LINE FILTER FOR PROGRAM SYSTEM 383 



This low-pass line filter, with its associated high-pass filter, makes it 

 possible to use the open-wire lines simultaneously for wide-band 

 program service and for commercial carrier telephone service, without 

 impairing the quality of the program. It represents an improvement 

 over older types of line filters, as well as an advance in the technique 

 of equalization in filters. In cases requiring careful delay and loss 

 equalization, it has been the usual practice to design the filter first 

 to supply the required discrimination or filtering action, and then 

 design a delay corrector to correct for the delay distortion in the 

 filter, after which a loss equalizer is designed to correct for the ampli- 

 tude distortion in both the filter and the delay corrector. The loss 

 equalizer introduces a small delay distortion which usually can be 

 anticipated and corrected in the delay corrector. In the wide-band 

 program filter the functions usually performed by these three separate 

 types of networks have been combined, with a consequent saving in 

 cost and space. 



Requirements to Be Met by Program Filter 



To function effectively as a line filter, the low-pass filter must 

 provide sufficient discrimination against carrier currents to make their 

 effect completely inaudible in all the receivers connected to the pro- 

 gram system. Discrimination varying from 46 to about 90 db is 

 necessary to accomplish this end. Because of the presence of an 

 auxiliary low-pass filter ^ which supplies considerable loss in the fre- 

 quency ranges where the requirement is unusually severe, each line 

 filter need furnish discrimination varying only from 40 to about 60 db. 



From the standpoint of program quality, it is essential that the 

 line filter, while furnishing the foregoing discrimination, shall not 

 introduce any appreciable distortion into the program. This require- 

 ment would call for nothing unusual in the way of filter design if there 

 were only a few filters in the system. Long open-wire program 

 systems, however, may extend as far as 3,000 or 4,000 miles, and may 

 contain as many as 50 low-pass line filters. A program that has 

 traversed such a circuit still must be comparable in quality to a 

 program that is broadcast from the point at which it originated. 

 Since the system contains much other apparatus, such as equalizers 

 and amplifiers, each low-pass line filter can be permitted to introduce 

 not more than about 1/100 of the distortion that can be tolerated 

 in the whole system, assuming 50 filters in the system. 



There are two types of distortion that must be controlled very 

 carefully in the program filter: these are (1) amplitude distortion, and 

 (2) delay, or phase, distortion. Amplitude distortion is introduced 



