Video and Computer Signals over CAT5
Here at Projectionfreak, we were early adopters of this technology. It’s now come into it’s own and is quite popular. With the advent of higher-end home theater installs as well as industrials, tours, and museum/corporate installs, we’ve all seen the need for getting signal to far away places. The thing is, it’s not only for distance. With tour budgets tightening and space/weight at a premium, structured cabling has definitely solved a lot of problems. But what is it all about? Let me just start by saying that this area of the signal transmission industry is HUGE. From manufacturers of cable all the way to people that wire their own home theaters and “swear this will work”, the opinions and info will vary greatly. Your individual project will have unique requirements and you should weigh all the options before purchasing or trying to rig up a system. Keep in mind too, that different cable/balun combos will be able to transmit higher resolutions different distances- so again check the spec! We’ll cover the raw basics:
Using CAT5 (the generic term for structured cabling) or UTP/STP to get signal from point A to B involves using twisted pair cable (duh) and a pair of baluns (BALanced/UNbalanced).
Let’s start with cable: the spec for twisted pair cable commonly referred to as “CAT5″ is laid out in ANSI/TIA/EIA-568a-568b and usually consists of 8 conductors (4 twisted pairs) of stranded or solid copper in a sheath. Most (but not all) CAT5 is UNshielded, that is, no extra layer of insulation or shielding to protect the inner cords from outside RF. Some are shielded and some applications require shielded so it’ll help to know beforehand- shielded TP cable is PRICEY! All this cable- CAT5,5e,and 6, is usually terminated with an RJ-45 tip (or 8p8c modular plug) if you’re in the field, but in permanent installs, an 8p8c jack is usually used. In touring or military applications the Ethercon connector from the folks at Neutrik is the weapon of choice for it’s high durability and solid connection. As in all things, you get what you pay for, so skimping on cable will only hurt in the long run. Every application will have it’s different requirements so check the spec! Buying 10,000′ of UTP CAT5e would be a drag when you actually needed shielded CAT6! Now, the UTP/STP spec for 568a/b used to apply for transmission of ethernet at 10baseT, 100baseT, or Gigabit,(collectively referred to as 10/100/1000 for connections that support all speeds) but now that computer signals and video (or non-data) have entered the scene, bandwidth capability is critical. All CAT5E cable is spec’d to 100MHZ and some CAT6 is spec’d up 600MHZ….remember- bandwidth is everything.
What is a Balun? A balun is an impedance transformer that allows you to send a signal that requires a certain impedance value (such as 75 ohm for video) over a cable that has a different impedance (such as CAT5 that has 100 ohm impedance). You need one at each end, one to transform the original impedance to the impedance of the cable and the other to transform it back to the original impedance. These are usually small, powered boxes that look like this. It has a DSUB-HD15 connector on one side and an RJ45 jack on the other, although different formats will have different jacks. There are any number of combinations of DA’s, splitters, and conversion boxes to go along. Some matrix routers that Extron makes will start at 48×48 and go up to 64×64 and are infinitely expandable. It’s not uncommon for a large venue (like a modern stadium) to have multiples of these systems. Here’s a great graphic that shows the numbers of ways signal can be sent/routed through one of these large matrix routers. Keep in mind there are many baluns that allow for transmission of mono audio right along with the video. This can be quite convenient when having to satisfy the audio folks- for instance when a presentation laptop is onstage and all your projectors and audio processing are FOH.
What’s the quality like? That largely depends on a lot of factors, not the least of which is cable quality. Non-data signals sent down structured cabling suffer from what’s called “skew”. The 4 pairs of wires that are in CAT5e cable are twisted at different lengths to keep cross-talk down in data applications. It’s this reason that these cables, once installed for non-data use- should NOT be used for data! When we send video and non-data signals down these wires they arrive at the other end at slightly different times resulting in “skew”. A balun will equalize these times to make up for the differences and some better ones will have EQ/skew pots to adjust accordingly. In addition, a lot of manufacturers will make what’s called “skew-free” cable, cable that’s twisted with negligible differences in length to help this along. For instance, the spec on Kramer’s BC-XTP cable are thus:
Center Conductor: 24 AWG, Solid Bare copper.
Jacket Colors: Brown * Brawn/ white, Blue * Blue/ white, Orange * Orange/ white, Green * green/ white
Outer Jacket Color: Green w/white lettering.
Outer Jacket: PVC.
Center Conductor: 0.041 inches, 0.1mm.
Outside Diameter: 0.22 inches, 5.5mm.
Attenuation (dB/100 ft.):
* -0.6 dB @1MHz.
* -2.0 dB @ 10 MHz.
* -6.7 dB @ 100MHz.
* -8.5 dB @ 155 MHz.
* -9.7 dB @200MHz.
* -13.6dB @ 350MHz.
DC Resistance: 30.4 ohms per 1000ft, 100 ohms per km.Impedance: 100 ohms
Pair to PAir skew: 1.5ns/ 100M
The number to pay attention to is that last one. 1.5ns/ 100M ….you can do the math. It generally means a pristine image even up to 2k. There as many connector and signal adapters as there are manufacturers of this stuff. You can send HDMI (all resolutions), VGA-UXGA, S-video, audio, baseband video and everything in between. UTP/STP is far cheaper than traditional cable per foot, and the install costs are cheaper. Be careful when using it on temporary installs, however, as some structured cable is plenum-rated, meaning it’s only used in specific construction applications and not for every day, out-in-the-open use- vice versa for non-plenum.
Here’s a short list of manufacturers of everything from cable to baluns/switchers: