Phase Distortion in Telephone Apparatus * 



By C. E. LANE 



This paper shows that if, over its transmitting range, the phase shift, B, in 

 radians, of a four-terminal network may be written S = Oo + Oico (w = 2-wx 

 frequency in cycles per second), there is no phase distortion if ao = N, N 

 being any integer. However, there is a delay, for any signal, given by dB/dw 

 = fli (seconds). If N is not an integer, there is a delay, Oi, and in addition a 

 distortion, which distortion, generally for speech and music, may be neg- 

 lected. Typical phase characteristics for lines, filters and all-pass networks 

 are shown. In general over their transmitting range, such phase charac- 

 teristics 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 (wo^ + 0) between ^ = and 

 t = T and zero for all other time. A description is given of the means em- 

 ployed for reducing the amount of phase distortion in telephone cable and in 

 low-pass filters in circuits used for program transmission and regular tele- 

 phone 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. 



'^"^HE effects of amplitude distortion in the transmission of signals 

 -L has been taken into consideration in the design of telephone 

 systems for some time. Recently ^ increasing attention is being given 

 to the phase changes which waves undergo in the process of their 

 transmission. The necessity of this is, on the one hand, due chiefly to 

 the use of long distance telephone systems involving greater lengths of 

 loaded cable and numerous filters and repeaters in tandem, and, on the 

 other hand, due to the demand for improved performance. One place 

 where better quality has become particularly desirable is in circuits 

 for interconnecting broadcasting stations. 



This paper will present some general considerations of the relation 

 between the phase characteristics ^ of telephone apparatus and signal 

 distortion,'' show the types of phase characteristics that most fre- 

 quently require consideration and discuss the manner in which the 

 amount of phase distortion is controlled. Brief reference will also be 



* Presented at New York Section, A. I. E. E., May 1930. 



1 At the end of this paper, a bibliography is given containing references to previous 

 publications on this subject. 



^ For a definition of phase characteristic see Appendix I. The "insertion " phase 

 characteristic and "image transfer" phase characteristic are defined there. 



^ A companion paper by J. C. Steinberg deals specifically with the effects of phase 

 distortion on the quality of speech and music. Another companion paper by H. 

 Nyquist and S. Brand treats of the measurement of phase distortion. 



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