We could probably wire up one of the Rhombus TZB-series passive delay chips with a switch and do pretty well to fix this. http://pdf1.alldatasheet.com/datasheet-pdf/view/268382/RHOMBUS-IND/TZB12-10.html -- Pete Rittwage > Den Thu, 31 Aug 2017 21:03:18 +0200 skrev email@example.com: >> >> > On 2017-08-31, at 20:06, Mia Magnusson <firstname.lastname@example.org> 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: > > https://cdn.shopify.com/s/files/1/0986/4308/files/Cable-Delay-FAQ.pdf > >> 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. > > > -- > (\_/) Copy the bunny to your mails to help > (O.o) him achieve world domination. > (> <) Come join the dark side. > /_|_\ We have cookies. > > Message was sent through the cbm-hackers mailing list > Message was sent through the cbm-hackers mailing listReceived on 2017-08-31 21:02:28
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