Testing a Video Signal

On a sunny day (Tue, 19 Jan 2010 14:38:11 -0800 (PST)) it happened Mark wrote in :

Very often H and color subcarrier are not even locked. And just do some calculation what difference in H you need to get a few cycles of in color subcarrier. The PLL that locks to the burst in the monitor or receiver does not look at H at all, except for opening a gate to gate out the burst.

"Mark"

yes agreed, but since Hsynch and subcarrier bust are phase locked to each other, the error expressed in % should be the same for both.

A monitor's H-synch has a wide tolerance while the colour carrier synch range is very small.

You have no clue how any of the circuits work.

Piss off

- you F****IT GOOGLE MONKEY !!

.... Phil

true but irrelevent.. if you reacall I was suggesting the op measure the H sync error in order to determine the color subcarrier errors. They will be the same when expressed in % and the H sync is easier to measure.

take care Mark

On a sunny day (Wed, 20 Jan 2010 09:28:50 -0800 (PST)) it happened Mark wrote in :

Thats is nonsense.

Total crap, any frequency counter will measure the free running Fc, only if it is way of will it cause the PLL not to lock.

Maybe you do not know, but just for the fun and the others who want to learn and not put out crap here, in the old VHS system there could be huge variations in H timing due to the mechanical nature of reading from a tape with a rotating head, while the Fc was exactly within specs. How is this done: The color is recorded as a lower frequency superimposed on the FM modulated BW video, the FM behaving like like RF bias was in audio tape recorders, and that frequency was obtained by mixing *down* the chroma with an oscillator locked to a multiple of H. On playback that oscillator was also locked to H, and used to mix-up the chroma back to 4,43 MHz. As H changed due to mechanical issues, and so did frequency of the chroma from the heads, the mixed product was stable again. In the whole process there is NOWHERE a need for Fc to be locked to H, H frequency would vary wildly, and Fc would be constant.

In PAL one of the quadrature modulators is switched 180 degrees every line. That causes a burst phase change of +45 / -45 degrees every line. In the receiver, when the PPL is locked, that signal appears on the PLL output as a H/2 (7.8kHz) signal. This 7.8 kHz is logical ANDed with that from a H flipflop that flip flops every H pulse, to cause it to flip in the same phase as the transmitter's, it is used to switch one of the demodulation axis 180 degrees. The level of this 7.8 kHz signal is used to activate the color circuit. If the PLL is *not* locked, there will be no 90 degrees burst swing detected, no 7.8kHz, and the color will be switched off. You need a good lock for color, apart from a s***ty noisy digital Tek scope (a cheap analog 10MHz displays video better), there is too little burst it seems, so the color killer comes in. Now I hope you just learned something but I doubt it.

If I was the manufacturer of that card, and sombody came complaining withh a s***ty signal like that generated by a totally absurd S-video interface, I would take the card back as I do no want to sell to idiots.

No you are!

LOL! Thanks. I hadn't been called an idiot today, yet. :-)

You are indeed fortunate to be able work in an area where you can use the best theoretical techniques at all times and are not required to do things by the cheapest method that gets the job done.

I'm not sure why you are fixated on the combiner when I have stated several times that the problem still exists when I connect the S-Video cable /directly/ to the S-Video input of the monitor.

Some more information for anyone who's interested:

I was able to get lock on the colour burst and measure it with the cursors on the Tek. I get 4.44MHz on the good card and 4.42MHz on the bad ones. Unfortunately this is only a one step difference of the cursor position so is not an accurate indication.

I could discern /no/ difference in the timing of the H-sync between the 2 cards.

The chroma level is within the measurement error of my Tek between the 2 cards as well.

The manufacturer is looking into the problem.

Yes, early on, but it is not necessary since I can see the chroma signal at the other end of the cable and the same cable was used for both the good and bad cards.

My comparison is between one card made by ECS which works fine and 6 cards made by Leadtek which do not work properly.

Both cards look nearly identical in layout and have probably just been copied from the reference design.

I have previously used 35 of the ECS cards without any problems, but can't get any more of them.

"Mike Warren"

As I alluded to before, the lock-in range of typical PAL TVs and monitors is only as far as the internal 4.43MHz crystal can be pulled by a small amount of capacitance - ie maybe +/- 25 Hz.

According to your testing, a CRT monitor locked briefly when it was cold - so the sub carrier burst is likely to be on the low side of the correct frequency.

The only way to measure it is to tweak a monitor's crystal so it DOES lock - then measure the frequency of oscillation with a counter.

..... Phil

On a sunny day (Wed, 20 Jan 2010 21:48:26 +0000 (UTC)) it happened "Mike Warren" wrote in :

Wonder what a 'theoretical technique' is, about the same as your S-video interface technique? Do you not think that before complaining about a thing you at least should connect it in the correct way?

