Testing a Video Signal

S-video has seperate Luminance & Chroma channels. You need both to get a color image. If you are feeding a composite monitor, you need a S-video to composite adapter.

shows a simple circuit.

The color burst in NTSC is only about seven cycles of 3.57954545 Mhz per frame. there is no way to read it during that time. You would need to read it in the monitor after it is regenerated in the chroma circuits

--
Greed is the root of all eBay.

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.

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- Mike

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.

--
Greed is the root of all eBay.

"Mike Warren"

** The symptoms fit with a colour sub carrier frequency that is too far off for your monitors to lock onto.

The allowed error ( for a broadcast signal ) is only 5 Hz in 4.4MHz or about 1ppm - which is way better than an un-trimmed crystal can provide.

Being only a 10 cycle burst means measuring it directly is not possible.

If the PCI card uses the computer's clock frequency for all timing, then you are stuck.

If there is an on board 4.43MHz crystal - maybe you can tweak it a bit.

.... Phil

On a sunny day (Mon, 18 Jan 2010 02:14:05 +0000 (UTC)) it happened "Mike Warren" wrote in :

start: The burst is way too small, possible teh color killer activates.

Feed it trough an amp that peaks above say 2 MHz, to icrease chroma amplitude, to see if the collor killers de-activate.

goto start

On a sunny day (Mon, 18 Jan 2010 03:01:53 +0000 (UTC)) it happened "Mike Warren" wrote in :

The color burst amplitude should be the same size as the syn pulse. (About 300mV). Your S-Video interface sucks.

That's what I was guessing, but hoped there would be an easy way to confirm.

There is only one crystal. I'm guessing it's the master clock and all the other clocks are PLLs.

I don't fancy getting involved in hardware mods to 100 video cards so I'm hoping there is a software trim available. I've asked the manufacturer and are awaiting their reply.

Thanks for your reply, Phil.

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- Mike

I don't believe this is the case here since the good card has exactly the same signal level. Also, I've had others with even lower colour burst still work fine on my test monitors.

Thanks for your reply.

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- Mike

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.

--
- Mike

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

On a sunny day (Mon, 18 Jan 2010 13:39:56 +0000 (UTC)) it happened "Mike Warren" wrote in :

Well religious beliefs is not a subject I will argue about.

On a sunny day (Mon, 18 Jan 2010 13:42:48 +0000 (UTC)) it happened "Mike Warren" wrote in :

That sucks even more.

Didja try another s-video cable? Maybe the chroma pin is loose in a connector.

-- Paul Hovnanian snipped-for-privacy@hovnanian.com

---------------------------------------------------------------------- Have gnu, will travel.

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.

"bg"

** Completely WRONG.

The phase of the colour sub carrier is what produces hue shifts.

Explains why NTSC is so prone to hue shifts in transmission and why PAL was invented to solve the problem.

If the TV receiver or monitor cannot phase lock onto the sub carrier burst, then normally no colour will appear.

.... Phil

the

well I can't see your pics for some reason but the problem has got to be either freq or amplitude of the chroma info...

It sounds like you are using a small cap for a crude combiner, but consider this, the cap will pass the chroma signal and sum it with the luma, but it depends on what the output Z of the luma signal is. so instead of using just a cap, you may want to insert some small Z like a resistor or inductor in series with the luma output, like a crude diplexer.

Regarding the freq, the color bust freq and the V and H should all be locked together. you can probably count the H and it should be

15.734. kHz for NTSC. Another trick is to take a known good video signal and loosly couple the UUT video to it and you will see the video sync drift through. If the H sync drift less then 1 frame per sec or so, it is probably close enought that a monitor should lock to it. The FCC spec for TV stations is a lot tighter then most monitors require, consider they work with cheap VCRs.

Please do come back and let us know what you find when you solve it. Mark

"Mark"

Regarding the freq, the color bust freq and the V and H should all be locked together. you can probably count the H and it should be

15.734. kHz for NTSC.

** The OP is in Australia an so using PAL monitors.

Another trick is to take a known good video signal and loosly couple the UUT video to it and you will see the video sync drift through. If the H sync drift less then 1 frame per sec or so, it is probably close enought that a monitor should lock to it.

** Horizontal synch is NOT the problem.

Colour sub carrier synch IS !!

..... Phil

If the frequency was off but close enough to allow the oscillator to partially lock to it, wouldn't that cause the hue to alternate?

"bg"

** Oh dear....

Go look up PLL on Google an learn something.

.... Phil