Techie Question: If it’s a digital video signal, why does the video quality degrade when using ethernet cabling?

Monday, December 15th, 2008 at 1:33 pm by DigitalSignageBlog, 4 Comments »

Just recently, I was involved in a discussion with a couple of friends about whether one of them should install network cabling throughout his new home. We started brainstorming reasons why he should do it:

1. You’re able to connect to your Local Area Network with faster and more reliable speeds than wireless

2. It will be more secure than wireless

3. Cheaper to install the cabling before the walls go up

4. You’re able to use the network cabling to extend video and sound

Now, whilst we were talking about point 4, we ended up having a heated debate whether you degrade video quality when extending a DIGITAL video signal. One my friends raised a good question which was ”If you send 0 & 1′s down the ethernet cable, you would expect those 0 & 1′s to come out the other end. So why would you loose video quality if it’s digital?” – that was a really good question and I was quite reluctant to answer because I didn’t have an answer.

So for those techies out there, how would you have responded to this question?

Feel free to respond to this question by adding a comment.

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Tags: cabling, network, quality, techie, video, video extenders

Category: Digital Signage Blog Info, Digital Signage Techie

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4 Responses to “Techie Question: If it’s a digital video signal, why does the video quality degrade when using ethernet cabling?”

Eric
December 15th, 2008 at 3:13 pm

Video-over-CATx systems I’ve used send RGB similar to a VGA cable, not data like on your computer network (hence you can’t mix them or throw in a D-Link somewhere in the middle). The good one’s will take a VGA signal and send it up to 2000ft. @ 1920×1080. The issue when you’re sending a video over long distances is that the red, green and blue signals travel at different speeds. The longer the distance, the more pronounced the lag between colors. This leads to distortions in the image like ghosting. That said, these systems include the possibility to tune them–adjust the speeds of the signals and other characters to get a perfect image. The same issue happens with any long video cable, that’s why commercial LCD DID’s include long cable compensation and the same type of tuning capabilities. I’m not familiar with systems that take digital signals (i.e. HDMI), so I don’t know if any of the above would apply.
Jim
December 15th, 2008 at 11:15 pm

Exactly as Eric said, distributing analogue video over CATx cables is quite common, but not all offer the same level of quality due to the way they pre compensate and post process the signal at the reciever end. A skew adjustment is often included to compensate the CATx which is a multi strand twisted cable (resulting in some wires being slightly longer than others). CATx is commonly used due to its low cost, ease of installation and proven long term reliability.

Distributing true digital signals (ie DVI/HDMI) is much harder due to the bandwidth required to carry the video. Its kind of like tipping a bucket of water down a hose pipe. There are a few devices out there which will allow you to extend Digital Signals, but be careful as many of them are compromised in some way (ie, reduced colour depth). There are even some devices which take a digital input, convert them to analog (VGA), then reconvert to digital at the remote end! Whats the point?

It is this issue of bandwidth that limits the extension of digital signals. Yes, you could stream MPEG without degradation (beyond the obviouse comporomise of MPEG that is), down CATx but here we are talking about uncompressed digital video…there are simply too many 0′s and 1′s to fit the pipe easily. There are a few solutions out there which reguire more than one CATx, but I saw an exciting new product at IBC in september being demonstrated by Adder. They have managed to engineer a true no compromise DVI extender and carry USB control over a single CATx line. Knowing a bit about this type of application, trust me, its a real achievement! I cant see it on their website at the moment, but I guess this is where it will appear: http://www.adder.us/KVMExtenders.aspx
DigitalSignageBlog
December 18th, 2008 at 1:31 pm

Great answers Jim and Eric.

Would I be right in assuming that rather than using CATx a better solution to extend video is Fibre? That way you can achieve a better bandwidth to carry the video?
Andy
December 20th, 2008 at 4:26 am

Eric’s answer correctly sums up the issues with sending video down Cat-X cable using an analog (RGB) technique. Fiber is a better way to get more bandwidth and longer distances (just like for data) but don’t expect to use standard network appliances with either simply because most video extenders are analog or proprietary digital formats. I don’t know whether the Adder extender that Jim references is ethernet protocol or a custom digital technique, but SVSi demonstrated its voLANte video-over-ethernet system at Infocomm in June that sends multiple HD video streams down one Cat-X cable in true ethernet format. As such, the video could be sent through common network switches to an unlimited number of displays. Although the SVSi video system is limited to 300-ft between switches, since it’s ethernet-compliant, inexpensive switches and network fiber exenders can be used to go even further (up to 10km). More info at www.easysignage.net.