Re: GO64 tech stuff?

From: Nicolas Welte (
Date: 2000-08-26 09:29:17

Richard Atkinson wrote:
> I've just realised what this hack can be used for - making 60Hz PAL mode
> games! Many games consoles can be hacked to run in 60Hz and I've even seen

I also suggested this in my little review of the effect for GO64: With
using this mode, PAL computers can display 60Hz video with all its side
effects _if_ the display supports it: NTSC games are often made for 60Hz
refresh, i.e. they run slower on PAL machines, including gameplay and
music playback. My old favourite Ghostbusters is an example, it is
really noticable if you run it on an NTSC machine. The 60Hz screenmode
can also help to fix the pixel aspect ratio problem: Graphics that have
been designed for NTSC look like they have been vertically compressed on
PAL: again Ghostbusters and the title picture is a good example, on NTSC
the stop sign with the ghost is a prefect circle! But a display that has
a true 60Hz mode is needed: my Sony TV does it, and the 1084-P and
1084S-P do it. My 1702 doesn't do it, plus the vertical zoom is not
adjustable from outside.

> Then I realised it might be possible to cause the line counter to reset
> mid-frame (as you do directly on the plus/4 to change the screen size),
> but without generating frame sync pulses as there is no raster line state
> machine operation during TEST bit set. Thus you could perhaps create
> "extra" bits of border in an NTSC machine, emulating a 312 line 50 Hz PAL
> display. Bingo - 50Hz NTSC for the NTSC games people to try :)

I tried to do this, but failed. My idea was to generate the 50Hz
screenmode mostly in software, and to construct it from two 60Hz frames,
by skipping the sync signal in the first frame, skipping most of the
second frame, and then release the test bit in the lower border of the
second frame, to add the extra border lines to the first frame, plus the
sync signal. It didn't work, the ghost picture appeared :-(

I'm very interested in new attempts to this problem, but of course a
8564 chip is needed for that! Unfortunately my source of new 8564 chips
somehow disappeared, it was, they used to carry spare chips.
If they don't answer my email I might phone them.

> How ghostly is this picture? It's important to distinguish between an
> artifact of the TV display, caused by *very* non-standard signals at its
> input (which may vary completely between TV sets) and an actual VIC-IIe
> effect. This also makes me wonder why the original author stopped at
> 64Hz; was that an estimate of the greatest out-of-spec signal "most" TVs
> would accept?

I admit I didn't try a second TV set, I was happy enough to have one
that shows NTSC in color and allows me to do experiments with the
timing. Maybe ghost image is the wrong term, it has the same intensity
as the main picture, so it doesn't look at all like something that is
created by the TV. Also, the ghost image vanishes if the raster line
where the test bit is activated is changed. And this does not change the
video timing at all. So it is an effect of the 8564.

Guenther Walter calculated his routines to use 62Hz (as was reported on
IRC, and this is also printed on the GO64 title), but my analysis showed
that it is actually 64Hz. I'd send you the article, but it is in German,
so you have to wait three weeks or so and order the English issue :-)

> (at least, it will produce odd display-dependent effects such as all out
> loss of colour) because the colour subcarrier PLL inside the TV decoder
> will simply lock onto the "new" alignment of the PAL lines during the
> altered parts of the frame (it will take a few lines to re-synchronise)
> and then lock back onto the normal alignment at the end of the altered
> parts of the frame. You have to always maintain an even number of lines in
> each frame because the VIC always reverts to the same PAL line type (eg.
> U) at the top of each frame, if it's anything like the TED.

It is clear that the correct colors will be restored at the start of
each frame, but in my experiments the new colors are stable on a whole
frame. New colors can be seen by running the NTSC versions of the demo
code on PAL machines, which will create modes with little more than 50Hz
or so,  and the new colors will be in the lower border, IIRC.

> Finally, we can look upon the VIC-IIe as a truly _usefully_ expanded
> version of the VIC-II. Hooray for the C128!

I always liked the C128 better than the C64, but most Germans don't
think so, the acceptance of the 128 is very low here. Some readers
expressed their disappointment already that the effect is only for the
128 :-(


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