1432 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1957 



equipment produce a cumulative gain-frequency distortion which is 

 noticeable in television circuits. Present practice is to correct for this 

 by standard video equalizers after the FM signal has been demodulated 

 to baseband. In connection with the experimental eciualizing program 

 to be described, parabolic gain equalizers operating on the FM signal 

 before demodulation were used. 



III. RESIDUAL DISTORTION 



After correction of the known shapes discussed above, there remains 

 a certain residual gain and delay distortion which results from a random 

 summation of many minor sources. The shape of this distortion is not 

 predictable, but its statistics are known. Examination of typical delay 

 versus frequency characteristics have shown that these may be reason- 

 ably well approximated by six cosine terms: a 40-mc fundamental and 

 the next five harmonics. Similar gain terms are needed. However, the 

 gain and delay distortion, when examined within the 20-mc band of 

 interest, do not have a minimum phase relationship. This is to be ex- 

 pected because of the presence in the system of the delay eciualizers, 

 which are non-minimum-phase networks, and of amplifiers with com- 

 pression. 



The magnitude of the residual distortion is small enough so that trans- 

 continental TD-2 circuits provide television and telephone transmission 

 of commercial quality. Some effects, such as cross modulation, are 

 sufficiently marginal so that improvement would be desirable. To deter- 

 mine whether this could be achieved by improved gain and delay eciuali- 

 zation, the development of an experimental adjustable equalizer was 

 undertaken. The considerations outlined show that such an equalizer 

 should approximate the desired characteristics with independent gain 

 and delay terms of the harmonically related cosine type. Equalization 

 to reduce cross modulation in telephone channels and differential phase 

 in color television must be performed before demodulation of the FM 

 signal to base band. The equalizer was, therefore, built to operate in 

 the 60- to 80-mc IF band. 



IV. TRANSVERSAL EQUALIZER 



One method of obtaining independent control of the loss and delay 

 characteristics of a network has been achieved in the transversal filter." 

 Equalizers have been designed on this principle for the equalization of 

 television circuits. ' This type of equalizer, referred to here as a trans- 

 versal equalizer, provides a flexible means of synthesizing any loss char- 



