PRIVATE LINE DATA TRANSMISSION 1481 



cycles. This contains an element of conservatism inasmuch as the strict 

 requirement is really fully implied only on the nominal effective band 

 (1,200-2,000 cycles). The signal power is reduced in the roll-off and 

 vestigial bands, respectively 1,000-1,200 and 2,000-2,500 cycles, and 

 some corresponding liberality may be expected there. 



The delay distortion constitutes a more serious problem with a faster 

 system as compared with a slower one, in part because of the wider 

 frequency band occupied by it, and in part because 0.4 signal element 

 represents a more severe tolerance in microseconds for a shorter element 

 than for a longer one. Consequently, the limits given represent about as 

 severe tolerances as may be expected to be needed with the use of a 

 telephone channel. 



The distortions of various circuits have been considered to estimate the 

 order of the problem involved in meeting the proposed requirements over 

 Unks of 100 to 500 miles. 



The following conclusions are reached first for the vestigial sideband 

 signal, and after this for the slower systems. 



3.4.1 Facilities Requiring No Treatment 



As already noted, K2, LI, and L3 carrier, and TD-2 microwave, use 

 "A" channel banks to separate the individual channels, and these give 

 the dominant delay distortion. This amounts to a maximum of about 

 200, and a minimum of 150 microseconds, according to the exact com- 

 bination of filters used. This figure is for one link of transmitter and 

 receiver. A single section delay equilizer can cut the maximum residual 

 to about 80 microseconds. It is concluded that these facilities present no 

 important delay distortion problems. An Nl carrier link gi^'es a maxi- 

 mum delay distortion of 220 microseconds, which can be reduced to 50 

 microseconds by one section of equalizer. This, then, also presents no 

 serious problem. 



3.4.2 Facilities Treated by Simple Prescription 



The delay distortion of H-44 voice frequency cable in the 1,000- to 

 2,400-cycle range runs to slightly under 900 microseconds for 300 miles, 

 if the cable is of standard toll capacitance (0.062 mf per mile), and to 

 slightly under 2,000 microseconds if of higher local plant capacitance 

 (0.084 mf per mile). The use of about one section of ecjualization per 100 

 miles reduces the residual to less than 330 microseconds for the low 

 capacitance cable. For the higher capacitance cable, about three sections 

 are needed per 110 miles. The J-2 carrier uses A channel banks, but has, 

 in addition, directional separation filters at each repeater. This gives 

 maximum and minimum distortions, respecti^'ely, for 100 miles, of 

 slightl}'' under 300 and slightly under 160 microseconds. The precise 



