Difference in luma-chroma delay of C64/C128 compared to standard S-video

From: Mia Magnusson <mia_at_plea.se>
Date: Thu, 31 Aug 2017 20:06:27 +0200
Message-ID: <20170831200627.00007147@plea.se>

As many people already know, C64 is older than the consumer S-video
signal format, and doesen't comply completely to that standard.

The result is that if you make an adaptor from C64 to a S-video input
the luminance will be slightly more to the left than the chrominance.
If you on the other hand feed a S-video signal to an old Commodore CRT
monitor the opposite will happend, i.e. luminance slightly to the right
of the chrominance.

Is the exact timing of this some kind of known fact, or is it still
something that needs to be calculated or measured?

Some 15-20 years ago I modified a CRT TV set to give correct picture
both with a standard S-video source (i.e. CD32) and with a C64, using a
thumb wheel switch to select one of the 8 possible different delays in
a TDA4565 IC that were used in that TV. I don't remember which setting
I used with C64 and with CD32 though.

I see four approaches:

1: Compare the schematics of some TV sets with S-video input to the
schematics of a Commodore CRT monitor with "C64 S-video" input, and
figure out the difference

2: Find some spec on S-video timing. I've googled but haven't found

3: Measure colour bars from a C64 and a known S-video source (for
example CD32)

4: Extrapolate from looking at a picture (the mistiming seems to be
approximately 2 pixels, so two cycles of the pixel clock would be about
the mistiming)

5: Experiment with different delays.

My idea is to figure out the optimal delay and then just calculate what
cable length gives such delay (afaik it depends on what kind of
insulation the cable uses so maybe a few different lengths could be
calculated for different common types of 75 ohm coax cable). Then
anyone who wants a perfect picture could just route the luminance
through a cable of the correct lenght.

If I'm not mistaken it would probably be a cable lengt of about 30
metres (100 feet) +/- 50% or so. That seems like a rather long cable
but it's not that bad to hide under a desk or behind a TV.

The point is that it would be a simple thing anyone with a soldering
iron could do, without any need for some fancy electronics. Just pick
up a spool of enough tv antenna coax cable and solder it in, and get a
real picture improvement.

IIRC the picture with a correct delay and a TV/monitor with CTI (Colour
Transient Improvement, or some similar function) looks just as good as
with a RGB signal. (For signals with higher resolution, like 640 pixels
wide on CD32, anything that is only one pixel wide will be black and
white, otherwise that will also look as good as a RGB signal).

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Received on 2017-08-31 19:00:08

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