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BELL SYSTEM TECHNICAL JOURNAL 



band have been available for some time.^^ New amplifier designs have been 

 worked out for use with balanced lines. In general, the problem is to provide 

 approximately zero loss and constant delay between unbalanced terminals a mile 

 or more apart. Thus, an unbalanced to balanced amplifier is required at the 

 sending end, the converse at the receiving end. If the circuit is long, balanced 

 amplifiers are most convenient for use at intermediate points. The equalization 

 problem has been successfully met even if ordinary telephone cables are used. 

 A series of variable equahzers have been experimented with which have several 

 degrees of flexibility. A variety of circuits ranging in length up to 9 miles have 



Fig. 13 — Photograph of Video amplifier 



been equalized with this arrangement with considerable success. A typical 

 amplifier, equahzer, and power supply are shown in the photograph, Fig. 13. 



Telephone Cables 



Ordinary fine wire paper insulated cables have very high attenuation at the 

 frequencies required. Typical values for loss and net loss after amplification and 

 equalization are shown in Fig. 14. Experience has shown that the noise levels 

 in such cables even at the higher video frequencies are rather high so that ampli- 



