OT: Video over Mic Cables?

RalphM wrote on 5/17/2009, 12:43 PM
I need to get an SD video signal (no audio) to an A/V booth where it can then be fed to a ceiling mounted video projector. I can run a video driver and string some wire on the floor, but would rather not.

I noticed there are some unused microphone cable runs (about 60 ft in length) installed that would be easy to access. Before I buy adapter cables for each end, has anyone done this sucessfully? If so, did you run composite or S-video?

Thanks,
Ralph

Comments

farss wrote on 5/17/2009, 12:51 PM
If you use video baluns at each end it might just work. If not I really doubts it's going to work. Baseband video needs 6MHz bandwidth i.e. co-axial cable of the correct impedance correctly terminated.

Bob.
musicvid10 wrote on 5/17/2009, 1:15 PM
I agree with Bob. Audio XLR cable is 600 ohms with low bandwidth. The impedance would be much higher at video frequencies. Grain and noise would be the result without amplification except on very short runs.

Get yourself 100 ft. rolls of standard RG59-U 75 ohm video cable at a hardware store, get the appropriate F-to-whatever adapters at Radio Shack, and it will work for a composite feed. I do it all the time for pit monitors, even video recording.
China wrote on 5/17/2009, 5:16 PM
Hi Ralph, I'm new to this forum, but I had been providing video services in a 1500 seat theatre in Brisbane for a couple of years. We used Sony FX1's and used adaptors out of the S-Video outputs, through a small converter to Cat5 cable, then it does a 60 metre run for either recording or through an old harware vision switcher, (S-Video in) then back out the same method to a video projector about 30 metres away. All works perfectly and the adapters were about $45 per pair. Cat5 cable is also pretty cheap. I haven't tried it with mic cables though. Good luck. Hope this helps.
Cheers,
China.
gpsmikey wrote on 5/17/2009, 5:47 PM
Yes, but Cat-5 cable is typically 100 ohm and rated for 100 mhz or better. You still have an impedance problem, but not nearly as bad and it is designed for much higher frequencies than typical mic cable. Be an interesting experiment, but I would certainly have a "Plan B" handy :-)

Recognize also that cat-5 is a balanced twisted pair while coax is an unbalanced line. I would expect lots of smearing of images with loss of edge definition on things with edges with the typical mic cable.

mikey
Steve Mann wrote on 5/17/2009, 6:26 PM
"Recognize also that cat-5 is a balanced twisted pair while coax is an unbalanced line. I would expect lots of smearing of images with loss of edge definition on things with edges with the typical mic cable."

I just ordered a pair of baluns from

http://www.securitycamerasearch.com/surveillance-products/cctv-connectors/bnc-video-baluns-cat-5.html

The spec for the baluns say they will work over CAT-2 or any twisted pair cable. Mic cables are twisted pair shielded, so it just may work.

I'll let you know.
musicvid10 wrote on 5/17/2009, 6:39 PM
CAT5 is lower impedance and higher bandwidth than 600 ohm balanced audio cable.
With the matching transformers, it should work acceptably (expect some ghosting) up to maybe 100ft. max.

For people who need more than 15 ft. of USB run, there are baluns / adapters that work with CAT5, also. Used a lot in presentations, etc.
baysidebas wrote on 5/17/2009, 7:24 PM
I hope it works, but the electrical engineer in me has to point out that the rate of twist in the audio cable is much looser than that in the CAT-5, even looser than CAT-6, and that the untwisting at the connector isn't as rigidly limited or observed in the audio cable as it is in the data cables. The shielding should help though.

Please do report your results though. We're holding our collective breath here.
RalphM wrote on 5/17/2009, 8:40 PM
Thank you all for the replies. The discussion has suggested another possible approach. I suspect that the old twisted pair drivers I have may work over the installed mic cable. I have used them to run video over what may be (at best) CAT 3 cable.

It will be a couple of weeks before the test can take place. May I suggest that breathing resume during this interval?

Thanks,
RalphM

johnmeyer wrote on 5/17/2009, 9:29 PM
Cat5 is definitely the way to go. I do it all the time. I use the Unicom VAA-U501-VA. It converts two unbalanced audio and one composite video to balanced. The audio uses a balun, the video does not, so if hum is a problem, you will also need to get a humbucker for the video (which I have), or get a better unit. You can also get versions that accept S-Video instead of composite. I regularly run video about 30-40 meters. I use these all the time in auditoriums and theaters to get the feed from the soundboard to one of my cameras. I've never noticed any ghosting or ringing or any other video artifacts, other than the ground-loop hum bar which, as already noted, can be fixed by using an isolation transformer, or purchasing a more expensive unit than the one I got, which includes the balun for video as well as audio.