Skew Compensation « SY Electronics

Skew Compensation

Skew correction is the removal of different delays in the individual colours caused by the twisted pairs in the cat5 cable having different twist rates which makes each of the pairs have slightly different lengths. These variations in length, though physically small, result in different time delays for the signals travelling along them and makes the colours appear on the display screen at slightly different positions. All our CatXQ products use XSQ Technology™ that allows the installer to eliminate these skew effects by using two continuously adjustable controls.

The Skew62 is also available if you require skew correction in situations where a CatXQ receiver is not a practical soluion.

What is skew?

When the individual colours do not line up correctly due to timing differences in the catx cable they are termed as being skewed, the following image shows an exaggerated skew problem:

See the next section to find out how to correct this problem using any CatXQ receiver product.

How to correct

Each CatXQ product has three continuously adjustable controls labelled Tune, S and Q that are used to trim the displayed image to the highest possible quality. If the Skew62 is in use then there is just the S qnd Q controls provided. There is no complex set up procedure and there are no DIP switches to set just three simple adjustments and the picture skew is corrected.

Firstly, adjust the Tune control in order to obtain as sharp an image as possible on the display screen. Now the skewed image can be corrected by adjusting the S and Q controls individually until the colours are properly aligned, as shown in the following image:

Please note that because the pixels on the display screen are rendered side-by-side that the colours in the first thin line will always appear slightly offset. The correct skew adjustment is achieved when the individual red, green and blue signals in the entire image can no longer be seen as being separate colours by checking that the black and white bars in the centre of the test card have clearly defined edges.

What causes skew?

This skew effect is caused by the cat5 cable itself because each of the twisted pairs has a different twist rate that results in each pair having slightly different lengths. This problem gets worse as the catx cable gets longer.

The following table shows the twist rates for each conductor pair in the cat5e cable. The information in this table is of a representative cable for illustrative purposes only and may not match any cable used in a specific installation, see the cable manufacturer’s data for more detailed information on a specific cable.

Pair	Cable pair colour	Turns per metre
1	Blue	62.1
2	Orange	81.3
3	Green	53.2
4	Brown	75.2

The more turns per metre there is in the cable pair, the longer that pair becomes as the overall cable length increases. This creates differences in the catx cable pairs that result in small timing differences between the individual colours at the CatXQ receiver, which results in the colours being displayed on later pixels than intended and the colours are no longer aligned with each other.

Skew62

The Skew62 can be used in situations where a CatXQ receiver is not a procatical solution as this device is fitted between the receiver unit and the display device. You will need to refer to the manufacturer’s documentation for the receiver being used to determine how to obtain a sharp image before adjusting the S and Q controls on the Skew62.

SY Testcard Images

Below are links to the SY testcard pattern in several common resolutions, if the resolution you require is not listed then youn may download the Windows program at the bottom of this page to generate a testcard for your particular requirements.