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Smooth Scrolling and Crawling problems
Question
ID
124
Category
General - Product Support
Date Created
18/02/2004 16:07:38
Date Updated
18/03/2009 11:30:13
1) When overlaying computer generated graphic crawls over NTSC video I am experiencing a problem that every 16 seconds (approximately) there is a glitch in the crawl. (background video is OK). I am also not able to get “smooth” crawls with any Vertical refresh other than 60 HZ.

The VGA output is smooth at all refresh rates.

Answer
1) This is the equivalent of a 'beat pattern', that you get when two frequencies are slightly different.

In this case, the VGA card is running at 60Hz and the NTSC output from our unit is running at 59.94Hz (this is the NTSC standard and cannot be changed). This gives a difference of 0.06Hz, which is one 'beat' every 16.6 seconds.

Every scan convertor works by storing the complete frame from a VGA source in memory and re-sending it out at a different rate. When there are fewer output frames going out than coming in, it has to duplicate the last frame - again, this is what all scan convertors do. Hence every
16.6 seconds, using the above example, you'll get a slight glitch.

The only way to avoid this completely is to use exactly the same number of frames going in and out - ie. 59.94Hz in, 59.94Hz going out. This is virtually impossible, but the closer you get the fewer glitches there are. Sometimes it's more acceptable to have more glitches per second,
instead of the odd one that comes along every few seconds or so that is more annoying.

In short, smooth-scrolling is a virtual impossibility with scan convertors. TV stations converting PAL (50Hz) to NTSC (60Hz) often merge several frames together to avoid these glitches - which is why some fast moving credit titles appear blurred. The same could be done for scan convertors, but you'd get a nasty latency when images moved.

Also see Frame rate conversion in CORIO2 products
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