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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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chatGPZ
Registered: Dec 2001
Posts: 11360 |
Quote: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
"doesnt matter" as in "no idea which it is, but its irrelevant to explain what happens" yes :=)
Quote:what does the term field mean from the tv electronics viewpoint?
field==halfframe. one field has all odd scanlines and the other has all even scanlines.
Quote:how does the vic only light up the odd (forex.) lines on the tv screen, and a little bit from the other ones?
the chrominance part of the signal is reused for two scanlines.
Quote:are there really 2 modes in the tv electronics for odd / even fields
modes? no, the length of a certain scanline at the start (or end, kind of matters how you define it) of each halfframe determines wether odd or even lines are used. (see how the "true" interlace on c128 works - it modifies the length of this scanline)
Quote:why does computer graphics flicker a hell more lot while "normal" tv screens looks as steady as my ass?
pal/ntsc is optimized for "natural" pictures, and only works well with not-so-sharp images. (again, sideeffect of the reusing of chrominance due to limited bandwidth)
uh, its a bit more complicated than that actually :) ask your favourite tv technican :=P
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dahl
Account closed
Registered: Feb 2003
Posts: 2 |
- I believe the VIC doesn't support bobbing because of its low vertical resolution; e.g "dontcare".
- http://www.100fps.com covers fields etc.
- check answer 1
- don't understand the question
- your computer graphics flickers more on a computer monitor because the pixels lits down faster (going from say white to black). The pixels on an ordinary TV needs slightly more than a 50th of a second to shut down completely giving a more stable picture. |
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chatGPZ
Registered: Dec 2001
Posts: 11360 |
let me try to draw a whacky chart, might be easier:
normal pal (tv) signal:
odd field: AAAAAAAAA
even field:BBBBBBBBB
odd field: AAAAAAAAA
even field:BBBBBBBBB
odd field: AAAAAAAAA
even field:BBBBBBBBB
fields are displayed alternating (interlace). the chrominance is reused for ever other line in the _same_ field.
vic signal:
odd field even field
AAAAAAAAA BBBBBBBBBB
--------- ----------
AAAAAAAAA BBBBBBBBBB
--------- ----------
AAAAAAAAA BBBBBBBBBB
--------- ----------
*again* in each field the color signal is reused for every other line, but since the fields arent actually using different scanlines the "black" scanlines don't actually appear as black, but as dim (2/3rd) "copies" of the previous scanline.
mmmh, does that make sense? =) |
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Oswald
Registered: Apr 2002
Posts: 5086 |
does the TV know anything about odd/even fields, or that is entirelly driven by the incoming signal? if so exactly HOW?
why does the electron beam light up half lines it shouldnt when it has old chrominance info ?
"the fields arent using different scanlines" huh??? |
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JackAsser
Registered: Jun 2002
Posts: 2014 |
"does the TV know anything about odd/even fields, or that is entirelly driven by the incoming signal?"
It's driven by the signal. The half PAL line you sometimes see on the bottom of one of the fields actually makes the next field jump down half a raster line. C64 never outputs this line, thus the fields never jump up and down => progressive display.
About chrominance, that's simply that the color data has half y-resolution (per field). The chrominance data is not "re-used" between different fields.
One field consist of 720/(576/2) luminance pixels and 720/(576/2/2) chrominance pixels.
On the C64, the same field is output twice without any half PAL line on the bottom, thus u effectivly get 720/576/2 resolution, progressive with 720/576/2/2 chrominance resolution.
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chatGPZ
Registered: Dec 2001
Posts: 11360 |
Quote:About chrominance, that's simply that the color data has half y-resolution (per field). The chrominance data is not "re-used" between different fields.
ah yes "re-used" is the wrong term. what happens (originally, in older, analog tv) is that the chrominance while beeing used for the current scanline it is also feed into a delay-line (in the simpliest case this is literally just a very long wire) and then in the next scanline (of the current field) the signal coming out of the delay line is used for chrominance.
confusion complete? =D |
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Oswald
Registered: Apr 2002
Posts: 5086 |
groppie:I've checked wiki, and it says the chrominance averaging/delaying stuff is done to eliminate errors (transmission bugs?) originally it was NOT used in the early tvs.
edit: thus the chrom data is not truly half reso, rather avged with prev line. |
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Oswald
Registered: Apr 2002
Posts: 5086 |
jack, how do I interpret all those magic numbers?:) |
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chatGPZ
Registered: Dec 2001
Posts: 11360 |
the wiki is wrong :) the chroma signal is half resolution to save bandwidth. the averaging thing is true however, the current scanline chroma will always be a mix of 1 third of the previous scanline chroma and 2/3 of the current scanline chroma. (no, its not like this, its more complicated, but i forgot =D) |
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MagerValp
Registered: Dec 2001
Posts: 1074 |
To understand how a CRT display works, stop thinking about pixels and lines. It's an analog display, and everything is driven by timing. Lines are not horizontal, but drawn in a zig-zag pattern. There are no pixels, but an electron cannon firing continuously at a phosphor layer that lights up for roughly 1/50th of a second. There are no dim 2/3rd lit up half scanlines inbetween, that's just how VICE approximates the effect of CRT dot pitch on high resolution PC displays.
Talking about odd or even fields on the VIC is also pointless, as you only have fields in interlaced video modes. The VIC outputs 50 frames per second, and no fields. Not odd, not even. The VIC does not output a spec conforming video signal, but it works because CRTs are not intelligent - they have no concept of odd and even fields. An interlacing video output device (Amiga's Denise, your TV receiver, whatever) "tricks" the CRT into moving every other field half a scanline down by delaying video signal 32 microseconds.
Please go read some articles on how CRTs, and especially interlaced video, works.
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