Switch 60hz on PAL machine

[LeCaravage]

Hello,

Context : Amiga 500 KS1.3 (512/512Ko).

I tried the game Wings of death and I noticed the display frequency is 60hz even if the game is from Europe (so PAL native I guess).
It's cool, because you've got a nice arcade feeling, plus the animations are smoother even at 30hz (compared to 25hz).
As my Amiga is OCS, I guess there's a trick with copper or something like which allows to switch monitor display to 60hz.

So my question is :

Does the program lose cycles if your PAL Amiga displays NTSC compared to a PAL display ?

[alexh]

Originally Posted by LeCaravage

As my Amiga is OCS, I guess there's a trick with copper or something like which allows to switch monitor display to 60hz.

I think it uses a CIA timer interrupt to trigger the CPU to re-write VPOSW to fake 60Hz.

Originally Posted by LeCaravage

Does the program lose cycles if your PAL Amiga displays NTSC compared to a PAL display ?

Yes.

[Snoopy1234]

iirc the NTSC titles on a PAL machine are running slower than a native NTSC machine because the clock frequency of the cpu is tied into either PAL or NTSC despite the display frequency.

[ross]

Originally Posted by alexh

I think it uses a CIA timer interrupt to trigger the CPU to re-write VPOSW to fake 60Hz.

A Copper IRQ (at level 3) is being used; the CIA IRQ would be a bit too imprecise and out of sync with the electron beam (and also phase dependent).*
Not that it's impossible with CIA, but a little more complex and slow.
The rest is right

EDIT: * just checked, it's done 'the slow way'..
In practice, when the IRQ is triggered, it waits a defined position polling the beam, recalculates with a CPU sync and then rewrites VPOSW.
This way syncing with the Copper is useless, so even an IRQ with the CIA would have been fine...

Originally Posted by Snoopy1234

iirc the NTSC titles on a PAL machine are running slower than a native NTSC machine because the clock frequency of the cpu is tied into either PAL or NTSC despite the display frequency.

This has nothing to do with the question.
Here we are dealing with using a video frequency of 60Hz on a PAL machine (therefore effectively a PAL60 signal).
The same thing done on an NTSC machine (so a 60Hz without tricks and with NTSC frames) would have practically the same effect and speed for the game (an imperceptible difference, <1%).

[Snoopy1234]

Originally Posted by ross

This has nothing to do with the question.
Here we are dealing with using a video frequency of 60Hz on a PAL machine (therefore effectively a PAL60 signal).
The same thing done on an NTSC machine (so a 60Hz without tricks and with NTSC frames) would have practically the same effect and speed for the game (an imperceptible difference, <1%).

You are correct. I didn't read the question other than losing cycles. I assumed a game programmed for NTSC would in turn be programmed to run at the faster clock frequency.

[LeCaravage]

Too bad we lose some cycles because for pure arcade 2D games feeling, this NTSC display rocks.

[ross]

Originally Posted by LeCaravage

Too bad we lose some cycles because for pure arcade 2D games feeling, this NTSC display rocks.

Unfortunately it's not "some" but a lot of cycles..
If you compare long frames, in PAL50 you have 71051 cycles, in PAL60 59701.

Also they are 'good' cycles, i.e. with little or no DMA competition for the display, as a wider border is usually used in PAL50.

[LeCaravage]

Originally Posted by ross

Unfortunately it's not "some" but a lot of cycles..
If you compare long frames, in PAL50 you have 71051 cycles, in PAL60 59701.

Also they are 'good' cycles, i.e. with little or no DMA competition for the display, as a wider border is usually used in PAL50.

Gosh, I must confess I didn't suspect so much difference.