518 BELL SYSTEM TECHNICAL JOURNAL 



television circuits.^^ The necessity in both cases is similar. In picture 

 transmission the frequencies between 900 and 1700 cycles were in- 

 volved and a maximum departure for the system as a whole of dB/doi 

 from a constant value of .0005 seconds in this frequency range was found 

 permissible. In television it was considered desirable to transmit 

 frequencies over the range from 10 cycles to 20,000 cycles such that 

 the value of dBjdw as a function of frequency was constant about 

 ±.00002 seconds over all but the lowest part of the range. There at 

 the very lowest frequency ±.001 seconds was considered permissible. 



Bibliography 



Carson, J. R., and Zobel, O. J. Transient Oscillations in Electrical Wave Filters, 



Bell Sys. Tech. Jour., July, 1923. _ 

 Carson, J. R. Building Up of Sinusoidal Currents in Long Periodically Loaded 



Lines, Bell Sys. Tech. Jour., Oct., 1928. 

 Clark, A. B. Telephone Transmission for Long Cable Circuits, Bell Sys. Tech. Jour., 



Jan., 1923. 

 Decker, Hans. Dampfungsentzerrung und Phasenverserrung, E. N. T., Vol. V, 



1928. 

 KiJPFMULLER. Die Erh6hung~der Reichweite von Pupinleitungen durch Echosper- 



rung und Phasenausgleich, E. N. T., Vol. Ill, 1926. 

 KiJPFMULLER AND Mayer. Uber Einschwing Ver Gauge in Pupinleitungen und ihre 



Verminderung, Wiss. Ver. a. d. Siemans-Konzern, Vol. V, 1926. 

 KiJPFMULLER. iJber Beziehungen Zwischen Frequenzcharakteristiken und Aus- 



gleichsvorgangen in Lineoren Systemen, E. N. T., Vol. V, 1928. 

 LiJscHEN. Die Technik der Telegraphie und Telephonie in Weltverkehr, E. T. Z., 



July, 1924. 

 Mead, S. P. Phase Distortion and Phase Distortion Correction, Bell Sys. Tech. 



Jour., Apr., 1928. 

 Sandeman, E. K., Rendall, A. R. A., and Turnbull, L L. Phase Compensation, 



Elec. Comm., Vol. VII, Apr., 1929. 

 Zobel, O. J. Distortion Correction in Electrical Circuits, Bell Sys. Tech. Jour., 



July, 1928. 



Appendix I 



Phase Shift Defined 



Networks with a pair of input terminals and a pair of output 

 terminals such as lines, filters, equalizers, phase correctors, transform- 

 ers, etc., are designed to work between a source of e.m.f. £„, of impe- 

 dance Za, and a receiving device of impedance, Zh. The source of 

 e.m.f. is generally spoken of as the generator and the receiving device 

 as the load. Such a network, N, connected to a generator and a load 

 are shown in Fig. 28. Terminals 1 and 2 are the input terminals and 

 3 and 4 the output terminals. 



For any frequency let Za = Wa be the image impedance -^ at ter- 



^^ Production and LItilization of Television Signals, F. Gray, J. W. Horton and 

 R. C. Mathes, Bell Sys. Tech. Jour., October, 1927; Wire Transmission Systems 

 for Television, D. K. (iannett and H I. Green, Bell Sys. Tech. Jour., October, 1927. 



-^ The image impedance at either end of a four terminal network is given by the 

 square root of the product of two impedances at that end, one measured with the 

 opposite end short circuited and the other with it open circuited. 



