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Stingray
Account closed
Registered: Feb 2003
Posts: 117 |
Raster Lines 312.5?
On a PAL 64 are there 312.5 raster lines? (everything I have read seems to say 312).
Dont laugh, serious question. |
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Graham
Account closed
Registered: Dec 2002
Posts: 990 |
no to both. the tv doesn't think it gets odd/even fields, and the tv doesn't move every second frame one half rasterline down. you would see the flicker, just take a C128 in 80 character mode and switch on the interlace flag, or try the same with an amiga... on amiga it's even the other way round: many programs which directly access the hardware registers for this fail in producing a proper interlace vsync, so on some monitors their lace modes will not have the rasterline shift. |
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Stingray
Account closed
Registered: Feb 2003
Posts: 117 |
Thats basically what my original question was. The specs for the c64 say 312 lines. I was reading a number of books on television signals at the time and not one of them motioned anything about a 312 or noninterlaced mode. I did read somewhere (on the internet) that the noninterlaced mode is like an unofficial mode, don't know if thats true or not but it would explain why the text books didn't mention. The C64 does use 312 lines noninterlaced (as White Flame and Graham said at the beginning of this thread) and so do many other early 8bit home computers. I have done an experiment with generating the 312 line image and it works perfectly and is also simpler to create then an interlaced image. |
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JackAsser
Registered: Jun 2002
Posts: 2038 |
@Graham: Interesting! So, the PAL/NTSC signal contains some information whether it's and odd or even field? That would explain much. |
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Kisiel
Account closed
Registered: Jul 2003
Posts: 56 |
@Graham: So if we have 504x312@50Hz picture, we can do full animation in IFLI 25Hz (I mean 50/2xfli screen)?? It's correct?? (of c. in theory) |
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uzzy/entropy
Account closed
Registered: Nov 2005
Posts: 4 |
Basically the 312 line thing is a cheat as far as TVs go and will overdrive an old one that's not too well made (there are sound transmitting reasons why we have an odd number of lines, and older manufacturers sometimes took advantage of that and made the designs a bit "tense"). I had a really old TV (very nice 17" with beautiful sound) where the C64 totally killed the sync generator.
That said, a friend inherited his parents' old 17" because its tuning was stuffed and it worked fine on the 64 for years.
Ah, analogue... those were(n't) the days... |
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Oswald
Registered: Apr 2002
Posts: 5127 |
Graham, dont let us guessing. Instead telling your "no's" rather enlighten us how the odd/even line stuff works on TV's normally. Are odd/even lines shown on same "rasterline" or..? How is an odd/even field seperated in the signal?, and hows that overdriven by a c64 ? :) Why does amiga interlace flicker and tv interlace not ? |
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tlr
Registered: Sep 2003
Posts: 1814 |
Quote: Graham, dont let us guessing. Instead telling your "no's" rather enlighten us how the odd/even line stuff works on TV's normally. Are odd/even lines shown on same "rasterline" or..? How is an odd/even field seperated in the signal?, and hows that overdriven by a c64 ? :) Why does amiga interlace flicker and tv interlace not ?
Amiga interlace flickers only because of the contents, i.e narrow horizontal lines etc... It's exactly the same with TV, only here we usually see smooth natural images. The lines are separated by half a line between the frames.
See:
http://en.wikipedia.org/wiki/Interlace
The article is somewhat vague on the fact that the frames are also taken from different points in time. In fact the temporal resolution is 20ms, but only with half the vertical resolution. It appears (or is supposed to appear) to the viewer as full vertical resolution with the time resolution 40ms. (PAL)
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HCL
Registered: Feb 2003
Posts: 731 |
Without knowing anything.. :). Isn't it just some sync-bit that tells if this is an odd or even screen, and that makes the TV/Monitor put the gfx either half a pixel lower, or at its normal position (?).
@JackAsser: How does HZ's interlaced upscroller work? The gfx is made to look good when it moves one pixel vertically each 2:nd frame. If it would stand still, it would flicker hellish :). |
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White Flame
Registered: Sep 2002
Posts: 136 |
Look at this pic again:
http://www.white-flame.com/interlace.gif
The top signal is interlaced, and the bottom one is not. The crucial difference is the location of the vertical refresh pulse thing. The horizontal axis is time, but rotate it in your head and you can see how the rasterlines line up.
Oswald: As tlr said, if a TV signal had dark and light alternating rasterbars (like a thin font or window borders on the Amiga do), it would flicker, too. Computer graphics made for TV broadcast are blurred vertically so that interlaced fields are more similar and don't flicker too badly against each other. |
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Krill
Registered: Apr 2002
Posts: 3098 |
Quote: Without knowing anything.. :). Isn't it just some sync-bit that tells if this is an odd or even screen, and that makes the TV/Monitor put the gfx either half a pixel lower, or at its normal position (?).
@JackAsser: How does HZ's interlaced upscroller work? The gfx is made to look good when it moves one pixel vertically each 2:nd frame. If it would stand still, it would flicker hellish :).
I am pretty sure that this is just what we call an optical illusion. Due to the steady movement of 1 pixel per frame and the 25hz interlace, the human brain interpolates in-between-pixels and it all appears as if being in double resolution. What also helps is the pretty low contrast, try switching the scroller to white and see it flicker again. |
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