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

From: Pete Rittwage <>
Date: Thu, 31 Aug 2017 15:56:18 -0400
Message-ID: <>
We could probably wire up one of the Rhombus TZB-series passive delay
chips with a switch and do pretty well to fix this.

Pete Rittwage

> Den Thu, 31 Aug 2017 21:03:18 +0200 skrev
>> > On 2017-08-31, at 20:06, Mia Magnusson <> 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 :-)
> Yes, I know that the level for the chroma is a bit off and that the
> output stages doesen't really have the correct impedance either, but
> that usually works fine as it is. Almost (?) every TV has AGC for the
> chroma, adjusted by the level of the color burst so it's probably not a
> big problem. :)
>> > 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.
> Well atleast the CRT TV I modified had a "s-video" control signal that
> just selected between two different delays. Selecting or not selecting
> delays in a Commodore CRT would probably be easy to spot.
>> > 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.
> Well, the composite video seems to have standard timing, and we know
> that the modulator just mixes the luma and chroma signal (with a RC
> filter which might change the timing slightly but that's easy to
> measure), so we could probably assume that the wanted delay is the
> difference between the specs for composite video and s-video.
>> > 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.
> I don't have that kind of equipment, just a (modern digital)
> oscilloscope so I'd have to look at the waveform. The chroma-luma
> timing should be visible that way too.
>> > 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.
> Yes, but with my current setup it's obvious that there is mistiming.
> And many modern color decoders can do tricks to detect a transient and
> improve the bandwith of the transient, so it looks better.
>> > 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.
> A 75 ohm coax shouldn't have any impedance matching problems if the
> TV/monitor has a proper 75 ohm termination. (I've seen 82 ohms in many
> cases, probably because that was cheaper, but it's rather close). As
> long as the reciever has a correct match the transmitter might be a bit
> off without any real problem.
>> > 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.
> It seems like for most RG cables there are three different progagation
> speeds, so it would be easy to have three different legnths for a user
> to try:
>> Bear in mind that there is also attenuation.
> Yes, but that could probably be ignored at such low frequencies.
> Atleast TV aerial cables would only attenuate a few dB with tens of
> metres and more than 100 times higher frequency, and the cables sold
> for parabolic dishes would have even less 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 ;-)
> Well, the point of using a long cable is that the cable is available in
> shops almost everywhere so you don't have to order stuff.
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Received on 2017-08-31 21:02:28

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