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Video signal DC level adjustment

Discussion in 'LEDs and Optoelectronics' started by HANKMARS, May 30, 2022.

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  1. HANKMARS

    HANKMARS

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    Jul 28, 2019
    If I receive a video signal thru wire that is ~500 ft long, I'll be concerned about its DC level. Is it practical to capacitively couple the signal to my video amplifier or will an attenuation factor of the caps degrade the signal. I'm too lazy to do the calcs I suppose but also I'm uncertain of signal freq.
     
  2. crutschow

    crutschow

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    May 7, 2021
    Don't understand your concern about the DC level.
    What DC level?
    Is power being sent along the same wire?
     
  3. HANKMARS

    HANKMARS

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    Jul 28, 2019
    Power to camera will be in same cable but not same wire as video signal wire.Video signal typically - & + from camera out. Typically power ground and - side of signal tied together at camera. Seems like at amp I could tie - side of signal to power ground but in last iteration this wasn't done. At the moment I don't recall why. What may be my gnd level at camera may drift some over a 500 foot run of wire, particularly if on a spool, say 250' straight line and 250' spooled. It takes very little voltage drift to start chopping one side of signal resulting in screen blackout or whiteout.
     
  4. kellys_eye

    kellys_eye

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    Coax? I'd be using a balun and CAT5 over those distances.
     
    hevans1944, davenn and HANKMARS like this.
  5. HANKMARS

    HANKMARS

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    Jul 28, 2019
    Coax is too rigid. Last prototype used 5 or 6 conductor (twisted pairs) shielded cable. Worked nicely. For whatever reason (I will guess DC level drift ) monitor had a tendency to white out. The famous microsoft fix (cycle power) would almost always bring back image.
     
  6. HANKMARS

    HANKMARS

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    Jul 28, 2019
    Surprisingly, baluns did not seem to be necessary. Good resolution until white out.
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    A standard conforming composite video signal needs a defined black level reference at 0.339 V. This black level is lost when AC coupling the signal. On the other hand it will also be lost if the DC level of the signal is disturbed.
    The solution is AC coupling together with a circuit that restores the black level. Here's an exemplary description of the process.Your receiving end may or may not be equipped with such a circuit.
    If it is, AC coupling will be a no brainer.
    If it isn't, you will possibly lose synchronisation and/or get a murcky image.
     
    HANKMARS likes this.
  8. CircutScoper

    CircutScoper

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    Mar 29, 2022
    What he said! Check out fig. 1 in the link below to see how simple video DC restoration off the horizontal sync pulse is -- just a cap and a diode.

    https://www.analog.com/media/en/technical-documentation/application-notes/an-1603.pdf
     
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  9. Harald Kapp

    Harald Kapp Moderator Moderator

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  10. CircutScoper

    CircutScoper

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    Mar 29, 2022
    What he said!
     
  11. hardyflower

    hardyflower

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    Sep 6, 2021
    that's a controversial subject
     
  12. Harald Kapp

    Harald Kapp Moderator Moderator

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    Not helpful. Anything you want to add to your statement?
     
  13. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Query....

    Not the first time I've see this in a post but I can't replicate it by using the quote function so show a screen cap of the output that's displayed on my screen when I look at that post.

    Note the inclusion of two (spurious?) IP addresses that have appeared. Anyone explain why this is happening?

    Screenshot from 2022-06-20 14-17-04.png
     
  14. Martaine2005

    Martaine2005

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    I wondered the same thing but forgot to ask about it.
    Clearly local addresses though.


    Martin
     
  15. Harald Kapp

    Harald Kapp Moderator Moderator

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    No such IP addresses on my screen. Not in Chrome, not in MS-Edge.
     
  16. Martaine2005

    Martaine2005

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    May 12, 2015
    65BBB283-5ECB-4057-8449-20A847F6F1AD.png
     
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