Composite video through CAT5e cable? - Page 1
Forum section: electronics / projects

author: hlavac
author_rank: Frequent Contributor
timestamp: September 30, 2012, 07:25:40 pm
content: I'm working on a little project that requires transmission of composite video (75ohm source) over a 100ohm cable(one twisted pair of CAT5e cable).My first thought was some sort of balun but then I realized composite video is not AC balanced :-(So I neeed to convert the 75Ohm source to 100ohm cable and then back to 75ohm output on the other end.On the video source side, I figured I will simply add a 300Ohm resistor in parallel to the line to match the cable to the 75ohm of the video source. Can't afford anything more on that end (size / weight constraints).On the other end, I'm thinking 100ohm termination and a buffer with 75Ohm output impedance to pass the video to a monitor.However, buffering video is not so easy it seems, I don't have any fancy fast 120MHz opamps and the jelly bean ones suck (bandwidth in kilohertz ffs!).Does anyone with some video experience have a nice simple circuit to do the video distribution amplifier with a few transisors or so that would work with 100ohm termination on the input instead of 75ohm? I do have some pretty fast BJTs on hand...I have only single +5V power supply available, and power consumption is not much of an issue (5V/1A DC wall plug adapter).

author: SeanB
author_rank: Super Contributor
timestamp: September 30, 2012, 07:39:02 pm
content: Security suppliers have a balun that is passive, and does what you want using any cable from CAT2 upwards ( and it will work over phone cable as well for short lines) for up to100m of cable with little degradation. Simple to use and they do work, I have used them to do cameras over a 60m run ( and powered the camera using the other 3 pairs as well) with no problems. Handles at least PAL video, and does work with a 640x400 camera. Pretty cheap as well.There are also powered units that will do over a kilometre of cable with ease.

author: hlavac
author_rank: Frequent Contributor
timestamp: September 30, 2012, 07:53:31 pm
content: Should be simple- if only I had an opamp with unity bandwidth of at least 10MHz...

author: kripton2035
author_rank: Super Contributor
timestamp: September 30, 2012, 07:54:39 pm
content: something like this :http://dx.com/p/passive-video-audio-balun-for-cctv-network-rj45-pair-37659

author: SeanB
author_rank: Super Contributor
timestamp: September 30, 2012, 08:03:00 pm
content: Correct, though the ones I used had screw terminals for the UTP cable side, but otherwise the same. They were slightly cheaper than that though..........

author: hlavac
author_rank: Frequent Contributor
timestamp: September 30, 2012, 08:19:15 pm
content: I was thinking of making some baluns (13:15 turns). But then you need some magic on the other end to shift the signal back above ground. With a "chopper" diode or something. Sounds like a lot of distorsion. I want nice quality

author: T4P
author_rank: Super Contributor
timestamp: September 30, 2012, 09:31:37 pm
content: How about a THS6012 CFB opamp? It has a bandwidth of 315MHz at -3db with 100 Ohm load

author: vk6zgo
author_rank: Super Contributor
timestamp: October 01, 2012, 04:14:34 am
content: Use the Cat5 as a draw wire,& pull arealvideo cable through!Seriously,though,it is quite commonly done.A balun would be the best idea,as you need to get the benefit of the balanced pair.Youcanlose the DC component in a video signal & restore it at the other end,or get away without restoration.You may,in fact, find that the Picture Monitor you use has a DC restorer built into the input circuit,so try AC coupling first.If you do need it,DC restoration can be simple or complex----you probably can get away with simple.Google for DC restorers & Video clamp circuits.

author: ee851
author_rank: Regular Contributor
timestamp: October 02, 2012, 04:33:44 pm
content: Quote from: vk6zgo on October 01, 2012, 04:14:34 amAmen, brother!It's all about impedance matching. For a quality signal at the output, you need baluns to make the input impedance equal the output impedance everywhere the transmission line changes, from coax. to Cat5 and back again at the other end. There will be a limit, in meters, of how fat you can run that video signal on Cat5 cable and still get a decent-looking signal out at the output end. It will be up to you to find that limit.If you don't take time to carefully match input impedance to output impedance through testing at each juncture, the limit will be low.Even with perfect impedance matching at every juncture in the transmission line, the limit on the length of Cat5 cable will probably be in the tens of meters, because Cat5 cable is not shielded. And that limit could be different on each Cat5 cable that you run, due to interference. Keep the Cat5 cable as far away from electrically conducting materials as possible. Keep it far away from transformers and things with magnetic fields as well, such as electric motors.

