Phase Phiddling (1974)

By I. Switzer, Switzer Engineering Services Limited

The system of harmonically related carriers (HRC) used on the St. Catharines system is based on studies begun in early 1970. A paper presented at the June, 1970 convention of the National Cable Television Association, in Chicago, reported on applications of phase lock loop (PLL) technology in cable television. Maclean-Hunter had successfully used PLL techniques to overcome some "direct pick-up" problems in CATV systems. The paper reported these results and speculated on further applications of PLL in CATV. One of the applications considered was the third order intermodulation problem (triple beat) in CATV systems. The paper suggested that PLL techniques could be used to generate carrier systems with uniform spacings. Third order intermodulation products would then be "zero beat" and their visibility would be significantly reduced, Not much was known about the problem at the time and the speculative discussion in the 1970 paper erred on the conservative side in assessing the importance of third order intermodulation in CATV systems. Third order intermodulation has since been shown to be more important in CATV system picture degradation than was suspected in the 1970 paper.

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