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

> On 2017-08-31, at 20:06, Mia Magnusson <mia@plea.se> wrote:
> 
> Hi!
> 
> As many people already know, C64 is older than the consumer S-video
> signal format, and doesen't comply completely to that standard.

Well, it doesn't comply to /any/ video standard if we want to tell the truth :-)

> 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

The differences may be rather big and please note that it might not be obvious where the timing difference is handled as well as that it might come from some analogue "tweaks" rather than a pure design difference.

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

There are "Rec."s about the video timings but I don't see how this alone could help. You still need to measure things.

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

I just (few weeks ago) wrote a small proggy for the 64 to display the quasi-standard colour bars over the whole screen. I wrote it to measure some other aspects but can be used to measure the difference. Just connect the 64 displaying the bars to a good waveform/vectorscope measurement set and compare it to a known standard source (like the broadcast test signal generator). As I wrote a minute ago in another thread I once ran a studio and I still have all of those (scopes and generators) if needed.

> 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)

Chroma has lower resolution than luma so there will always be some mistiming when we talk one pixel for example. I remember doing those things (finding the best relation between luma and chroma using the studio equipment while looking at both the picture and the WFM. It was required especially when working with non-Betacam material.

> 5: Experiment with different delays.

Until your WFM shows what you want it to.

> 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.

I am not sure what you mean. The propagation times over properly matched line is close to negligible. You may get some quasi-impedance-matching by trimming the cable to a specific length but a) it's a mother of bad ideas when it comes to impedance matching and b) we talk relatively low frequencies here, where it doesn't work that well.

> 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.

Ah... with tens of metres of length difference you might get some propagation time difference but you'd still need to do this mostly by trial and error and you'll get different results with different cables. Bear in mind that there is also attenuation.

> 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.

I think it's still better to make a small circuit for that :-) and you might get better connectors on the output side while you're at it ;-)

-- 
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