A500: How important is matching oscillator frequency to PAL/NTSC?

[polpo]

I have an NTSC Amiga 500 that I've modded to PAL by swapping in a 8371 Agnus and 28.37516 Mhz oscillator. It works perfectly when running European demos and games and the screen size is perfect. However, I'd still like to run games that were originally targeted to NTSC by American game studios such as Defender of the Crown at their original screen-filling aspect ratio. So I've purchased an 8372A Agnus chip and intend to make a switch so I can switch between NTSC and PAL. But that has me wondering – how important is it for the oscillator frequency to match? I'm thinking of making a board that goes where the A500's oscillator goes that can switch between enabling a 28.6363MHz oscillator when in NTSC mode and 28.37516 Mhz when in PAL mode. Or would this be a lot of effort for not much reward?

[gdunlapsd]

It only matters if you're using an A520 RF modulator to get color composite or RF video out; it uses the clock (which is present on the RGB port) to derive the color clock for NTSC or PAL (depending on which version of the A520 you have). If you have the PAL clock crystal and try using an NTSC A520, you'll just get a black-and-white picture.

If you're using RGB video, or an RGB2HDMI it makes no difference at all.

[polpo]

I use RGB output to a RetroTINK 5X so sounds like it will work perfectly fine! Are there any known timing issues that would arise with the slightly lower clock speed when using the PAL clock frequency on NTSC titles?

[pandy71]

Originally Posted by polpo

I have an NTSC Amiga 500 that I've modded to PAL by swapping in a 8371 Agnus and 28.37516 Mhz oscillator. It works perfectly when running European demos and games and the screen size is perfect. However, I'd still like to run games that were originally targeted to NTSC by American game studios such as Defender of the Crown at their original screen-filling aspect ratio. So I've purchased an 8372A Agnus chip and intend to make a switch so I can switch between NTSC and PAL. But that has me wondering – how important is it for the oscillator frequency to match? I'm thinking of making a board that goes where the A500's oscillator goes that can switch between enabling a 28.6363MHz oscillator when in NTSC mode and 28.37516 Mhz when in PAL mode. Or would this be a lot of effort for not much reward?

There is no NTSC/PAL switch - all you can switch is VSync frequency but not CPU clock and most importantly HSync frequency - i can imagine software written for NTSC which may not work correctly on PAL and vice versa - particularly on those areas where HSync and available amount of CPU cycles is important.
So for 100% you need probably two separate Amiga systems or perhaps wait for new implementation of Agnus able to truly switch between NTSC/PAL. Not sure how WinUAE address this issue.

[jbilander]

Originally Posted by polpo

how important is it for the oscillator frequency to match?

Switching a PAL-machine to NTSC and vice versa without also changing the main 28 MHz crystal will work just fine. It won't be true native mode when you switch, but close enough. You can use software to switch e.g. Degrader (on Aminet) or using the two-finger-mouse-button-salute on boot to trigger the early start-up control (with KS 3.0 or later) and select mode there.

Originally Posted by polpo

I'm thinking of making a board...Or would this be a lot of effort for not much reward?

You could do that but there is an easier way. You can actually feed an alternative 28 MHz clock via the DB23 d-sub Video port pin 1 XCLK and assert pin 2 (/XCLKEN). That is what a genlock does and what my little adapter here can do.

https://github.com/jbilander/Amiga_DB23_video_adapter

Then your machine can be true native PAL or a true native NTSC-machine with a flick of a switch. No need for two separate Amiga systems. All ECS Agnuses can switch between PAL/NTSC in software. The 8372A has a pin (41) which controls start-up mode NTSC or PAL. On a NTSC Rev 5 motherboard I believe this pad is grounded so you need to put a piece of kapton-tape on the chip-pin (to isolate the pin) in order to have the start-up default to PAL (weak pull-up inside the chip). On a Rev 6 motherboard there is a jumper JP4 where you can easily set your preferred start-up-mode or solder a switch-wire to the pad.