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Composite video ghosting, 50 ft run will an amplifier cure it?

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Brian Ehlers:
"Overwhelming the reflections" is not really what happens.  "Not contributing to the reflection problem" is more like what might happen with a different amplifier.  Or it might get worse.  Regardless, if the root problem is in the cabling, any improvement you achieve with a different amplifier is just a band-aid.

I'm going to have to bow out and let Brad or someone else with more practical, real-world video installation experience offer suggestions.  My expertise is in high-speed circuit design in general, not video installation in particular.

Arnold B. Krueger:
Jerrybosun wrote on Tue, 08 June 2010 15:42

Your explanation made a ton of sense. It cleared up a few misconceptions that I have had since the 1980's.  So this amp may actually work but it will be doing so by overwhelming the reflections that are causing the ghosting smear.

1. Would a filter and or impedance matcher help in any way?

You can't filter out ghosts because they are at the identical same frequencies as the signals that cause them. A ghost is due to a time-delayed reflection of the original signal. Other than the time delay, the ghost can mimic the origional signal amazingly well. Anything that filters out the ghosts will also filter out the signal that you want to see.

The problem you are having is exactly due to poor impedance matching. So, if you solve the impedance mismatch problem, you would resolve the problem with ghosting.

Here's what I think is happening. Video signals of the kind you are working with are traditinally handled with equipment and cable whose characteristic impedance is 75 ohms. Standard audio cable may not have a specified impedance, but tends to run around 110-120 ohms. Since 120 ohms is almost twice 75 ohms there is a very significant impedance mismatch. The impedance mismatch causes reflections from the ends of the cable that bounce back and forth along the cable, are delayed by the length of the cable, and mix with the signal. Voila! Ghosts.

If the cable is short, then the reflections are so close together that they tend to merge with the video signal and are harder to see. They can even get lost in the limited bandwidth of the receiving equipment.

If the cable and the sending and receiving equipment are perfectly impedaqnce matched, there are no significant reflections.

If there is a minor mismatch, then the reflections are smaller and thus harder to see.

There are passive impedance-matching devices called baluns that can possibly match 75 ohm equipment with 110 ohm cable. However, it is usually preferable and less costly and complex to simply use the right cable to start with.

The video cable of choice for your application would be something like RG 6, preferably with a 18-20 solid copper center conductor and quad shielding.  This cable is readily available at electronics and appliance stores. For a 50 foot run just about any kind of RG6 that you can find will work very well.

If you can't use a standard pre-terminated cable and have to add your own connectors, the connectors of choice are called "Compression fittings" and are available for the kind of connectors on your video equipment.

Tim Padrick:
Belden 8281 or 8281F would be the cable to use for this application.

Thanks for all your help.  FYI the unit did not fix the problem.  The units claim that it was an amp was misleading..  less than 1% amplification per 3 way split.  Looks like I going to have to open the walls again.  At least I can use old wire to pull the new.
Can an Hdmi cable run that far with no degradation? if not I'll cat5 with some baluns/

Brian Ehlers:
An amplifier doesn't have to boost the voltage to be called an amplifier.  In particular, a distribution amplifier typically splits the signal and buffers each output individually, no more.

I'm still confused over exactly what flavor of video signal you are trying to send.  Regardless, it sounds like it is single-ended and is supposed to be transmitted over coax cable, not cat5.  Arnold and Tim have made suggestions for the proper type of coax.  I'll leave it to you to verify that you use the correct impedance (probably 75 Ohm?).

Ideally, the equipment at each end would use BNC connectors, not RCA.  But you probably don't have any choice in the matter.  You may not be able to terminate the coax cable yourself with a soldering iron.  Typically, special tools are used to strip and crimp the connections.  Yes, this kind of stuff really is important if you want to avoid the type of problems you've already got.  For component video, it's also important that each of the wires be the exact same length.

You know, if it were me, I'd be calling in a local electrical contractor with A/V experience.  He'll know exactly what kind of cable to use, have the proper tools, and stand behind his work.


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