Improving Image Quality in Twisted Pair Transmission
What is Skew?
In twisted pair video systems, skew is a distortion in the image caused by the late arrival of one or more of the color signals. The delayed color signal is shifted to the right of the other colors in the image, resulting in very poor image quality that can cause viewer fatigue.
Interestingly, the source of skew is the cable itself. Data grade twisted pair cable was developed for computer networks where the main concern is not skew, but crosstalk. Ethernet networks consist of two signals, transmit and receive, running long distances in close proximity. To minimize the crosstalk between individual twisted pairs, the rate of twist is varied among the pairs of wire. This has a side effect of making the twisted pairs vary in length. The pairs with more twist per inch will have a longer length and signals sent down each pair will arrive at the receiver at different times. Computers have little trouble resolving this time differential, but high resolution video systems are less forgiving.
Skew is not a problem when twisted pair cable is used for composite video or S-video signals, because the signals are sent on a single pair of wires. High resolution video signals, on the other hand, are subject to skew, particularly at long distances, because the transmitter processes the signal and sends each color down a separate pair.
So, how much of a time delay is too much? The Cat 5e standard specifies a maximum delay skew limit of 45 nanoseconds (ns) over the entire length of the cable used. This is not acceptable for analog RGB signals. For high resolution video transmission, skew delay must be as close to zero as possible. Measurements taken here at Extron of a typical 1024 x 768 signal with a refresh rate of 60 Hz (pixel clock of 65 MHz) showed the pixel duration to be approximately 15 ns. For an image running at 1280 x 1024 pixels, with a refresh rate of 60 Hz (pixel clock of 135 MHz), the pixel duration was approximately 8 ns. Depending on the resolution, the delay skew resulting from a length difference of three feet will most likely need compensation.
For example, imagine you're working with a brand of Cat 5e cable with a documented delay skew of 1.451 ns per foot. On a moderate cable run it wouldn't be unusual to find a difference of five feet between the shortest and longest wire pairs, resulting in a cumulative delay skew of 7.255 ns (5 feet x 1.451 ns). This would be very close to one pixel width off at the 1280 x 1024 rate and half a pixel width off at the 1024 x 768 rate.
Avoiding Skew: Skew-Free™ Cable
The best way to handle skew is to keep it out of the system altogether. And the best way to do that is install a twisted pair cable specifically designed for video transmission, like Extron Enhanced Skew-Free™ AV UTP Cable.
With our Enhanced Skew-Free cable, the length of each twisted pair is uniform so the red, green, and blue signals arrive at the receiver simultaneously. This cable is comprised of four, color-coded twisted pairs of 23 AWG wires and can be terminated with RJ-45 connectors.
The plenum version of the Extron Enhanced Skew-Free™ AV UTP Cable has a plenum-rated jacket constructed with special fire protectant agents for use in applications where National Electric Code (CL2P) cable is required. Plenum cable can be run through open air spaces, eliminating the need for conduit and reducing installation costs.
Of course, it's not always possible to use Skew-Free cable. Twisted pair video systems are often installed using existing data grade cable. If such systems require high-resolution video to be transmitted long distances, skew is bound to be a problem.
Compensating for Skew: Two Methods
Skew Compensation Cables: Since the cause of the problem is the varying distance between the individual wires, one solution is to add a length of cable (a skew equalization cable, or patch cable) to the wire that is shorter, to equalize the distances. Coaxial cable is preferred as an equalization cable since it maintains proper impedance.
The good news is that twisted pair cable test equipment measures and reports wire pair length, and the test results on the various pair lengths can be used in selecting the right length of skew compensation cable. Extron skew compensation cables are made in 10 different sizes, from two to 20 feet long.
Skew Equalizers: Extron manufactures two devices that use a time delay circuit on each color input that can be independently adjusted from 0 to 39 nanoseconds. The only difference between the two is the type of output connectors used. The SEQ 100 BNC and SEQ 15HD Skew Equalizers each feature three rotary switches on the front panel for adjusting the time delay. These independent rotary switches allow each color signal to be delayed to compensate for the different lengths of each pair of wires.
You can connect an Extron SEQ 100 Skew Equalizer between the twisted pair transmitter and the video source or between the twisted pair receiver and the display device, whichever is more convenient. The SEQ 100s can be mounted in a rack or under furniture and are equipped with external international power supplies.
As with skew compensation cables, twisted pair cable test equipment can be used to measure the lengths of the twisted pairs and determine the appropriate settings on the skew equalizer. If cable testing isn't feasible, some trial and error is required to get the settings just right. Set up the system with a test pattern displayed on the screen. Examine the test pattern for loss of horizontal registration and make adjustments to the skew equalizers until the image is optimized.
Clearly there are numerous potential pitfalls when designing a twisted pair system for high resolution video. The advantages are also numerous, and Extron has a wide range of options for overcoming any technical challenges you may encounter.