Film-Tech Forum ARCHIVE: Video Promo Trailers via RF

This is topic Video Promo Trailers via RF in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Frank Angel (Member # 248) on 10-21-2006, 02:59 AM:

We are saddled with a down-and-dirty video distribution system for out lobby promotion trailers. It uses coax cable carring standard NTSC video/audio RF to distribute program material to to cheap TV sets that are used as monitors. Sure, I would like to do it differently, but no one is going to approve pulling all new cabling all over the building. So I am stuck with this system. It is what it is.

Thing is, the suits seems pleased as punch with the image quality (except me). But there is one real problem that even the laymen complain ahout and that is slight to moderate herringbone that beats across the image. It's not terribly bad, but it drives me crazy, especially on the distant TVs where enough adjacent channels (2 & 4) must be picked up by the coax and blead into the line.

So I am thinking, if there are any frequency band pass filters available, I could insert them at the antenna input of each set and the coax cable. That should allow only channel 3 frequency to pass and attenuate channels 2 and 4, no? If I can get the herringbone pattern out of the video, I think I could live with the limited quality that the RF delivers without pulling out any more of my already sparse hair.

Anyone know if such things exist and if so, where to get a bunch?

Thanks

Posted by Demetris Thoupis (Member # 833) on 10-21-2006, 03:15 AM:

There are the known "traps" that you can use to lower the channels 2 and 4 in your case down to 50db or less which will not give interference to channel 3. But what if you try changing the transmission channel to one more "clean"?
D

Posted by Louis Bornwasser (Member # 3063) on 10-21-2006, 08:23 AM:

I worked many years in the MATV industry. I agree. Look for a completely clear video channel......dead quiet.

Then buy a channel modulator for that channel. If you are on good terms with your local cable people, they may have an old one for free. If you actually order, look for one from a cable supply company, not from Radio Shack or even from "high end" sources.

You will not only get herringbone free pictures, but this will help with other picture issues as well. The "on board" modulator on vcr/dvd is a major weak link.Louis

Posted by Bruce Hansen (Member # 281) on 10-21-2006, 06:30 PM:

I agree. Buy a REAL modulator. The one that is built into a VHS deck, or the cheep crap that Radio Shack sells are junk.

But, if you are not able to do that you might try putting one of those 10db RF amps that Radio Shack (and others) sells in the line.

Posted by Frank Angel (Member # 248) on 10-27-2006, 02:02 AM:

Thanks guys. Didn't even need to go the re-modulation route, which would have had to go into the UHF frequency range because this is NYC & every VHF center frequency is used. A tech at the TV center went into a junk box and came up with industrial RF distribution amp/modulator on Chan 3/4 and evidently its signal is pumping out quite hefty because it there isn't even a hint of beating against Chan 2 any more. And I must say, without the herringbone, the picture looks just fine for the intended purpose. Everyone's happy and I won't have to be crawling thru miles of hang ceiling to rerun video cable. Sweet.

Oh, and aside is in order here -- for one of the far distant runs, there was no way to get coax across a huge expanse of building -- the two theatres are in a big u shape. So, even though I was very skeptical, I got one of these "balum" things that you stick composite video and L&R audio channels into one end of the cigarette pack size black box and stick an RJ45 on a CAT5e cable into the backside of this box and you can get that video and audio to travel almost 1000 feet over our existing CAT5 computer network infrastructure. On the other end, you stick another one of these "balums" (dorky word for nothing more than a transformer, I believe) and out comes your composite video and the two audio channels totally unscathed. Not to mention that of course CAT5 is unshielded, and it's a really long run which passes through at least 4 comm closets filled with electrical equipment to say nothing of all the miles of AC running in the drop cielings along with this punney little CAT5 wire carrying both video and audio! And the black box balums are not even powered! Yet it seems not at all to matter. The picture on the other end is hot dang, clean as a whistle. Pretty sweet, I must say. The two balums run under $100.

In the same vein, I also came across a similar pair of electronic boxes (these are powered) that piggy-back an VGA monitor signal from on computer to a box which can then travel to a companion "conversion" box and into a VGA monitor more than 800ft away, all on unshielded CAT5 wire -- and the box outputs as clean a VGA monitor signal as the originating monitor! Why is this so totally amazing to me? Well it because look at the size of the cable that carries the video from the computer to the monitor. Look at the humungus shielding around it and the ferrite cores they wind it around and you think, gee, this is a pretty fragile signal if it has to be protected with such a heavily shielded and fairly short monitor cable. Then here they get this same VGA signal to travel over UNshielded CAT5, again, running without much concern where or what in the hang ceiling crosses it and it gets over 800 ft away with narry a hiccup. Pretty impressive says I.

Posted by Robert Minichino (Member # 3495) on 10-27-2006, 10:23 AM:

Just as an aside, it's a "balun" which is short for balanced/unbalanced transformer. Coax is unbalanced and twisted-pair is balanced.