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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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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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Oswald
Registered: Apr 2002
Posts: 5086 |
Quote: @oswald: any more loose ends?
all my ends are on loose, I simply gave up. |
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Graham
Account closed
Registered: Dec 2002
Posts: 990 |
@Magervalp:
The problem is that the deinterlacing does not work. Some movies are 25 fps, some are 50 fps, and sometimes it's even 25 and 50 fps mixed. So even IF you have a deinterlacer, the better movies and animations (50 fps) will look jerky. |
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ChristopherJam
Registered: Aug 2004
Posts: 1408 |
Well, unless it's got some kind of motion prediction, a 50Hz scroll will ghost as badly on a 100Hz display as a 25Hz scroll does on a 50.
I saw someone trying to play some Amiga games on a 100Hz TV once. It wasn't pretty. |
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MagerValp
Registered: Dec 2001
Posts: 1074 |
Of course, deinterlacing is a complicated process, and only ever works most of the time, never always. High end displays do a good job with natural images. They are completely unsuited for displaying computer images though.
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