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Demos Amiga Demoscene Archive Forum / Coding / PAL on NTSC Amigas


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#1 - Posted: 28 Aug 2007 12:18
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Hi Everyone!

I would like that my PAL code runs on as many NTSC Amiga as possible, and
prints an error message on those unable to run in PAL mode.

I have some questions about NTSC Amigas, and how to make them run in PAL mode.
As far as I understand, the ability to display PAL or NTSC or both depends
only on the Agnus chip. The first versions of Agnus displays either PAL or
NTSC, while Agnus 8372A and later versions (including Alice) can display both
switching bit 5 of register BEAMCON0. If that is correct, I would have expected
that from Agnus 8372A on there was just one Agnus chip, used in both PAL and NTSC Amigas.
But that's not the case: of any version of the Agnus chip (including Alice) there is
the PAL type and the NTSC type. So my questions:
1) the PAL display I get on a NTSC Amiga (i.e. with a NTSC Agnus) is it the same as
a PAL display on a PAL Amiga (and viceversa for NTSC display)? I always believed
so, but I never had the chance to check on a real NTSC Amiga.
2) if the answer to previous question is YES, why there are distinct PAL and NTSC Agnus?
3) the 8372A Agnus, capable to display both PAL and NTSC, is the (first) ECS Agnus mounted on
A3000, A600, A500+ together with SuperDenise. However, there are A500s and A2000s which have
the ECS Agnus but the old Denise. Is it possible in this case to display both PAL and NTSC?
In WinUAE, the answer is YES (setting ECS Agnus as chipset option) but again I would like
someone to confirm this happens on real machines, too.

my best regards!
Sir Garbagetruck
#2 - Posted: 30 Aug 2007 01:59
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Ok, your older machines, say an NTSC A1000, like all of mine, have the older Agnus chip, and cannot display PAL, no matter what sort of bit flipping you do. You can install a newer Agnus, I think. Most folks however just bought an A500 and either cut the pin to force it into PAL mode upon boot, or ran something like BPD (boot PAL, Dammit) which was a program that put itself into the coolcapture vector and flipped the bit to PAL _prior_ to initialization of Intuition.

This is the important part here - if you are just wanting the display to go into PAL mode, and you don't care about being system legal/Intiution knowing the size of your screen, etc - you can twiddle the bit on and off as much as you like in your program and it will flip between the two, and so long as the monitor can deal with it, it'll display. (Used by Dr. Skull, I believe, in one of the effects for VD's "Full Moon" demo. He told me this on IRC once when I was saying that there was no way in HELL that anyone in their right mind would use NTSC, what with it having 25% less raster time and 20% less screen real estate.)

But if you want INTUITION and various graphics portions to know the REAL dimentions - that is, that you are in PAL mode and your screen has more space - then you'll need to go to PAL mode before it initializes.

Remember how on 1.3 cli would boot on a PAL machine with 20% of the screen not taken up by the window, but you could resize it? Well, on an NTSC machine, you can't resize it unless you initialize the machine in PAL mode.

So anyway to answer your question - yes you can twiddle that bit on the 'fatter' Agnes (at least that's what we called it) and it will flip PAL/NTSC to your heart's content. Just remember to be kind to the system if you're doing actual SYSTEM level work.

Oh, and obviously observe the rules of less raster time: Don't time anything to the vblank; remember the refresh is at 60hz not 50hz and compensate; and if you DO draw anything in the overscan area below, chances are it will be lost on some monitors - so don't.

As for the error message - I am not aware of any way to detect this, to be honest. I certainly never SAW anything that said "you should buy a PAL machine" - other than of course demos which had scrollers and effects where we couldn't see 'em. But we also needed monitors that would handle that, which is another thing you should be aware of - if someone is using an Amiga on a TV set in the states, for example, flipping into PAL mode will give an 'error message' of sorts: a lovely screen flipping, with some high-pitched whining and a lovely sense of "I just busted my TV, oh crap oh crap oh crap."
#3 - Posted: 31 Aug 2007 09:15
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I don't think it matters from a software perspective, but I think an NTSC amiga switched to 50Hz is actually displaying NTSC50 and a PAL amiga switched to 60Hz is actually displaying PAL60, so there are still differences between PAL and NTSC Amigas. (I could be wrong, though?)
#4 - Posted: 31 Aug 2007 09:54 - Edited
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PAL and NTSC amigas run their chipsets on slightly different clock frequencies (at least that was the case for the A500 family, and I suspect that is the case for all OCS/ECS/AGA machines).

With regards to output video signal, I have successfully run my PAL Amiga in "NTSC mode" against a TV that didn't support a true NTSC signal.

PAL vs NTSC video signals have two main components:
1) the encoding of color, when encoding RGB -> composite signal
2) the horizontal/vertical refresh rates

My TV did handle the NTSC-style refresh rate, but not the NTSC color encoding.

I too believe that the PAL/NTSC machines have different hardware solutions for composite signal encoding, and only point (2) is affected by the flippable PAL/NTSC bit in the machine. That's how you end up with PAL60 and NTSC50.

Best way to resolve this is probably to send over a test executable to someone who has an NTSC machine (Truck? Trixter? Phoenix?). Unless someone happens to know _for sure_...
#5 - Posted: 13 Sep 2007 23:38
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thanks to all for the info!
So, tu sum up, if I set an NTSC Amiga in PAL, I get NTSC50, which is a 50hz display but with NTSC color encoding. Does NTSC50 has the same resolution as PAL?
If it isso, I belive that if the monitor is connected through the RGB port this is exactly the same as PAL, right? And if the monitor is connected through the video port but is capable of displaying both PAL and NTSC, it can probably display NTSC50 as well.

What about those A500/A2000 having the ECS Agnus (8372A) but the OCS Denise? Is my speculation that they can display both PAL/NTSC true ?


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