[cctalk] Re: interlace [was: NTSC TV demodulator ]

On May 20, 2024, at 10:51, Paul Koning via cctalk <cctalk(a)classiccmp.org> wrote:  VT100? I don't think so. And yes, it has a setup menu, but that's setup of the terminal functionality, not the monitor part. The earliest monitors could only handle one format. A major innovation was "multisync" where the monitor would determine the horizontal and vertical sweep rate and line count, and display things the right way. The first PC I owned had one of those, and as far as I can remember it had nothing that one would call a "setup menu". The reason interlace matters is not the very slight slope of the scan line in analog monitors, but rather the fact that alternate frames are offset by half the line spacing of the basic frame, so each frame sweeps out the gaps in between the lines scanned by the preceding frame. It matters to get that right, otherwise you're not correctly displaying consecutive rows of pixels. In particular, when doing scan conversion (from analog format to a digital X/Y pixel raster) you have to offset Y by one every other frame if interlace is used, but not if it isn't. paul