Re: GO64 tech stuff?

From: Nicolas Welte (
Date: 2000-08-25 11:01:05

Richard Atkinson wrote:
> Something just came up on #c-64 IRC channel. Apparently there's an
> interesting article about a new hack found with the VIC-IIe in the German
> GO64 magazine. Has anyone read this article yet?
> <ThunderBl> the VIC with 62Hz was great...
> <ThunderBl> vortexion: on C128 in C64 mode it is possible to kick the VIC
> into leaving away rasterlines!
> <ThunderBl> MrX: exactly that. displaying less rasterlines, thus
> increasing screen refresh. 62 Hz or something. on some monitors it doesn't
> work, but on those on which it does, it's very cool :) especially for
> interlace pics etc.!
> I'm most intrigued. Perhaps there are hidden registers, something like the
> plus/4s TED registers. Or maybe it's a completely different hack of the
> VIC-IIe's implementation.

I read it, and I also did the NTSC fixing of the demo program for them, and
I wrote an explanation what the program does. 

Guenther Walter from Austria experimented with the TEST bit in $d030 of the
VIC-IIe, and found that while this bit is activated, the internal raster
counter is incremented once per machine cycle! This way raster lines can be
skipped completely, and with very simple programming you can achieve up to
64Hz screen refresh on PAL and NTSC machines. Of course the effect is much
more visible on PAL machines, but since GO64! is now an international
magazine, it was important that they have NTSC compatible programs and that
at least some effect is also visible there. This also leads to a problem
that I expected: rumor spreads pretty fast (well, I have the magazine for a
few weeks now and just got the next issue) on the net, and subscribers to
the English language GO64! will have heard from the effect before they get
the magazine. It's worst for US subscribers: they have NTSC machines, so
the effect is not big, and they get the magazine very late (maybe two
months after Germans get the German issue).

Some demo programs for viewing interlace pics are also provided on the
magazine disk, and it indeed looks cool! Ninja/The Dreams wrote the new
interlace viewers with PAL/NTSC detection and optional 64Hz mode, and they
have very stable timing.

On PAL machines there's an additional effect that some people already know
from the TED: If you skip an odd number of scanlines, the color carrier is
out of phase and you get another set of colors.

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