DISTORTION CORRECTION 483 



The effect of adding these two sections successively to the cable 

 circuit is shown in Fig. 15. It will be seen that the first section, 

 besides equalizing the attenuation up to about 2000 cycles per second, 

 produces as well approximately ideal results on the time-of-phase- 

 transmission at the lower frequencies. The complete circuit attenua- 

 tion departs less than .2 T.U. from a constant value everywhere 

 over the assumed frequency range. If desired, the time-of-phase- 

 transmission could be improved also at the upper frequencies by the 

 addition of proper phase networks. Such a type of correction will be 

 made in the following application. 



4.4. Distortion Correction in Open-Wire Television Circuit 



The networks to be described here were designed by the writer 

 especially for the particular open-wire circuit which was used for the 

 television demonstrations from Washington, D. C, to New York City 

 on April 7, 1927. They were designed entirely from calculated data, 

 some of which had previously been derived from measurements on other 

 similar lines, as the complete circuit was not available for measure- 

 ments until later. 



The circuit had a total length of about 285 miles, being made up 

 principally of 276.4 miles of 165-mil open-wire pair together with 

 8.43 miles of necessary entrance, submarine and underground 13- 

 gauge carrier-loaded cable (C-4.1). The iterative impedances of these 

 two types of lines are very nearly the same in the frequency range 

 considered and were so assumed in what follows. Hence, the propa- 

 gation length of the circuit was taken as the sum of the propagation 

 lengths of the parts. In order that such a circuit be suitable for 

 television transmission it must be made to have extremely high quality 

 over a very wide frequency range by means of distortion correcting 

 networks. The requirements which the design of the present net- 

 works aimed to meet follow. 



Design Requirements 



1. An impedance of 600 ohms is to terminate the line at each end. 



2. The attenuation, or insertion loss, of the corrected circuit is 

 to be constant within ± 1 T.U. over the entire frequency range from 

 10 to 20,000 cycles per second. 



3. The time-of-phase-transmission of the corrected circuit is to be 

 constant within ± 500 microseconds (lO"*") from 10 to 400 cycles per 

 second, and to be the same constant within ±10 microseconds from 

 400 to 20,000 cycles per second. 



4. Provision is to be made for distortion correction under various 



