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BELL SYSTEM TECHNICAL JOURNAL 



Coaxial units of the size discussed above have been used in a few cases a mile or 

 so in length. Even for such distances, however, it has been found desirable to 

 reduce the power interference by balancing it out. One method which we have 

 used is shown in Fig. 15. This has given an improvement at power frequencies 

 of the order of 50 db in certain cases. 



Balanced Shielded Cables 



The ideal type of transmission line for video signals combines the balance 

 feature with low attenuation and a high-frequency shield. The distance over 

 which such cables could be used appears to depend upon the perfection of balanced 

 video amplifiers and the equalization, although power interference may also present 

 difficulties. Such cables have been built using a pair of wires and a disc type of 

 insulation analogous to the coaxial structure described above. Attenuation 

 measurements on a 3-mile test length installed in New York City are shown in 

 Fig. 14. This figure also shows the net result after amplifiers and equalizers were 



Fig. 15 — 60-Cycle balance on coaxial line 



added. The attenuation of this special type of cable is such that amplifiers would 

 be required at only about 5-mile intervals. The useful range of such a cable for 

 video transmission has not been determined but in any case it should be con- 

 siderably greater than that of the paper-insulated telephone cable circuit. 



Experiment in Network Broadcasting 



During the last week of June 1940, the proceedings of the Republican Conven- 

 tion in Philadelphia were broadcast in New York by television. The facilities 

 used included the 3-megacycle coaxial system plus certain video connections at 

 each end as shown in Fig. 16. 



Because of the interest in this circuit and its good performance in transmitting 

 441 -hne television, the overall attenuation and delay characteristics are given on 

 Fig. 17. It will be noticed that a net band of about 2j megacycles was transmitted 

 and that over most of that band the delay distortion did not exceed ±0.2 micro- 

 second. The random noise, modulation and other distortions introduced by the 

 wire line network appeared to be unimportant when viewed on a commercial 

 television receiver. 



