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Testing a Video Signal

Discussion in 'Electronic Design' started by Mike Warren, Jan 18, 2010.

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  1. Mike Warren

    Mike Warren Guest

    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.

    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.

    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.
  2. Mike Warren

    Mike Warren Guest

    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.
  3. Phil Allison

    Phil Allison Guest

    "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
  4. Mike Warren

    Mike Warren Guest

    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.
  5. Mike Warren

    Mike Warren Guest

    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.
  6. Mike Warren

    Mike Warren Guest

    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.
  7. Mike Warren

    Mike Warren Guest

    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.
  8. Phil Allison

    Phil Allison Guest

    ** 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
  9. Phil Allison

    Phil Allison Guest


    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

    ** Horizontal synch is NOT the problem.

    Colour sub carrier synch IS !!

    ...... Phil
  10. Phil Allison

    Phil Allison Guest


    ** Oh dear....

    Go look up PLL on Google an learn something.

    ..... Phil
  11. Phil Allison

    Phil Allison Guest


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

    ** Meaningless drivel.

    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


    ..... Phil
  12. Mike Warren

    Mike Warren Guest

    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.
  13. Mike Warren

    Mike Warren Guest

    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.
  14. Phil Allison

    Phil Allison Guest

    "Mark Fucking Idiot Kolber "

    ** It is in fact the ** MOST ** important issue here.

    You fucking GOOGLE MONKEY !!

    ** Wot a completely asinine suggestion -



    ...... Phil
  15. Phil Allison

    Phil Allison Guest

    "Mike Warren"
    ** You bet is not accurate enough.

    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

    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
  16. Mike Warren

    Mike Warren Guest

    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
    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".
  17. Mike Warren

    Mike Warren Guest

    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.
  18. Phil Allison

    Phil Allison Guest

    "Mike Warren"
    ** You mis-comprehend.

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

    ** You already have your evidence.

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

    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
  19. Mike Warren

    Mike Warren Guest

    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
    going as far as I have.
  20. Mike Warren

    Mike Warren Guest

    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
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