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Author Topic: Composite video ghosting, 50 ft run will an amplifier cure it?  (Read 18655 times)

Jerrybosun

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I have  4 RCA lines run that I am taking a composite video signal from an HD cable box (1 dig audio & 3 video RBG)  and running it to a large screen LCD. I am getting some ghosting on what appears to be more than one color with different ghost horizontal smearing. I used high quality wire and soldered my own connections, (I am a great solderer... forgive my little sin of pride) and there is no big RF/Power lines crossing the path.  The Digital audio is working fine.

Unfortunately these have been buried and drywalled over already. Sooo.... I have a few questions.

1. Will this amp/distributor help the ghosting.
2. Is there a cheaper solution.
3. Is this too cheap of a solution? (do I need a higher end 3 channel (RBG) video amp
4. Does HD make a difference on the type of amp I buy?

FYI ebay sale #120275840797   .. Title is    "3 in 9 Out Composite Video Splitter Distribution Amp"

index.php/fa/526/0/
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Brad Weber

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Re: Composite video ghosting, 50 ft run will an amplifier cure it?
« Reply #1 on: June 07, 2010, 05:12:17 pm »

What did you use for cable?  This is video, not audio, so it should be coax video cable and not audio cable.  And 50' is getting a bit long of a run for the tiny coax cables normally used with RCA connectors, with the longer run and cable buried in walls I probably would have used RCA-to-BNC adapters and full size coax.  That being said, I know people that have run video on audio cable for longer distances, however that was for composite video applications rather than component video.

This could be a tricky one to figure out as you are dealing with several potential issues such as the quality of the cable signal, how good an analog output the cable box provides, how the LCD display processes the component analog input and so on along with the cable itself and the result could reflect multiple of these factors.  If you could temporarily get the cable box closer to the LCD display and use a short, component video cable for the connection then that might let you verify if the problem is really, or at least primarily, the cabling.

Many distribution amps only provide gain to make up for the multiple output split, they do not necessarily provide any additional gain.  According to the eBay listing that particular one supposedly does provide additional gain but it is not clear how much.
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Jerrybosun

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Re: Composite video ghosting, 50 ft run will an amplifier cure it?
« Reply #2 on: June 07, 2010, 06:05:16 pm »

Yes I did use high quality copper shielded audio wire.
I tried  moving  the box closer and it works 100% with a 6 foot run,  
Yeah this unit is a true amp not just a splitter.  480Mhz but no db info..
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Brian Ehlers

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Re: Composite video ghosting, 50 ft run will an amplifier cure it?
« Reply #3 on: June 07, 2010, 07:25:05 pm »

For transmission of high-speed signals -- and yes, composite and component video are high-speed signals -- it is extremely important that the entire path be impedance matched.  The driving amplifier must be of high-quality and drive the signal with a particular impedance (typically 50 or 75 Ohms).  The receiving device must provide termination matched to this impedance.  And every inch of cable and every connector in between the driver and receiver must also be matched to that impedance.

Wait, I can hear everyone scream now, "But I've been using the cheap video cables forever and never had a problem."  Good for you.  But the more you push the envelope, the better chance you'll experience unacceptable degradation.

Sending video through "audio wire" (whatever that is) for 50 feet, and using RCA connectors (which are inherently not suited for video, despite being "standard") is indeed pushing the envelope.  What happens is that when the signal hits an impedance discontinuity, part of that signal reflects back the other direction.  When it hits another discontinuity, it reflects back again.  Do this over a long enough distance with analog video, and a ghost image is exactly what I would expect to see.  Do it over a very short distance (say, from your home VCR to TV) and you only get a slight smearing that no one notices.

It is possible that the distribution amplifier you are considering will do a better job of driving your wiring, but it's more likely you need to optimize the wiring instead.  Or use a different transmission method.
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Jerrybosun

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Re: Composite video ghosting, 50 ft run will an amplifier cure it?
« Reply #4 on: June 08, 2010, 10:42:58 am »

Thanks Brian,
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?
2. If so, what type?
3. If so, where would you located it in the signal path?
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Brian Ehlers

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Re: Composite video ghosting, 50 ft run will an amplifier cure it?
« Reply #5 on: June 08, 2010, 12:37:09 pm »

"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.
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Arnold B. Krueger

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Re: Composite video ghosting, 50 ft run will an amplifier cure it?
« Reply #6 on: June 17, 2010, 07:50:17 am »

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.


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Tim Padrick

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Re: Composite video ghosting, 50 ft run will an amplifier cure it?
« Reply #7 on: June 27, 2010, 08:31:14 pm »

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

Jerrybosun

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Re: Composite video ghosting, 50 ft run will an amplifier cure it?
« Reply #8 on: July 04, 2010, 02:41:50 pm »

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/
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Brian Ehlers

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Re: Composite video ghosting, 50 ft run will an amplifier cure it?
« Reply #9 on: July 06, 2010, 08:05:54 pm »

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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Arnold B. Krueger

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Re: Composite video ghosting, 50 ft run will an amplifier cure it?
« Reply #10 on: July 07, 2010, 03:42:00 pm »

Jerrybosun wrote on Sun, 04 July 2010 19:41

Thanks for all your help.  FYI the unit did not fix the problem.



No surprise there.

Quote:


 The units claim that it was an amp was misleading..  less than 1% amplification per 3 way split.



I suspect you misunderstand the basics of video amplification. Video isn't like audio where a little louder may not be a problem.

If you actually amplify video, the color and intensity changes. Amplifying is something like increasing the brightness and or contrast.

About the only time video is amplified is to compensate for losses in transmission. I think 10% (1 dB) amplification is the most I've ever seen.

Quote:


 Looks like I going to have to open the walls again.  At least I can use old wire to pull the new.



If things get sticky, don't forget about cable lubricant. That all said, in general you can't pull cables through a closed wall unless it is inside conduit.

Quote:


Can an Hdmi cable run that far with no degradation?



HDMI cables are usually pre-terminated and are constructed for their given  finished length.  After about 30 feet in-line amplifiers may be involved.

Quote:


if not I'll cat5 with some baluns/


If you are going like 50 feet or more, line extenders with the specified level of CAT-XX  cable (may be up to 2 sets of CAT 5E or CAT 6) depdending on the length and range extender will be advisable.
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Tim Padrick

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Re: Composite video ghosting, 50 ft run will an amplifier cure it?
« Reply #11 on: July 10, 2010, 04:44:58 pm »

Tim Padrick wrote on Sun, 27 June 2010 19:31

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


8281 F can be soldered if you are using (oversized) RCA plugs (which I'd use only if that's what the gear you are interconnecting has).  I've not used 8281.

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Re: Composite video ghosting, 50 ft run will an amplifier cure it?
« Reply #11 on: July 10, 2010, 04:44:58 pm »


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