Zig-zag artifacts on PAL image

Andrey Demenev

What is the reason of these artifacts on propeller generated PAL image? The zigzag lines move up the screen slowly. Not the moire - that is because of camera close to TV screen - but between color bars

Toby Seckshund

The horizontal line rate and the chroma subcarrier are supposed to be related together by 1135/4 + 25Hz offset, this gives the "chroma crawl". When the SubCarrier to Horizontal relationship (SCH) is so then this gives the least objectionable sort of crawl. On full broadcast bars the crawl can be seen on the magenta to green transition, where the phase has to shift 180 degrees, if you get close up and sweep your finger down the screen, over about 3 seconds, you will lock onto this. Subcarrier systems were only really intended as a bodge for moving pics with fairly large areas of simular colouration. Fine detail or static pics do not work so well. If the subcarrier is not locked to the horizontal, as most of the early computers were, then all you could hope for was for it to be close enough to lock the monitor.

ADDITion

Some LCD monitors etc will try to bring back this SCH and by doing so will cause the chroma to shift along in timing, a bit, then when a whole cycle (235nS) has been used up it will reset and start again. This gives a sawtooth look to the colour edges.

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Why did I think a new, more challenging, job was a good idea ??

Post Edited (Toby Seckshund) : 8/1/2010 9:11:12 AM GMT