Delay Equalization of Eight-Kilocycle Carrier Program Circuits 



By C. H. DAGNALL and P. W. ROUNDS 



This paper clescrit)es the e(|uaIization of delay in 8-kc program systems trans- 

 mitted over broad-band carrier telephone facilities. Use is made of a condenser- 

 l)late ])otential analog which i)rovides a ready method for blocking out the basic 

 design and arriving at the final equalizer constants. Most of the equalization 

 is accomplished at audio frequencies, and the remainder at carrier frequencies 

 with quartz-crystal equalizers. 



IX TRANSMITTING programs for radio broadcasting over the I'nited 

 Slates, an extensive network of wire circuits has been established by the 

 Bell System. Most of the additions to this network since the war have 

 employed a single-sideband carrier system^ applicable to broad-band carrier 

 facilities. The selection of a single sideband requires sharp frequency dis- 

 crimination, and when this discrimination is achieved with minimum-phase 

 structures, it is of necessity accompanied by delay distortion. 



In one or two carrier links, each including a transmitting and receiving 

 terminal, the delay distortion is sufficiently small so that no deterioration 

 in the program is noticeable. However, flexibihty of maintenance and 

 operation of an extensive program network requires that the network be 

 built up of a large number of links in tandem. When this is done, the effects 

 of delay distortion become quite conspicuous and equalization of the delay 

 is necessar}-. Furthermore, if the equalization is to be satisfactory between 

 any two points in the network, each link must be independently equalized. 



Most of the delay distortion arises in the carrier-frequency band-pass 

 filter which selects the lower sideband, the small remaining portion being 

 contributed by the amphfiers and repeating coils. Figure 1 illustrates the 

 unequalized delay in one terminal. Equalizers have been added to each 

 terminal to make the phase characteristic approach linearity and so permit 

 at least ten links to be operated in tandem without e.xcessive distortion. 



Theory of Design 



The equalization of delay distortion is accompUshed through the use of 

 all-pass networks which, in their most general form,- may be constructed 

 as a tandem set of lattice sections of the type shown in Fig. 2. An electro- 

 static analogy, developed during the late thirties,^ has been found to be of 

 great assistance in visualizing the performance of these networks and in 

 indicating a rational method of design. 



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