LONG DISTANCE CABLE CIRCUIT 



591 



of transmission through the combination of cable and networks for 

 all frequencies in the band to be transmitted. Fig. 19 shows the 

 delay characteristic of a section of cable 50 miles in length, with and 

 without the delay-equalizing networks. The delay is seen to be main- 

 tained within db 0.05 milli-second of a constant value. A schematic 

 circuit of these networks is shown in Fig. 20. With the greatest 



MUTUAL INDUCTANCE BETWEEN 

 THESE TWO WINDINGS 



THIS COIL IN SAME POT WITH 

 OTHER TWO WINDINGS 



Fig. 20 — Schematic circuit of section of delay equalizer. For a 50-mile equalizer, 

 three kinds of sections are used which vary in the resonant frequency and in the 

 sharpness of resonance. The first three sections are of one kind, the fourth is of 

 another and the last seven are of the third kind. 



length of cable circuits which will be used in this country for program 

 transmission, this amount of deviation per section is not sufficient 

 to cause objectionable distortion. For a 50-mile section uncorrected, 

 the delay at 8,000 cycles would be 0.9 milli-second greater than at 

 1,000. A description of these delay-equalizing networks with the 

 theory of their performance is being presented in another paper so 

 that a more detailed description is omitted in this paper. 



Office Wiring 



Owing to the wide frequency range transmitted over the circuit, 



special care must be taken with the office wiring. This is to avoid 



excessive variations in the losses introduced by this wiring due to 



changing humidity conditions. A new type of insulated cable is used 



in which the textile material of the insulating wires has been very 



thoroughly washed to remove all traces of foreign substances, so that 



the absorption of moisture with its accompanying increase in loss is 



greatly reduced.^ The office cabling is also shortened as much as 



possible, the outside cable connecting directly to the repeaters without 



* H. H. Glenn and E. B. Wood, "Purified Textile Insulation for Telephone 

 Central Oftice Wiring," .4. /. E. E. Transactions, Vol. 48, April. 1929. 



