Delay Equalization of Eight-Kilocycle Carrier Program Circuits

N T R A N S M I T T I N G programs for radio broadcasting over the United States, 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 5 applicable to broad-band carrier facilities. The selection of a single sideband requires sharp frequency discrimination, and when this discrimination is achieved with minimum-phase structures, it is of necessity accompanied by delay distortion. 3 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, flexibility 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 necessary. 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 amplifiers 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 excessive distortion.