author: nctnico
author_rank: Super Contributor
timestamp: October 02, 2012, 05:35:33 pm
content: How long is is the cable and what kind of videosignaal do you need to transport? The required complexity of the solution depends very much on the answers to those questions

author: poodyp
author_rank: Regular Contributor
timestamp: October 02, 2012, 07:06:23 pm
content: This is a more appropriate price., or theebay special.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 03, 2012, 07:00:09 am
content: Quote from: ee851 on October 02, 2012, 04:33:44 pm"Silicon Chip" magazine published a construction article on using video through Cat5,some years back,& it is apparentlydone quite frequently for security cameras & the like.I worked for many years in Television,& I never saw anything but coax cable used for Broadcast standard video.I am sure if we could have used Cat5 to get equal results,the "suits" would have had us doing so,due to the cheaper costs.

author: johnwa
author_rank: Frequent Contributor
timestamp: October 05, 2012, 11:03:56 am
content: While you should ideally match all the impedances, you may find that you can get away without anything too fancy. I have successfully run colour PAL video over ~70m of cheap shielded audio cable, and I am sure that the impedance match would have been quite poor. I also managed to send the 12V supply to the camera down the cable. I just AC-coupled the signal with some large electros, and fed the power onto the signal wire through some large chokes at each end. I put red LEDs across the input and output to clip the transients when the coupling capacitors charged.I can't really remember how good the sharpness was with this system, but there was certainly no ghosting or anything. I think you should be able to get quite acceptable results with Cat5, and a passive balun at each end.

author: hlavac
author_rank: Frequent Contributor
timestamp: October 05, 2012, 03:45:06 pm
content: Quote from: nctnico on October 02, 2012, 05:35:33 pmStandard composite video, PAL and NTSC. Cable length is about 5 meters.Tried without impedance matching, and it is not acceptable (loss of color, artefacts...).Tried matching a bit with 300Ohm in parallel by the source and 24Ohm in series on the other end, got some color back, but still bad quality and artefacts.I have bought some decent opamps from UK (OPA350PA, MCP631-E/SN, OPA353UA).I wish local exectronic shops here in CZ would carry these...I will now try to make a proper impedance matched buffer amp as in the circuit I posted above.

author: nctnico
author_rank: Super Contributor
timestamp: October 06, 2012, 12:45:06 am
content: If its only 5 meters I'd get 75 Ohm cable. Its much easier to pull in a piece of cable than to build all kinds of amplifiers.

author: hlavac
author_rank: Frequent Contributor
timestamp: October 08, 2012, 03:43:34 pm
content: I need the other pairs for power and control data for the camera. And I don't have room for another cable.It´s a light helmet mounted face-camera-on-a-stick for motion capture,I can not afford anything heavy on the end, inertia is a bitchI'm stuck with the cat5e cable. Amp time!

author: SeanB
author_rank: Super Contributor
timestamp: October 08, 2012, 04:15:00 pm
content: So you only need VGA quality video, which a balun can do. Will probably be lighter than the amp as well.

author: sorin
author_rank: Frequent Contributor
timestamp: October 11, 2012, 05:30:51 pm
content: http://myhometheater.homestead.com/vgacable.html

author: elal1862
author_rank: Newbie
timestamp: October 14, 2012, 12:51:28 am
content: Quote from: sorin on October 11, 2012, 05:30:51 pmAt the expense of a terrible impedance mis-match. But hey, some people don't mind double visionMy 2 cents:Don't try to fix something that isn't broken, let alone trying fixing things that you're not even sureifit's broken!Plan ABuy those cheapo passive baluns from DX/ebay. For that little money, you can't go wrong.If it works:Stop right there; don't waste your time on getting 'the ideal solution' if 'less-than-ideal' works well enough.Plan BDoing differential in/outputs with standard opamps is a pain in the a$$ with regard to resistor tolerances. You're better off using dedicated differential amps (with on-chip resistors laser trimmed for high CMMR) like the MAX4142 + MAX4144

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 15, 2012, 12:02:22 pm
content: The CCTV baluns sound like exactly what you need, they are cheap enough and small enough too. I have used them for lots of projects, just remember, baluns for component video have a polarity, so if they don't work at first, swap the wires and then give it a go.With good baluns, a good source, and good cable and no interference you can expect about 50m with no noticeable quality loss, and even then it will probably be hidden by the horrible quality of the camera/lens/lighting/everything.