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Author
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Topic: RF signal converted to composite video ?
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Frank Angel
Film God
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Posts: 5305
From: Brooklyn NY USA
Registered: Dec 1999
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posted 04-15-2008 08:24 AM
Presently we are distributing attractions to 3 CRT monitors (32in TV sets to be exact) in our grand lobby via RF on standard 75 ohm coax cable. Yeah, yeah I know...it's down and dirty but it works and it was cheap. The run from the source to the farthest monitor is about 75 feet. The run to the other monitors is only about 30 ft. The 30ft runs, the monitors looks fine. The longer run seems to have some very slight herringbone, probably the coax or even the set picking up channel 2 (the RF is on chan 3).
At first I thought, just up convert to a UHF channel to eliminate the interference, but then I thought, why not just go composite video.
Question is, can a standard 1v p-p composite video signal run on coax, #1 and #2, can it sustain a distance of 75 ft without loosing so much high frequency so as to make it worse than the slight herringbone? (which, btw, no one, my boss included, seems to be able to see, even when I point it out, which I find unfathomable since it drives me to ). Luckily there is audio distribution and speakers happen to be in the ceiling right above the monitors, so I can still get audio to play when I loose it in the sets themselves if I go to composite instead of RF.
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Bruce Hansen
Jedi Master Film Handler
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Posts: 847
From: Stone Mountain, GA, USA
Registered: Dec 1999
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posted 04-15-2008 06:26 PM
Go to Radio Shack, and get a 10db RF amp. If you are coming out of a VHS machine, or a small modulator, the RF out is only meant to go a few feet into one TV. Also remember that energy cannot be created or destroyed, only changed in form. If you put energy into a two way splitter, each output will have only HALF the energy (minus losses) that was put into the splitter.
If you use composite video, you MUST use a DA, unless your monitors have looping inputs with 75 ohm terminators that can be turned off. If you "Y" the inputs of two monitors together, each one will only get half the signal level, and each will look rather "dark". If you "Y" three monitors together, you may not have enough sync level to lock the picture at all. A 75 foot long cable is nothing for composite video. Look for a video DA that does not even have a gain control, and gives you "unity gain". That way you don't have to use a scope to set the gain. You could go several hunderd feet with RG-59U type cable without any noticeable change in picture quality on a monitor, with composite video.
John, what type of scope were you looking at HD-SDI on? HD-SDI runs at 1.5Gb/s, most scopes will not come even close to desplaying that, and there is no sync, just ones and zeros (highs and lows). Could it be that you were looking at component analog HD.
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Mark Ogden
Jedi Master Film Handler
Posts: 943
From: Little Falls, N.J.
Registered: Jun 99
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posted 04-16-2008 03:44 PM
Only 75 - 250 feet of double-shielded before a VDA? Wow, that's a lot of processing. Our studio building in NYC is an entire city block long, there are routinely video runs of 500 - 1000 feet with no equalization 'till just before the master routing switcher input, and generally everything looks fine. Sometimes, though, it's the distribution amps themselves that cause a problem in the older analog studios; as they get up there in years, their frequency response falls off at different rates, so you sometimes get variations in signal quality from similar sources depending on their path to the main production switcher.
Back in my NASCAR tour days, I used to take composite video feeds out of the infield at places like Daytona and Talledaga on three thousand feet of 8281, and with just one Telemet Clamper and a frame-sync made a very broadcast-able picture. Was it absolutely dead-nuts flat out to 4.2 MHz? Not quite. Could anyone tell? Nope.
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