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Video signal DC level adjustment

HANKMARS

Jul 28, 2019
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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.
 

crutschow

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

HANKMARS

Jul 28, 2019
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Don't understand your concern about the DC level.
What DC level?
Is power being sent along the same wire?
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.
 

HANKMARS

Jul 28, 2019
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Coax? I'd be using a balun and CAT5 over those distances.
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.
 

Harald Kapp

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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.
 

CircutScoper

Mar 29, 2022
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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.

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
 

kellys_eye

Jun 25, 2010
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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
 

Martaine2005

May 12, 2015
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I wondered the same thing but forgot to ask about it.
Clearly local addresses though.


Martin
 

Harald Kapp

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