A NEW TELEPHOTOGRAPH SYSTEM 573 



of these paths are 180 degrees out of phase with the currents in the 

 others, and hence would result in infinite attenuation of the echo 

 were it not for small unbalance currents. Measured crosstalk losses 

 for the echo paths in excess of 70 decibels have been obtained for 

 these bridging networks manufactured with ordinary tolerances. 



Certain auxiliary features may also be incorporated in telephoto- 

 graph networks to assist in their operation and perform other related 

 functions. For example, telephotograph methods are not efficient in 

 their present form for the rapid exchange of operating instructions; 

 therefore telephone facilities may be associated with a telephotograph 

 network for use by the customer in coordinating the operation of this 

 system. Arrangements may be used whereby such voice communica- 

 tion may be carried on over the telephotograph circuit between 

 picture transmissions, and loud speakers may be bridged on the circuit 

 for monitoring purposes. 



Acknowledgment 



The attainment of this objective in telephotograph development 

 and the establishment of the present leased wire network has engaged 

 the initiative and resourcefulness of several score of individuals at the 

 Bell Telephone Laboratories, Inc., the Western Electric Company, 

 and the American Telephone and Telegraph Company. In reviewing 

 the advances which have been made, the practical limitation of space 

 has made it impossible to discuss in greater detail the various phases 

 of the work and to render individual recognition to all who have 

 contributed to the solution of the problems involved. Among those 

 most intimately concerned and through whose efforts the many details 

 have been worked out and correlated are W. A. Phelps and P. Mertz 

 of the Bell Telephone Laboratories, Inc., and I. E. Lattimer of the 

 American Telephone and Telegraph Company. 



Referenxes 



1. "The Transmission of Pictures Over Telephone Lines," H. E. Ives, J. \V. Horton, 



R. D. Parker, and A. B. Clark, Bell System Technical Journal, v. 4, April 1925, 

 pp. 187-214. 



2. "Synchronization of Television," H. M. Stoller and E. R. Morton, Bell System 



Technical Journal, v. 6, Oct. 1927, pp. 604-15. 



3. "Synchronization System for Two-Way Television," H. M. Stoller, Bell System 



Technical Journal, v. 9, July 1930, pp. 470-6, and discussion, p. 477. 



4. "A Permanent Magnet Light Valve," G. E. Perreault, Bell Laboratories Record, 



V. 10, Aug. 1932, pp. 412-16. 



5. "Certain Topics in Telegraph Transmission Theory," H. Nyquist, A. I. E. E. 



Transactions, v. 47, April 1928, pp. 617-44. 



6. "A Theory of Scanning and Its Relation to the Characteristics of the Transmitted 



Signal in Telephotography and Television," Pierre Mertz and Frank Gray, 

 Bell System Technical Journal, v. 13, July 1934, pp. 464-515. 



7. "An Oscillograph for Ten Thousand Cycles," A. M. Curtis, Bell System Technical 



Journal, v. 12, Jan. 1933, pp. 76-90. 



