MEASUREMENT OF PHASE DISTORTION 523 



For example, telephotography,* which requires a relatively narrow 

 band of frequencies, can allow so little phase distortion within this 

 band for satisfactory transmission of a picture, that the use of H-174* 

 side circuits for distances greater than about 100 miles requires the 

 use of some means for correcting the phase distortion introduced by 

 the loading on the cable pairs. The transmission of programs for the 

 interconnection of radio broadcasting stations requires a wide fre- 

 quency band for satisfactory quality; and unless corrected, effects 

 due to phase distortion outside the usual telephone range of frequen- 

 cies, which would not be very disturbing for telephone conversation, 

 make the program transmitted unsatisfactory to the listener. Tele- 

 vision, of course, with its very wide frequency band and its very rigid 

 requirements regarding phase distortion, does not allow even the use 

 of open wire circuits for its transmission for any great distance without 

 the aid of phase correction.'* 



It is the purpose of this paper to describe and discuss various meth- 

 ods which have been devised for the measurement of phase distortion. 

 Phase distortion and its effects, as well as methods of correcting for 

 jj- 6,6, 7, 8^ are considered here only sufficiently for the understanding of 

 these measuring means. It is, of course, necessary that before the 

 correction can be designed, the amount of distortion be known, and 

 that, after the corrective apparatus has been built and applied to 

 the circuit, the overall system be checked to find out how complete 

 the correction has been. The devices described below are for this 

 particular purpose, and before describing them, the fundamental 

 theory upon which they are based will be considered. Some of the 

 principles underlying particular methods of measurement will be 

 considered in the description of the devices themselves. 



After the discussion of the various measuring devices, certain data 

 will be given which give the results of various measurements made on 

 actual telephone circuits with some of these measuring devices. 



^ "The Transmission of Pictures over Telephone Lines," H. E. Ives, J. W. Horton, 

 R. D. Parker, A. B. Clark, B. S. T. J., Vol. IV., p. 187, 1925. 



* In designating loading systems, the initial letter denotes spacing, H denoting" 

 6000 feet and B denoting 3000 feet; the first number denotes the inductance of the 

 loading unit in the side circuit in millihenries; a second number denotes the inductance 

 of the loading unit in the phantom circuit in millihenries; the letter A'^ following the 

 number denotes non-phantomed pairs. 



*"Wire Transmission Sj'stem for Television," D. K. Gannett and E. I. Green, 

 A. I.E. E. Transactions, Vol. XLVI, p. 946, 1927 and B. S. T. J., Vol. VI, p. 616, 1927. 



' "Phase Distortion and Phase Distortion Correction," Sallie Pero Alead, B. S. T. 

 J., Vol. VII, p. 195, 1928. 



^ "Distortion Correction in Electrical Circuits with Constant Resistance Recurrent 

 Networks," O. J. Zobel, B. S. T. J., Vol. VII, p. 438, 1928. 



^ "Phase Distortion in Telejihone Apparatus," a companion paper by C. E. Lane. 



'"Effects of Phase Distortion on Telephone Quality," a companion paper by 

 J. C. Steinberg. 