I am not fixated on that interface, but you seem to have been for many years :-) Here analog video is long of the past, only some very small cameras I have still use it.

That Tek is a noisy piece of crap.

There is a Tek religion going on, and it has manipulated the brains of it's victims ^H^H^H^H^H^H^H devotees to the point that they are blind and do not even see how horrible it displays.

To measure the frequency difference you can put one signal on X and and one on Y. A Lissajous figure will appear that rotates the faster the bigger the difference. Or you could external trigger on one, and measure the other, and see how many periods pass by in a second. If there is a lot of difference that would be difficult.

Here is a cheap frequency meter, but the resolution is only 32 Hz: Should be enough for this purpose. A monitor oscillator I would expect to have near a kHz or more lock in range.

A good reference is also the Fc of a locked monitor or TV.

H sync frequency has nothing to do with it.

Tek?? Oh well.

:-)

You tell me. How would you do it?

Again, I did. I connected the S-Video output from the card to the S-Video input of the monitor using a commercial S-Video cable and got a monochrome picture. From all six cards under test. The other brand, and others before that, work fine.

Here too, and this is causing me problems because video cards with S-Video or composite outputs are nearly impossible to find now. However my customers have large numbers of old TVs installed in various venues. It would bankrupt them to have to replace all of them and the wiring etc. to get HDMI. And absolutely no one involved would care about the improvement in quality.

I'm on the lookout for HDMI to S-Video or composite converters, but it seems no one makes one.

I only have 10 cycles to view. If I had both cards running simultaneously I still wouldn't be able to sync the colour bursts to do that.

See above.

I have one that is accurate to within a few Hz too. The problem is getting it to read the burst. The only way I can see to do that is the way Phil suggests and measure it at the monitor's oscillator. That's more work than I want to go to at this stage, unless the manufacturer comes back to me and says there is no problem. Then, I will need to offer proof.

I know. I just did this to answer the suggestions of a couple of posters.

Do you seriously think that a monitor will have trouble with a chroma /level/ variation of a few percent?

I get that you don't like Tek, and I'm not all that impressed with them either, but I don't have the budget others do, especially if this is to help correct someone else's error.

All I needed to do here was connect the card to my monitor via the S-Video connector and tell the manufacturer that it didn't work. What I wanted to do was get a little more information as long as it didn't entail too much work so I could reduce some of the inevitable to and fro while the manufacturer tried to tell me things like "Make sure you have the card set to PAL, not NTSC" and "have you tried another cable".

Clearing enough space to disassemble the monitor is more work than I want to do unless I have to, so I'll wait and see what Leadtek have to say first. :-)

On a sunny day (Wed, 20 Jan 2010 23:23:03 +0000 (UTC)) it happened "Mike Warren" wrote in :

I would have to charge you, and, as you are so poor, you would not be able to pay. But there are a lot of examples on the web. Google is your friend.

I am NOT telling you what you to do - only pointing out what measurement method would likely work.

The PCI video cards do not work with the monitors you need them to work with.

Therefore, they are " unfit for the purpose intended " as the TPA says.

Even if you had a figure for the burst frequency error - it would not prove you case any better than the evidence of people's eyes.

.... Phil

At this stage I'm hoping that the error can be corrected by an update to the firmware as I can't find any other cards with S-Video or composite outputs. Everything has HDMI only. If I had another source, I'd not have even bothered going as far as I have.

not

the

s.

On any quality video gear the H and V and burst ARE phase locked together to minimize dot crawl and other artifacts. Its true that, they don't HAVE TO BE phase locked and it's true that they are not phase locked in consumer grade VCRs but on a computer video board they probably are.

Do you see see more then one clock ref on that board, if not then the H and V and color are phase locked.

Look at the block diagram of a sync generator.

If you can't measure the burst error, then measure the H error.

good luck.. Mark

This is a video graphics processor running (I guess) at somewhere between 200 and 500MHz. There is only one external clock, a 27.0MHz crystal. I expect that is only be being used as a reference for a number of PLLs inside the chip, but I don't actually know anything about what happens inside the chip.

It's all just guesses. The only thing I do /know/ for sure is that it is not doing what it's supposed to, and another brand of card which appears to have been built from the same reference design works fine.

As I posted a few hours ago, I can not measure any difference in the H-sync, but do see a very slight difference in the colour burst frequency.

Just for fun I added 5pF in parallel to one of the caps on the crystal and now all three of my monitors have colour.

I guess doing that can be a last resort, since it only takes a few seconds, but it's certainly not an ideal situation.

Tweaking the PCI card's crystal was suggested by ME to YOU just over THREE days ago.

Rub, rub rub .....

.... Phil

Yes.

LOL! :-)

Simply looking at the 4 scope traces is it clear that the cards=20 do not seem to produce proper PAL "phase alternate line" signals.

In particular the "cheaper?" cards may not bother to invert the=20 chroma burst as would be needed to maintain proper chroma.