Atari 130XE ANTIC swap - success??

[jaek_3]

Hello, 

 

I recently got my hands on a PAL Atari 130XE with a bad GTIA chip. I managed to fix it with a simple mod (do NOT use this one, by the way; the wiring picture contradicts the text and even itself and neither configuration works. Refer instead to this diagram). Now it works flawlessly.

 

I was a little annoyed at how slow my favorite cartridge games were playing at PAL speeds, though, so I decided to temporarily desolder the PAL ANTIC chip, solder in a socket, and plug in my NTSC Atari 400's ANTIC chip. I had read that this would change the CPU speed and monitor refresh rate to NTSC levels. It... worked, sort of; the speed and resolution are increased but the colors look washed out and just strange. Below are some screenshots. There's also seems to be some interference in the signal which is difficult to photograph.

 

My monitor seems to think this is an NTSC 4.43 which I suppose makes sense, since this is a PAL machine. Is there any way to fix the colors short of doing a full NTSC conversion? I had wanted to be able to just swap out the ANTIC chip as needed, but if this is the quality of "NTSC mode" I don't think I'll bother. 

[Simius]

You need also NTSC GTIA and 14.31818MHz crystal. 

[Rybags]

CPU speed will be unchanged though NTSC has more refresh and graphics DMA loss so will actually be a bit slower.

You could play with the colour adjust pot but likely won't change much.

I don't think you'll get NTSC colours with a PAL GTIA regardless of what you try.  Unsure if you can just plug an NTSC GTIA in since it will be receiving the PAL colourburst though supposedly that pin should be NC on NTSC since it uses the system clock as reference.

[jaek_3]

What exactly would one need to do to convert a PAL Atari 130XE to NTSC? I'm investigating the possibility of building some kind of PAL/NTSC switcher like the VIC-II² on the C64. I haven't built a prototype yet but in my testing I've determined an ANTIC switcher could work with a relay to toggle 5v between the PAL and NTSC versions of the chip.

 

I am still unsure how exactly PAL and NTSC Ataris differ in terms of their GTIA chips and clock crystals however.

[Rybags]

NTSC derives the colour clocking from the system clock (OSC pin I think)

PAL's colour clocking isnt' a direct ratio so it requires the second crystal which supplies the 4.43 MHz to the PAL input pin which is used to derive colourburst.

 

From what I can gather, doing NTSC -> PAL would be a much more complex conversion since there's extra stuff on the motherboard.

Going the other way is probably much easier.

Other considerations can be RF mod - different countries will have different frequency seperation between video and audio.  But who needs the RF output anyway?   So long as S-Video and Composite work should be enough.

 

Possibly you could just drop in an NTSC Antic and GTIA and see if it works.  The colourburst would be a bit slow since the PAL master clock is slower so might only show B&W.

But a positive there is that PAL crystals are somewhat rare but NTSC based ones practically fall out of trees.

[thorfdbg]

To be a bit more concrete, the PAL color burst is 5/4th of the frequency of the NTSC color burst, so the PAL versions of the Atari's first use a dual D-flip-flop to divide the master clock down to 1/4, and then use the output of this divider to synchronize the PAL color crystal which runs on the 5th harmonics of the divided frequency. This is also the reason why we don't have artifacting on the PAL machines (or rather, we do have some weak form of it were you need set hi-res pixels in a 5/4th pattern to get some "colorish" impression, though one can in principle create more colors than on the NTSC machines - just quite weak ones).

 

[Dinadan67]

On 12/10/2022 at 12:09 AM, jaek_3 said:

My monitor seems to think this is an NTSC 4.43 which I suppose makes sense, since this is a PAL machine. Is there any way to fix the colors short of doing a full NTSC conversion? I had wanted to be able to just swap out the ANTIC chip as needed, but if this is the quality of "NTSC mode" I don't think I'll bother.

I have a PAL 800XL with NTSC Antic, and the colors are perfectly fine, just the unaltered PAL-colors. I have used it with different TVs, a Commodore 1084 Monitor (PAL) and a LCD Monitor. The latter even displays "PAL 60" on screen when switching to composite input.  I think the problem is with your monitor, which seems to not recognize PAL-60 correctly.

[thorfdbg]

Note that ANTIC is not responsible for the generation of the TV signal. GTIA is. Antic "only" supplies the data to be displayed as DMA controller. As such, it should be responsible for the maximal height of the display and by that also the vertical refresh, but not on the color generation - that's GTIAs domain.

Thus, while I have not tried, swapping a PAL Antic to a NTSC Antic should give you 60Hz refresh, though with a PAL color carrier. It is not exactly following PAL specs, though most monitors or TVs will likely accept this signal as they just trigger the vertical refresh earlier.

