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Split video signal?

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#1 James Cunningham

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Posted 07 December 2012 - 08:05 PM

How much degraded does a composite video image become if there is just one split and is the image degraded if the splitter is on the signal but no video wire hooked to it? Thanks.

#2 Fish

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Posted 07 December 2012 - 09:59 PM

Good evening,

I will proceed under the assumption you are asking the following (2) questions:

--> Does a composite video signal degrade if passed through a splitter?
Well, if it's a passive device (not powered) then the peak voltage at each output port will be about half of the input. So long as the input signal is reasonably strong with minimal loss from the source, you will not see any degradation.

--> What about if only one of the output ports is used and the other left open?
Now this is where you can get into trouble. For best performance, you must terminate the unused port with a load equal to the characteristic impedance of the system, normally 75+j0 ohms. If you do not, the high impedance of the open port will reflect back the signal with a phase shift. This will be seen in the monitor of the connected port as ghosting and will degrade the image.

Composite video is a rather complex waveform and things have to be about right to pass it cleanly through any transmission medium, especially so when color is involved. Just ask anyone with TVs in the 60s and 70s who always had trouble with color balance, especially with greens!

Regards, Marc

#3 Larry F

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Posted 08 December 2012 - 12:09 AM

Rock warns against passively splitting the composite video output from his cameras.

To feed two screens, I was looking for a small, 12-volt composite video splitter I could mount on my scope, and found this:

http://www.active-vi...p/aca-vd122.htm

It works.

#4 James Cunningham

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Posted 08 December 2012 - 01:52 AM

So, its better to have video cables extend from both ports of the splitter. Do both of the cables need to be connected to something?

#5 Larry F

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Posted 08 December 2012 - 07:53 AM

If you are talking about a cheap, non-powered video splitter with F-59 connectors, like you use to take a 75-ohm antenna signal to two TV's (like this for example), but for some reason you want to only use one signal, you can buy terminators like this for the unused ports. Alternatively, you can take the spliiter out of the line and just use a coupler, like this (with RCA jacks) or these (with F59 jacks, top two on the page) or these (with BNC connectors) and they have them with combinations of the connector types and of course you can buy various adaptors.

If you are using the powered splitter that I posted before, I don't think you have to terminate the load on the unused output, but why bother to put any splitter in the line if you're not going to use it? If you do have a splitter so the video is connected to two devices, say two screens, and one of them is not powered, I think there is still a load on the output so you would not have to do anything if you were only using one device.

Rock suggests that if you want to preserve video quality but send the signal to two places, use both the composite and S-video outputs of the camera. The problem with that is only that most video monitors don't have an S-video input, and the cheap composite-to-S-video adaptors (like this) don't really give you the better S-video quality on the monitor. A powered adaptor, like a DVE or even a powered distribution amplifier/splitter (like this) would be better, but now you are talking about a bigger piece of equipment and more cost (and unused capacity in a 4:1 splitter, of course). Shop around on the Internet for these devices. Prices vary tremendously for what looks like very similar pieces of equipment.

#6 James Cunningham

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Posted 08 December 2012 - 08:11 AM

I wanted to use a splitter at the camera for one composite video signal to go to my monitor and the other to a small monitor at the scope for focusing. After getting a good focus, I was going to unplug the small monitor video and then just concentrate on the big monitor. Looks like I will have to unplug the splitter and then plug the composite video back into the camera. Thanks.

#7 Lorence

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Posted 08 December 2012 - 02:34 PM

I wanted to use a splitter at the camera for one composite video signal to go to my monitor and the other to a small monitor at the scope for focusing. After getting a good focus, I was going to unplug the small monitor video and then just concentrate on the big monitor. Looks like I will have to unplug the splitter and then plug the composite video back into the camera. Thanks.


Get a focuser and a long extension cord.

http://www.jmitelesc...s.htm#Celestron

#8 Fish

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Posted 08 December 2012 - 07:20 PM

James,

Why disconnect the small monitor after you get a good focus? Just leave it connected, on or off, and ignore it? Solves your problem!

Marc

#9 James Cunningham

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Posted 08 December 2012 - 08:33 PM

I wasn't worried about the image on the small monitor but I did want a good image without degradation on the big monitor. Having read the earlier posts, I will take off the splitter after I get a good focus.

#10 Larry F

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Posted 08 December 2012 - 11:01 PM

If all you are interested in is focusing, use the S-video output of the camera into one of those cheap adaptors and that into the on-scope focusing monitor. Forget the splitter.

#11 James Cunningham

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Posted 09 December 2012 - 08:03 AM

I have an MCV1-E video grabber which only works with S-Video.

#12 Larry F

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Posted 10 December 2012 - 06:50 AM

But the MCV1-E is needed to take camera output to computer. If you are doing camera output to video screen, then you need a composite signal (until Rock makes a camera with component or HDMI outputs!)

#13 mclewis1

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Posted 10 December 2012 - 10:22 AM

OR (as has been mentioned above) you use a video distribution amplifier to handle the complex stuff, don't unplug anything and leave the composite connection for the small local monitor ...

Mallincam > S-video > 20' cable > 5 port distribution amp >
- 3' cable > Speco crt monitor
- 3' cable > MCV1-E frame grabber
- 3' cable > LCD monitor
- 6' cable > DVR

Mallincam > Composite > 2' cable > small LCD monitor for framing/focusing






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