PHASE DISTORTION IN TELEPHONE APPARATUS 



517 



the practical solutions so far have been largely empirical. This is due 

 to the comparative simplicity of the experimental method. The 

 results of a number of adjustments in the terminating apparatus may 

 be observed experimentally while one is investigated mathematically. 

 This is due to the fact that one can tell, using an oscillograph, from the 

 direct observation of the received signal when a proper adjustment is 

 reached. It is not necessary, as in telephony, to make complete 



0.0007 



0.0006 



0.0005 



to 

 o 



z 



O 0.0004 



U 



UJ 



to 



0.0003 



< 



_l 



UJ 

 Q 



0.0002 



0.0001 



200 



400 600 800 ipOO ipOO 3p00 SpOO 



FREQUENCY IN CYCLES PER SECOND 



7p00 9pOO 



Fig. 27 — Insertion delay characteristics corresponding to the phase characteristics of 



Fig. 26. 



articulation tests or a large number of single frequency measurements. 

 The advent of the continuously loaded cable made possible by the use 

 of permalloy has simplified the problem. A discussion of detailed 

 methods of phase correction in telegraph cable cannot be gone into 

 here. Circuits in general use have been described in previous publica- 

 tions.^^ 



Two other important places where it has been necessary to control 

 phase distortion are (1) in circuits for picture transmission '-■* and (2) 



-'•' The Loaded Submarine Cable, O. E. Buckley, Bell Sys. Tech. Jour., Julv, 1925; 

 High Speed Ocean Cable Telegraphy, O. E. Buckley, Bell Sys. Tech. Jour., April, 1928 ; 

 The Application of Vacuum Tube Amplifiers to Submarine Telegraph Cables, A. M. 

 Curtis, July, 1927; Automatic Printing for Long Loaded Submarine Telegraph 

 Cables, A. A. Clokey, Bell Svs. Tech. Jour., Julv, 1927; J. R. Carson (U. S. Patent 

 1,315,539) and R. C. iMathes (U. S. Patent 1,311,283). 



-■•Transmission of Pictures over Telephone Lines, H. E. Ives, J. W. Horton, 

 R. D. Parker and A. B. Clark, Bell Sys. Tech. Jour., April, 1925. 