[jaek_3]

54 minutes ago, Dinadan67 said:

I have a PAL 800XL with NTSC Antic, and the colors are perfectly fine, just the unaltered PAL-colors. I have used it with different TVs, a Commodore 1084 Monitor (PAL) and a LCD Monitor. The latter even displays "PAL 60" on screen when switching to composite input.  I think the problem is with your monitor, which seems to not recognize PAL-60 correctly.

My monitor is a Sony PVM 1943MD. With the NTSC ANTIC installed it displays "NTSC 4.43". Is that different from PAL 60?

[Rybags]

Generally it's the same thing.  The display will still be just 525 line.  To do 625 lines ~ 60 Hz would require a somewhat faster scanline rate.

From what I've read over the last 20 years, it's more common for TVs and old monitors in PAL countries to run NTSC gear OK than the other way around.

[jaek_3]

3 hours ago, Rybags said:

Generally it's the same thing.  The display will still be just 525 line.  To do 625 lines ~ 60 Hz would require a somewhat faster scanline rate.

From what I've read over the last 20 years, it's more common for TVs and old monitors in PAL countries to run NTSC gear OK than the other way around.

Alright, as soon as my DIP sockets arrive I'll drop in an NTSC GTIA and see what happens. 

[Rybags]

I'm not sure how it will like receiving a PAL clock input on the pin that should be normally NC for NTSC.

If they're cheap sockets then maybe you could stack them and remove that pin on the upper one for testing purposes.

[jaek_3]

Socket soldered: as you predicted Rybags with an NTSC ANTIC and GTIA installed I get a black and white only picture. I guess all that's left to do is to replace the clock crystal? Is it as simple as replacing the PAL crystal with an NTSC one? My understanding is that PAL Ataris have additional circuitry not present in the NTSC models. Can all that stay in place? The only crystal I can find in my NTSC Atari 400 is 3.579575MHz.

[jaek_3]

The only crystals on my PAL board read 4.433618MHz (for PAL colorburst) and 14.31818MHz (the master crystal, I assume). I guess this means I won't be able to use my Atari 400 crystal after all, and will need to find a 14.31818MHz one instead?

Edited December 15, 2022 by jaek_3

[Rybags]

14.318 should be an NTSC crystal so providing the correct speed to be divided down for the NTSC clock.

 

If using an NTSC GTIA the PAL 4,.43 crystal should become irrelevant since it's not used for anything other than the PAL colour clocking.

Why you'd get B&W with the correct crystal and NTSC Antic and GTIA - I'm not sure.  Maybe there's changes that have to be made in the video generation circuit between GTIA and the outputs.

[jaek_3]

To clarify I haven't replaced any crystals yet: only the ANTIC and GTIA

[jaek_3]

Update: I've successfully built a PAL/NTSC ANTIC switcher 🙂

 

Not the most beautiful thing to look at perhaps but it is functional (the chip on top of the GTIA is an unrelated mod)

Edited December 15, 2022 by jaek_3

[Rybags]

Is the colour working properly though?

[jaek_3]

No, it just swaps the ANTIC chip for you automatically. I'm still waiting for parts/investigating implementation methods but if possible I'll make a GTIA/crystal switcher too so I can have full PAL and full NTSC available on the same machine at the flick of a switch.

[Mclaneinc]

The colours will be slightly different whatever you do as it will be an NTSC palette, most people just like the extra speed it gives but you may / will find it breaks some european stuff, thankfully you will be able to switch back, best of both worlds..

 

As for the board, it's inside, does the job, so job done.

[jaek_3]

I can now confirm that with an NTSC ANTIC, GTIA and 14.31818MHz master clock crystal in place, I'm getting a bright and colorful NTSC picture! Only annoyance is that to get proper colors in either mode I have to readjust the pot on the 130xe motherboard, but I think I can fix that with some resistors. I don't see any reason a full PAL/NTSC switcher shouldn't be possible to make now. 

[Rybags]

Could you put one on your daughterboard and redirect?

[chevymad]

12 hours ago, jaek_3 said:

No, it just swaps the ANTIC chip for you automatically. I'm still waiting for parts/investigating implementation methods but if possible I'll make a GTIA/crystal switcher too so I can have full PAL and full NTSC available on the same machine at the flick of a switch.

If the Sophia 2 is ever available again, it replaces the gtia and also will automatically switch between Pal and NTSC. So then you just need a crystal switcher. 

[jaek_3]

Update: I now have a fully functional PAL/NTSC switcher! I would have finished it sooner but this Christmas season has been particularly hectic.