Naked Science Forum
Non Life Sciences => Geek Speak => Topic started by: chris on 01/01/2013 22:20:02
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Does anyone have any reliable solutions to the problem of skew in video transmitted over long cat5/6 cables?
I have two video signals from a remote base-station to send over about 70 metres; the image is not readable owing to serious skew - the colours are al arriving at different times owing to the cable strands being different lengths.
What I'm after is anyone who has solved this problem and what unit they used which they would feel comfortable recommending?
Chris
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What about going with video over IP/ethernet? CAT-5 is non shielded, and would seem to be suboptimal for video transmission.
Pulling Coax?
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RF video link ? ... http://www.maplin.co.uk/HowTo/video_sender.htm http://www.maplin.co.uk/tv-and-satellite/video-senders
[ I don't think "video-sender" transmissions are encrypted, so maybe not suitable for security purposes : you could be providing a big-brother style "reality TV" channel for your neighbours [:0] ]
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Hmm, I've no specific experience but the web suggests that this sending video over cat-5 cable should present no particular problems with 70m lengths (some say up to 300m is OK). I am not sure what you mean by the "cable strands being of different length". Are you using a version of "Component Video" with the R, G and B signal on separate wires? In any case I would guess that the line lengths would be close enough to being identical. Are you sending the various signals differentially down the twisted pairs? You may well get very odd results if you are not doing so.
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Hi Chris,
I think you need to clarify the nature of the "video signal" and how you're using cat-5 cable with it.
Is this a TV-resolution/standard signal or megapixel computer-monitor-like signal?
It sounds like you're sending the picture as analogue...? Is this RGB-component then?
What's the bigger picture? What are you trying to acheive? What's your signal source? Are you looking for a "proper" solution or a merely a "better" (but preferably cheap/hacky) solution?
Ahhh - is the problem not so much that the colours are arriving at different times as you've got all sorts of "ringing"/"ghosting"/"shadows" (potentially different on the different components)... which would happen if the lines are not being driven and terminated properly at either source or destination?
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The "official" technical solution is to encode the video into IP right at the camera, and then transmit the IP packets as Ethernet frames across the 70m of cable (the Ethernet standard says that it should work over at least 100m).
However, cameras are often positioned outdoors, without a good power supply, and with no place to plug in a computer to do the video encoding.
Tips on Ethernet Cabling:
- Modern Ethernet cable has 8 wires, as 4 twisted pairs (http://en.wikipedia.org/wiki/Ethernet_over_twisted_pair#Cabling). Older Ethernet cables have fewer pairs.
- If two signals are sent down wires that are part of different pairs, they will suffer severe crosstalk interference. 1 signal=1 twisted pair.
- Ethernet is intended to be used with balanced/differential transmission. Transformers couple the signals onto each pair, at sender and receiver. This allows the receiver to be much more sensitive, and provides some electrical safety isolation.
- Each pair intentionally has a different twist rate, to minimise crosstalk. The different twist rate means that each pair will have a slightly different length.
- Sending unbalanced signals over Ethernet cable can result in pickup of external radio transmitters, and crosstalk, even if the pairs are correctly assigned. This can also produce earth loops.
- Transmitter and receiver ends must both be matched to the impedance of the cable, or you get reflections and poor signal transfer.
- Attaching the Ethernet connectors is a bit tricky - specialised tools are available to terminate the connectors.
- Ethernet achieves its distance by using digital transmission, which allows signals to be "cleaned up" by the receiver. Analogue signals cannot be corrected in this way.
For analogue and/or unbalanced signals, coaxial cable tends to work better than Ethernet cable, but it is bulkier and more expensive.
High-quality security cameras are available fairly cheaply these days, and come with remote power feeding - this is probably the cheapest solution for a "remote" camera. (For a "local" camera, a USB webcam is probably the cheapest, but USB is only rated up to 5 meters.)
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Ethernet up to 100 MBps, of course, only uses 4 wires of the 8 wires.
I believe Gigabit actually uses all 8.
Anyway, if you are using a 10 MBps or 100 MBps, typically the wires 1, 2, 3, & 6 are used (usually the orange and green pairs).
Wires 4, 5, 7, & 8 are unused (typically blue and brown pairs).
Some devices such as Ethernet Bridges, or Telephones are frequently powered over the extra wires.
You could likely power most low voltage ethernet devices using the extra pairs of wires, and may be able to also power the cameras.
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USB to Ethernet adapters are available , I purchased a pair from Amazon that are rated for use up to 45 meters but unfortunately they seem to be faulty and do not work when tested over 1 meter so I cannot comment as to how they would perform over 70 meters but I expect 45 meters is conservative.
Coax would work fine for a monochrome camera but you might need PAL encoders/decoders for colour.
These USB adapters incorporate 3156 Driver/Receiver chips and as has been suggested by SeanB may well be good for 300 meters.
ISL3156E - 16kV ESD Protected, RS-485-RS-422 Differential Transceiver with Full Fail-safe Rx
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If you buy a POE enabled camera and a POE switch or a POE injector then the camera can be powered over the cable easily. If you have a non POE camera then you can use a POE receiver to power it if it uses less than 10W. Composite video can go 100m over Cat5 cable with a balun at each end, 300m or more is possible with active amplifiers and receivers at the ends. Security suppliers have these as standard parts.
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I thought I'd update you on this situation, since I've solved it.
The video over cat5 systems I am using are made by Adderlink, who are a very good manufacturer; consequently I contacted them and explained my set up.
They told me that the cat 6 cables I was using are optimised for digital transmission and have a squaring off effect on the analogue signals their gadgets transmit.
They told me to find the cheapest cat 5 / cat 5e I could and try that. I was skeptical but I bought a reel of cheap and nasty dirt cheap cat 5, fed it through the duct and, hey presto, I have perfect resolution on both monitors. In fact, one of the senders, which is supposed to be rated up to 1900x1080 only is happy handling 1900x1200 at over 70 metres.
I am slightly dubious of the explanation; I suspect it's more to do with the fact that the cat 6 cable has a huge pitch whereas the Cat5e is literally twisted pairs running in parallel; consequently the actual cable lengths of the pairs in the cat5e are much more similar than in the cat6, where some of the cables might be a good 20 feet longer than others over the distance I'm running.
Anyway, a happy ending; and I thought my experience might help someone else so I'd post it here.
Chris
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cat 6 cables ... are optimised for digital transmission and have a squaring off effect on the analogue signals their gadgets transmit
I am also suspicious of this explanation - cat 6 cable with embedded signal processing?
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The twist differential on cat5 or cat5e cable is much less than cat6 - if you strip out a length of cat6 and measure the difference in lengths between the orange, green, blue and brown pairs it is around 15% (don't remember which is longer - green pair?) and that is why you get more skew on non-skew-corrected SVGA-over-catX extenders. Higher end models insert a sync pulse on the red,green and blue signals and then the receiver has a chance to correct the skew - Scene Double are a manufacturer who do this (they OEM for BlackBox).
Interestingly cat6a (or cat7 if you're an American) doesn't rely so much on the twist ratio (common mode rejection) in favour of PIMF (pairs in metal foil) - this gives a bandwidth of 650Mhz to cat6a whereas cat6 is only a 250Mhz cable, cat5e only 100Mhz. It's amazing you can fit 1000Mbits/s (gigabit) over a 250Mhz cable but this shows the power of channel conditioning/packing.
The really clever technology in this area is the KVM over IP technology used by people like Amulet Hotkey - but that's another story! (They also make Adderlink look a bit primitive).
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Thank you, FunkyWorm, I'm glad that my suspicions proved accurate; I'm also grateful for the other information you were able to supply.