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Back in 1979, I worked at a company that made devices to convert video signal from hospital CAT scanners to film hard copy. Basically a lens system projecting video from a CRT faceplate onto film. 'Twas a standard issue job at a typical company that made a boring product. Most were 525i black and white, but we did a few special jobs for 525p and maybe higher (it's been a while....). We were able to hack most of the horiz deflection to run at the higher frequency (mostly RC time constants in the Conracs we used). The flyback period tended to be an issue, in that the horiz interval would be shorter for 525p than 525i, but the capacitance loading of the high voltage winding on the flyback would show down how fast the deflection could get back to the other side of the CRT screen. This would result in overscan, as the left and right edges of the image would be clipped. The duty cycle of the flyback pulse vs the electron beam writing the video on the CRT screen would change some, but that wasn't too big a problem.
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