132 BELL SYSTEM TECHNICAL JOURNAL 



shows schematically the circuit of such an amplifier, but the actual 

 detailed design is not simple and involves great technical skill. 



Since the high amplifications just discussed are employed to offset 

 the equally high attenuation of the line structures, careful consideration 

 must be given to the stability or constancy of the attenuation caused 

 by the line structure. The fact is that as the temperature changes, 

 the attenuation of any line structure varies correspondingly. If the 

 line structure is underground in cable, the temperature changes are 

 slow in rate and the variations in line attenuation correspondingly 

 slow, but if the structure is in aerial cable or consists of open wire, 

 not only do we have variations in temperature with the season of the 

 year, but large daily variations as well. With an aerial cable, for 

 example, the change in loss of 19-gauge B & S non-loaded pairs through- 

 out the year for a circuit 1,000 miles (1600 km.) in length amounts to 

 approximately 500 db in the frequency band we propose to use. For 

 our long cable circuits operated on voice frequencies, we have developed 

 automatic regulating means, so that amplification is varied to com- 

 pensate for changes in attenuation. With the much higher attenua- 

 tions and equally higher amplifications involved in broad-band 

 systems, more refined methods of compensating for temperature 

 changes are under development. This is a very serious problem and 

 sets one limit to the use of such systems. 



I spoke of the new type of amplifier, employing negative feedback, 

 which became available at a most fortunate time. It is almost 

 equally fortunate that, due to continued research and development, 

 new and simpler forms of electric wave filters became available. 

 Time does not permit me to go into details, but in these newer types 

 of electric filters suitably cut quartz crystals are utilized. Develop- 

 ments have also made it possible to use inductance coils with iron 

 cores. As a result of these two changes the filter structure is simplified 

 and its size reduced. 



Fundamental to the whole broad-band transmission development, 

 there are many other elements which have required much research 

 and development effort such, for example, as modulators and de- 

 modulators, but I shall be able to discuss only the more striking 

 problems underlying all broad-band systems as they pertain to certain 

 specific applications. 



Cable Carrier Systems 



We plan as a first step to apply the cable carrier system to pairs in 

 existing cables. These cables were designed and manufactured with 

 the expectation that they would be required to transmit frequencies 



