Testing a Video Signal
in [Design]
|
From: Mike Warren on 18 Jan 2010 08:52 Michael A. Terrell wrote: > >Mike Warren wrote: >> >>Thanks for the reply. As I explained, I'm converting through >>a 1000pF capacitor. This have been sufficient for me for over 6 >>years, and anyway I have the same problem feeding into a >>S-Video input to the monitor. > > > That may be too much capacitance. You are working with siglas in the >3.5 MHz range and may be coupling too much of the Luminance signal into >the chroma channel. I've seen as little as 47 pF used. I did experiment years ago and found 1000pF to be the best compromise in practice from the values I normally keep in stock. I have never had the need to revisit the situation. I already /know/ that the converter is not the problem here because I get the same fault if I plug directly in to the S-Video input of the monitor. -- - Mike
From: Jan Panteltje on 18 Jan 2010 12:19 On a sunny day (Mon, 18 Jan 2010 13:39:56 +0000 (UTC)) it happened "Mike Warren" <miwa-not-this-bit(a)or-this-csas.net.au> wrote in <xn0gpawoqczhn6003(a)news.aioe.org>: >Jan Panteltje wrote: > >>On a sunny day (Mon, 18 Jan 2010 02:14:05 +0000 (UTC)) it happened "Mike >>Warren" <miwa-not-this-bit(a)or-this-csas.net.au> wrote in >><xn0gpaenva4zfj001(a)news.aioe.org>: >> >> >> >>>www.mike-warren.net/tmp/DSO1-ECS-web.jpg >> >>start: >>The burst is way too small, possible teh color killer activates. > >I don't believe this is the case Well religious beliefs is not a subject I will argue about.
From: Jan Panteltje on 18 Jan 2010 12:26 On a sunny day (Mon, 18 Jan 2010 13:42:48 +0000 (UTC)) it happened "Mike Warren" <miwa-not-this-bit(a)or-this-csas.net.au> wrote in <xn0gpawr9d35ww004(a)news.aioe.org>: >Jan Panteltje wrote: > >>Your S-Video interface sucks. > >It certainly does, but the same type of circuit is very commonly used, >and I've used it myself in hundreds of these machines since 2003 >without a single complaint of no colour. That sucks even more.
From: Paul Hovnanian P.E. on 18 Jan 2010 13:05 Didja try another s-video cable? Maybe the chroma pin is loose in a connector. -- Paul Hovnanian paul(a)hovnanian.com ---------------------------------------------------------------------- Have gnu, will travel.
From: bg on 18 Jan 2010 14:13
Mike Warren wrote in message ... >We have just purchased a quantity of PCIe video cards with >TV out (S-Video) and can only get a monochrome picture >from the S-Video output. > >I have so far tried 6 different cards from 2 batches and >all have the same fault. Here are my observations?: > >-Another brand of video card that used the same video chip >(nVidia 8400GS) works fine using the same driver. > >-The driver /is/ set up correctly. > >- The fault is there in both PAL and NTSC modes. Only one >of my monitors supports NTSC so I haven't done any extensive >testing in that mode. > >-These cards produce a B&W picture on 3 separate monitors >(2 LCD and one CRT) All monitors work fine with the other >brand. The 15 year old CRT monitor will produce a colour >picture from this card for a few seconds when cold. > >- Feeding the signal into a Canopus ADVC-100 video capture >device produces a colour picture on my computer. > >Here are some pictures of the output. In the oscilloscope >pictures the top trace is the composite signal after my >passive S-Video to composite converter (just a 1000pF cap) >and the bottom trace is the chroma signal from the card. >www.mike-warren.net/tmp/DSO1-ECS-web.jpg >www.mike-warren.net/tmp/DSO1-Leadtek-web.jpg >www.mike-warren.net/tmp/TV-ECS-web.jpg >www.mike-warren.net/tmp/TV-Leadtek-web.jpg > >This is a vectorscope signal produced using Adobe Premier >from the same colour bar signal. I haven't recorded the >output of the good card since I don't believe the >vectorscope will help with this problem. >www.mike-warren.net/tmp/Vectorscope-Leadtek.jpg > >What tests can I do to try and work out what is wrong with >the signal? I tried to read the frequency of the colour >burst but I haven't been able to lock the DSO (Tek TDS1002) >on it so far. > >The supplier claims there is nothing wrong with their cards >as no one else has ever complained. > >-- >- Mike For NTSC - As others have mentioned - - - The color burst looks to be about 1/3 of the level of the sync. You also seem to have quite a bit of noise in the signal which would make your color burst amplitude even worse. The spec should be 40 IRE peak to peak for both. I don't recall the limits for these amplitudes, but some monitors are very forgiving of a non standard ntsc signal, others aren't. I've run into problems like this when using an El Cheapo color bar generator on an El Cheapo monitor. If the frequency of the burst was off, the color hue would shift, but you would still have color. The spec there is 2.5 usec of burst which amounts to almost nine cycles. I think 10 cycles of burst is the magic number. No color = no burst. Wrong color = wrong frequency/phase The wrong burst position can cause color shift or loss of color. Your position looks about right. The sync pulse should be 4.7 usec, and there is another 4.7 usec space after the sync pulse for the color burst. The burst is usually centered in that space which leaves about 1 usec between the end of the sync and the start of the burst. |