doug@terak.UUCP (Doug Pardee) (09/17/85)
The RF modulator in my Commodore 64 died the other day, and those
familiar with the C64 know that the "stock" modulator is so bad that
this could be considered a relief :-)
To verify that it was indeed the modulator that had packed it in, I
connected the composite video output from the C64 to the "camera"
input on my VCR via clip leads and the shielded cable that Commodore
provides for connecting the RF. To my only slight amazement, the
resulting image was much clearer than I'd ever seen a C64 produce on a
standard TV set.
So after Radio Schlock opened for the day, I rushed down and bought
their new $25 super-duper RF modulator (intended for real video stuff
like satellite TV). After concocting a video/audio cable to feed it
with, I hooked it up and VOILA! -- the picture looked almost as bad as
with the stock C64 modulator. The 60Hz hum is gone, and so are the
various interference patterns characteristic of the C64, and the fading
shadow following an abrupt bright-to-dark transition, but...
The most infamous C64 video problem remains, even though it had been
much reduced in my VCR lashup. So I'm fishing for some advice here...
The problem manifests itself as a single "ghost" image, about 3-4
pixels (maybe 0.5 microsecond or so) to the right of the main image.
The ghost is almost non-existent with some color combinations, and
with others (e.g. red on blue) it makes text totally illegible.
The color of the ghost image is not necessarily the same as the
original. The ghost is just as prominent when viewed on a B&W TV.
Another problem (I assume it's related) is that with certain colors
(especially blue), a solid color screen has a "texturing" of vertical
lines at about the same 3-4 pixel spacing. These appear to be
intensity variations, perhaps saturation, but definitely not hue.
They are also just as prominent when viewed on a B&W set.
Certainly the pixel rate exceeds the bandwidth of the NTSC color
subcarrier, but the resulting "error" should be incorrect color for
the pixel, not a ghost. And Atari systems which use similar pixel
rates don't have this "ghosting" problem. And most importantly to me,
my VCR lashup didn't either. And the solid-color screen isn't subject
to the bandwidth limitations except at the edges.
Some clues to sift through:
The C64 produces separate luminance and chrominance signals, and then
mixes them to produce the composite video. It has been reported that
there is far too much chrominance in the resulting composite video.
I didn't open the C64 and disconnect the old modulator, so it is on
the video output in parallel with the new one.
I do know better, but... the cable I made up to connect the video
from the C64 to the modulator is 3 feet of audio-grade shielded cable.
On the other hand, the cable that I used in the VCR lashup doesn't
look any better (but maybe it is).
The new modulator has a switch to select between 75 ohm and 1K input
impedance. The 75 ohm position is virtually unusable; the resulting
picture is snowy and I can't even get good vertical sync. The 1K
position gives a solid picture.
The ghosting is much improved if one uses a composite video monitor
rather than a modulator/TV combination. More so if the separate
luminance signal is used for luminance, and the composite signal is
used just for chrominance. I don't know about the solid-color screen.
Anyone got any ideas?
--
Doug Pardee -- CalComp -- {calcom1,savax,seismo,decvax,ihnp4}!terak!doug