WIDE BAND TRANSMISSION OVER BALANCED CIRCUITS 3 



including overcoming of the variations due to the effect of temperature 

 changes. 



Stable and highly linear repeater gain can be produced so readily 

 now-a-days, thanks to the negative feedback amplifier invented by 

 Mr. H. S. Black,'* that the idea of such high attenuations is no longer 

 appalling even though on 16-gauge pairs it means repeaters spaced only 

 about four miles apart. Overcoming the transmission variations due 

 to temperature with automatic regulators introduces no fundamentally 

 new problems, but, of course, the complexity and precision of regulation 

 must be considerably higher due to the considerably larger variations. 



Crosstalk, of course, must be given special consideration. First of 

 all, it is necessary to restrict transmission of a given high-frequency 

 band to only one direction in a single cable; the other direction must be 

 supplied by another cable ^ or other separate transmission medium. 



Considering transmission in one direction only, if only one pair in a 

 cable is set aside for high-frequency transmission of, say, a million- 

 cycle band, most, but not all, of the crosstalk difficulty can be avoided. 

 However, the fact must be reckoned with that if one pair in a cable is 

 singled out and an amplifier is applied having 60 db or more gain at a 

 point intermediate between voice-frequency repeater stations, the 

 amplifier will have a strong tendency to sing due to crosstalk between 

 the pairs connected to the input and output and the other pairs in the 

 cable. If two cables are available, this difficulty can be avoided by 

 jumping from one cable to another every time the high-frequency 

 amplification is introduced. If two cables are not available, overcom- 

 ing the difficulty may call for the insertion of high-frequency choking 

 devices in some or all of the low-frequency cable conductors at points 

 where high-frequency amplification is introduced. 



Considering now crosstalk between two circuits in a cable trans- 

 mitting in the same direction, assuming the amplifier difficulty to have 

 been overcome, tests have been made on various cables in the field 

 from which these conclusions have been drawn. For telephone 

 message purposes it is probably uneconomical to apply million-cycle 

 frequency ranges to more than a single pair in an existing cable. 

 However, with television, the crosstalk requirements are much less 

 exacting. This is because the range of intensities necessary for a good 

 television image is much less than is needed to accommodate message 

 telephone subscribers and, therefore, a considerably larger ratio of 

 extraneous current to maximum signal current can be tolerated. 

 Tests indicate that two or more television channels, each a million 



* "Stabilized Feedback Amplifiers," H. S. Black, Elec. Engg., January, 1934; 

 Bell Sys. Tech. Jour., January, 1934. 



