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Oswald
Registered: Apr 2002
Posts: 5086 |
VIC and the odd/even fields
okay this is getting confusing for me now. in another thread on lemon64 groepaz says that the vic does always display the same field, but it doesnt matter if its odd or even, and it lights up both the odd/even lines.
some1 can tell me:
- does vic use odd or even or dontcare lines?
- what does the term field mean from the tv electronics viewpoint?
- how does the vic only light up the odd (forex.) lines on the tv screen, and a little bit from the other ones?
- are there really 2 modes in the tv electronics for odd / even fields
- why does computer graphics flicker a hell more lot while "normal" tv screens looks as steady as my ass?
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Radiant
Registered: Sep 2004
Posts: 639 |
JackAsser: Since it only retraces one half scanline, the beam only moves down 0.5 scanlines. Quite simple, no? :-) |
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JackAsser
Registered: Jun 2002
Posts: 2014 |
Quote: JackAsser: Since it only retraces one half scanline, the beam only moves down 0.5 scanlines. Quite simple, no? :-)
@radiantx: put in that way it all feels rather logical then. ;D |
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White Flame
Registered: Sep 2002
Posts: 136 |
As the raster beam moves down the screen, it's moving in the vertical direction at a constant rate. It doesn't scan purely horizontally, then jump down vertically, etc. So by default the scanlines would actually appear diagonally, in more of a sawtooth configuration, with the diagonal saw edges being a raster line and the quick snaps back being the horizontal refresh. The actual scanning guts are slightly rotated inside CRTs so that the raster lines are horizontal on the final display, afaik.
This should help explain how a vertical refresh in the middle of a scanline offsets the entire picture by half a scanline, interlacing 2 fields. It's 2 sawtooth patterns with a 50% phase shift. |
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JackAsser
Registered: Jun 2002
Posts: 2014 |
Quote: As the raster beam moves down the screen, it's moving in the vertical direction at a constant rate. It doesn't scan purely horizontally, then jump down vertically, etc. So by default the scanlines would actually appear diagonally, in more of a sawtooth configuration, with the diagonal saw edges being a raster line and the quick snaps back being the horizontal refresh. The actual scanning guts are slightly rotated inside CRTs so that the raster lines are horizontal on the final display, afaik.
This should help explain how a vertical refresh in the middle of a scanline offsets the entire picture by half a scanline, interlacing 2 fields. It's 2 sawtooth patterns with a 50% phase shift.
@whiteflame: that was exacly how I thought it worked too until gropaz and radiantx "corrected" me. Dunno who's right though... ;D |
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ChristopherJam
Registered: Aug 2004
Posts: 1408 |
I suspect the only reason the visible scanlines are horizontal is from the horizontal circuit operating on a slight diagonal, so it drags the beam back up a bit as it goes to the right, compensating for the vertical circuit providing a constant downward motion.
If you assume for the sake of argument the lines are on a slight angle when a CRT is first constructed, it will probably be a lot simpler to understand the interlace. Making them horizontal later is just an adjustment that can be done by rotating some coils or feeding some current from one circuit to another.
(Oh, and to answer an earlier question, the iron mask on a CRT doesn't line up with the pixels at all - it just ensures each beam can only reach the right colour phosphors. Have a look at a reversed space with a magnifying glass, and count the green dots. There almost certainly won't be eight of them!) |
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AmiDog
Registered: Mar 2003
Posts: 97 |
And how can all this be applied to modern CRTs with digital filters, 100Hz, etc? (Not to mention all video equipment which I assume will have to know if the current frame is an odd or even field. Doesn't even a simple genlock need to know that to be able to sync the two video sources properly?)
At some point one had to start detecting the horizontal position of the V-Sync and act upon it. So, how old (or cheap?) would a TV need to be to still rely on that half scanline shifting the display down half a pixel for free? |
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MagerValp
Registered: Dec 2001
Posts: 1074 |
Quote: And how can all this be applied to modern CRTs with digital filters, 100Hz, etc? (Not to mention all video equipment which I assume will have to know if the current frame is an odd or even field. Doesn't even a simple genlock need to know that to be able to sync the two video sources properly?)
At some point one had to start detecting the horizontal position of the V-Sync and act upon it. So, how old (or cheap?) would a TV need to be to still rely on that half scanline shifting the display down half a pixel for free?
100 Hz CRTs with digital filters are way beyond what we're discussing here. Just think of them as digital displays, like LCDs.
And yes, the beam moves down in a zig-zag pattern, with constant vertical velocity. However, the zig-zag is rotated and skewed:
On each hsync it goes a little more to the left, adjusting the slope of the zig-zag pattern.
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Graham
Account closed
Registered: Dec 2002
Posts: 990 |
100 Hz CRTs are the biggest marketting joke ever. Despite the fact that they have a field rate of 100 Hz, they still are bound to display 25 Hz interlace :D |
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JackAsser
Registered: Jun 2002
Posts: 2014 |
@oswald: any more loose ends? |
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MagerValp
Registered: Dec 2001
Posts: 1074 |
Quote: 100 Hz CRTs are the biggest marketting joke ever. Despite the fact that they have a field rate of 100 Hz, they still are bound to display 25 Hz interlace :D
A good 100 Hz CRT, just like a good LCD or DLP, displays a progressive image, using the usual deinterlacing techniques. Cheap 100 Hz CRTs are a joke though.
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