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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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JackAsser
Registered: Jun 2002
Posts: 2014 |
Quote: 1
12
32
3
^^^means the TV can shift scanlines "half" scanline down, but as someone said the TV knows nothing about interlace in itself so how can you tell it to shift one half scanline?
It's all rather simple actually.
When the TV hits the VSYNC at the middle of the last raster line it has to move up to line 0 immediately, however there is still half a raster line to go. So in effect the raster beam will continue it's journey for the remaining time (and also move down at the same pace as normal). So finally when it hits the HSYNC it will have moved down for 0.5 raster lines.
In numbers:
At line 313 on the even field it hits VSYNC in the middle.
even field:
1) x=middle y=313 VSYNC (jump to odd field)
odd field:
2) x=middle y=0
3) x=right y=0.5 HSYNC
4) x=left y=0.5
Remember the beam ALWAYS moved in this down-right slow slope, VSYNC and HSYNC simply resets either X, or Y. The slope remains.
Got it? |
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Oswald
Registered: Apr 2002
Posts: 5086 |
ah another confusing info ! :D
so the scanlines are not totally horizontal ??! :)
leftmost Y=0, rightmost Y= 1 ? :) |
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JackAsser
Registered: Jun 2002
Posts: 2014 |
@oswald: that's my impression of it yes. ;D |
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chatGPZ
Registered: Dec 2001
Posts: 11360 |
ehrm, the scanlines are horizontal for their visible part (beam moving left to right) but they are not for their invisible retrace part (beam moving right to left).
-- scanline
/ retrace
/
-- scanline
/ retrace
/
--
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Oswald
Registered: Apr 2002
Posts: 5086 |
jack, according to groppie you have to come up with a new explanation :) |
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JackAsser
Registered: Jun 2002
Posts: 2014 |
Quote: jack, according to groppie you have to come up with a new explanation :)
Well then, let's all join the club of major confusion. ;D Without having a constant slope I can't explain how interlace would have worked. I mean that slope could be counter acted by rotating the cannon a bit... But I'm not the TV-technician really. |
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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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