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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 |
oswald. listen. the CRT is all about timing as magervalp says. Since one of the fields is delayed due to the last line is longer on the previous field, that effectivly moves down the display half a line.
Also, CRTs doesn't "skip" every 2nd line. see it like:
OE
DV
DE
_N
1
1
12
12
32
32
34
34
_4
_4
The CRT it self doesn't know about fields, it simply displays frames, a new one on each vertical sync. In PAL 50 per second and by altering the length of the last rasterline every 2nd frame you effectivly wiggle the display up and down half a pixel, thus get a preceived higher resolution to the cost of 25hz motion instead (u don't want the two fields to be separated in time because that leads to weaving artefacts when moving).
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Oswald
Registered: Apr 2002
Posts: 5086 |
jack, now that is starting to make sense, but brings up new questions: why is every "2nd" line darker when the vic drives the CRT if the CRT effectively scans every line ?
2ndly according to wikipedia the fields are indeed seperated in time.
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JackAsser
Registered: Jun 2002
Posts: 2014 |
@oswald, simply because the raster beam moves in a zig-zag pattern. During a horizontal sync the signal is ultra-black and when that happens the TV starts moving the beam back to the left + moves it down a bit. Hence, if your beam is narrow enough and your screen has a dense enough fosfor-thingie then you'll see the individual scanlines quite easy.
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JackAsser
Registered: Jun 2002
Posts: 2014 |
Quote: jack, now that is starting to make sense, but brings up new questions: why is every "2nd" line darker when the vic drives the CRT if the CRT effectively scans every line ?
2ndly according to wikipedia the fields are indeed seperated in time.
@oswald, the fields are NOT separated in time. Either u misunderstand the wiki OR the wiki is wrong. The fields CAN be separated in time, but TV-signals generally are NOT. Badly re-interlaced movies by lame-ass converter-kiddies usually have the fields separated though. ;D
Of course every other field is shown 20ms after the other one, but the content it displays belongs to the same time frame when the camera snapped them.
You CAN separate the fields in time so that each field effectivly has a strickly forward time-pointer, but that will yeild weiving artefacts (plz see my twister in LCP Memories). The upside is that motion gets 50hz, downside is that effective resolution is halved, conclusion bandwidth is the same.
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MagerValp
Registered: Dec 2001
Posts: 1074 |
There are no "every 2nd line". The gap between scanlines depends on the CRT's dot pitch:
JA: please don't bring up digital effects and bandwidth, he's confused enough as it is :)
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Graham
Account closed
Registered: Dec 2002
Posts: 990 |
@JackAsser:
"Of course every other field is shown 20ms after the other one, but the content it displays belongs to the same time frame when the camera snapped them."
This depends on the camera and effect machines. Some cameras do 50 fps where each field is taken in 20 ms steps, but most cameras do it in 25 fps ofcourse. Same for genloc stuff put ontop. I have seen 50 fps scrollers put on top of 25 movies on TV. |
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JackAsser
Registered: Jun 2002
Posts: 2014 |
@Graham: yeps of course, but in general... |
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ptoing
Registered: Sep 2005
Posts: 271 |
Now one question remains. Is this technical stuff really relevant for us with what the demoscene does. I for one part would say no. The C64 works like it works, TVs work like they work. Unless you do hardware hacking stuff stays like it is. Done. |
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chatGPZ
Registered: Dec 2001
Posts: 11360 |
never wrong to understand WHY thinks work as they work :) |
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ptoing
Registered: Sep 2005
Posts: 271 |
Well I agree with that, tho people are explaining the same stuff over and over again here. |
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