Re: [stella] interlace

Subject: Re: [stella] interlace
From: Eckhard Stolberg <Eckhard_Stolberg@xxxxxxxxxxxxxxxxxxxxx>
Date: Sat, 24 Jun 2000 15:59:01 +0200
At 17:59 23.06.2000 -0400, you wrote:

>Anyone remember why can't you do calculations during VSYNC?  I remember the
>discussion (a couple years back), but I don't remember the details.

It was probably me, who did most of that discussion. ;-) I noticed that
some 60Hz NTSC games would roll on my PAL VCS/TV while most others worked
fine. The games that rolled the screen all did something during the three
lines of VSYNC. Sometimes it was only so simple things as setting up the
timer for the VBLANK period between the second and third WSYNC.

I also noticed that 60Hz games will roll on my TV, if VBLANK isn't turned
on during the VSYNC period. Later I was able to tune the VHOLD on my TV
in such a way that most of the games that execute some instructions during
the VSYNC lines will sync properly. Only those games where the VBLANK is
turned off during VSYNC still don't work.

On RGVC people from the USA regularly report that some of their VCS games
roll the picture, even though they are clearly not PAL games. The games
that get mentioned in those posts are always games that don't leave the
VCS alone during VSYNC, so I'm assuming that it is the same problem
as with my TV. I don't have an explanation for this phenomenon though.

Ciao, Eckhard Stolberg

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