354 BELL SYSTEM TECHNICAL JOURNAL 



making the necessary operating observations and a predistorting net- 

 work and associated amplifier. 



At the intermediate station are included line filters for separating 

 the carrier currents and program currents and directing them to their 

 proper channels, two adjustable attenuation equalizers for correcting 

 for the attenuation distortion of the line wires and associated appa- 

 ratus, gain control attenuator, line amplifier and associated monitoring 

 equipment. At intermediate stations where it is necessary to provide 

 branches to radio stations or to other program circuits an amplifier of 

 a special type having several outlets is inserted immediately in front 

 of the line amplifier. 



At a receiving terminal, the layout employed is very similar to that 

 utilized at intermediate stations except that an additional low-pass 

 filter and a restoring network are inserted ahead of the receiving 

 amplifier. 



A novel feature is provided in this program system for minimizing 

 its susceptibility to interference at higher frequencies. It consists in 

 predistorting the transmission at the sending end of the circuit so 

 that currents above 1,000 cycles are sent over the line at a higher level 

 than if this arrangement were not employed, thus increasing the signal- 

 to-noise ratio at these frequencies. Such an increase in power at high 

 frequencies is permissible without overloading in the line amplifiers 

 in view of the fact that the energy content of the program material 

 above 1,000 cycles is materially less than at the low frequencies and 

 decreases rapidly as the frequency is increased. In order to restore 

 the program material to the same relations it would have if it were not 

 predistorted, a network is inserted at each point in the branches 

 which feed the radio stations and at the receiving terminal. This 

 network introduces attenuation and phase distortion which are com- 

 plementary to those introduced at the sending end of the circuit by 

 the predistorting network. The net reduction in high-frequency inter- 

 ference is equal to the discrimination introduced by the predistorting 

 network in favor of these frequencies, and is therefore equal approxi- 

 mately to the loss of the restoring network at the same frequencies. 



In the lower part of Fig. 1 is shown a level diagram, from which 

 may be noted the losses and gains introduced by different parts of the 

 system at a frequency of 1,000 cycles. The maximum volumes which 

 are permitted in the various parts of the system are also indicated 

 approximately by this diagram. 



Line Facilities 

 As is well known the open-wire lines employed in telephone and 

 program service do not have uniform attenuation for all frequencies, 



