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

> On 2017-08-31, at 21:26, Mia Magnusson <mia@plea.se> wrote:
> 
>>> 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. :)

The levels are off specs, the timings are off specs, the impedance is off specs, the sync pulses are off specs, the signal is non-interlaced, and so on :-)

Some time ago I was intensely brainstorming and pushing (even here) an idea to design an interface that would "legalise" all the above so that every piece of standard compliant (and expecting standard signal) piece of equipment could handle the video from the VIC-II equipped CBM machines. That is until I realised that it can't really be done.

> 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.
> [...]
> 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.

Now, I might be missing something but to me the composite video is a way of 2to1 encoding/muxing so that two signals can be transmitted over one line rather than two. Unless I am now mistaken, there should be no differences (other than unintended) in the luma/chroma timing between the two. Now that you say about your CRT TV, I start wondering whether I am really missing something.. like time needed to decode composite.. hmm.. but that still shouldn't affect interrelation between the two. I always assumed that what I was doing in the studio was due to unintended differences caused by non-broadcast quality of the equipment used rather than inherent differences.

>>> 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.

Yes. Good oscilloscope will be fine.

> Yes, but with my current setup it's obvious that there is mistiming.

Interesting. I tested my "colour bars" program on the real-hardware and s-video monitors and didn't notice anything obvious. Might it be that the reason is somewhere else? Like that your current s-video equipped display is not driven by analogue video circuitry anymore and therefore has problems with the non-standard-compliant signal from the 64?

> [...]
> 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.

I see. But still your "obvious" timing problem smells somewhat fishy to me. Never really noticed that kind of issues when using analogue s-video displays. Including the truly hi-res, broadcast studio types of. My bet is that yours is not analogue, is it? And if it is - might it be miscalibrated somehow?  OTOH you say you did this before for the same reason..

Well - then I probably don't have anything clever to say on that. I might help with the measurement and comparison between a reliable, standard (CD32 might not count as such) signal source and the 64. BTW - which board version you have? There were big differences between the modulators and the resulting output. From tack sharp to fully soapy for example. So this also might affect in a way.

-- 
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