Re: [stella] PAL VSYNC timings??

Subject: Re: [stella] PAL VSYNC timings??
From: "Eckhard Stolberg" <Eckhard_Stolberg@xxxxxx>
Date: Mon, 18 Nov 2002 14:18:00 +0100
Hello Dennis,

I'm using VSYNC to check the console switches. I just couldn't resist using
the 228 machine cycles in VSYNC for this. I'm also doing WSYNCs to give the
3 scan lines needed for VSYNC. I was mostly doing this because I thought
that's what Eckhard meant by "uninterrupted lines".

What I meant with uninterrupted was: have the VCS do nothing but
generating the sync signal by turning off the processor with WSYNCs.
I was thinking of a sequence like:

lda #$02
ldx #$00

My old TV has a VHOLD knob. When I had set it a little off-tune,
all PAL games and most NTSC games would play fine on the TV.
Some NTSC games would roll the screen though. The only thing that
all those games had in common was that they were doing something
during the three lines of VSYNC. Most of them were setting the
VBLANK period timer there.

I don't know what exactly causes the screen rolling. It might be
the accesses to the timer, accesses to the RIOT in general, or
just having the VCS do anything at all. But on RGVC there were
always people complaining about some of the problematical games,
like Desert Falcon, rolling on their NTSC TVs, although they
were 100% sure that the game was NTSC too.

When I had figured out that the problem was my TV, I was able
to tune the VHOLD in such a way that it would work with all
games (except for Artillery Duel NTSC, which is just doing a
too high frequency). Therefore I'm sure that the problem is not
affecting too many TVs out there, since most of them don't have
a VHOLD knob anymore. But unless you really really have to use
the cycles during the VSYNC for something I'd still advice to
leave the VCS alone during that time, just to be safe.

The last I saw of this topic, Chris Wilkson was going to run some tests.
Does anyone have these results? I can't find them in the archives.

Unfortunately the Atari related projects that Chris plans to do
and the ones that his time allows him to actually finish aren't
matching very well. And there's no-one who regrets that more than
Chris himself, I'm sure. ;-)

Ciao, Eckhard Stolberg

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