteristics which usually are curved, may be regarded as the sum of two characteristics, a straight line having a slope corresponding to the minimum slope of the original and which introduces a delay without distortion and a curved portion to which all of the distortion of the signal may be ascribed. Oscillograms are given showing the distortion for a loaded line and for band filters for a signal which is of the form y sin (ω0 t + θ) between t = 0 and t = T and zero for all other time. A description is given of the means employed for reducing the amount of phase distortion in telephone cable and in low-pass filters in circuits used for program transmission and regular telephone service. Also, phase distortion in repeaters and transformers is described. Brief reference is made to the problem of phase distortion in telegraph, picture transmission, and television circuits.

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