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Automatic Skew Compensation for Video Transmission over Unshielded Wire Pairs

IP.com Disclosure Number: IPCOM000127370D
Publication Date: 2005-Aug-25
Document File: 9 page(s) / 469K

Publishing Venue

The IP.com Prior Art Database

Abstract

This article sets forth a technique for automatically correcting for skew when video is being transmitted on 3 pairs of wires, such as in a CAT5 cable. The skew error results from the cable having multiple pairs of slightly different in length, causing the colors to arrive at the monitor at different times. This produces ‘separation of colors’ resulting in blurry images with color tinted outlines. The method described automatically provides an additional delay in some of the twisted pairs in the cable to equalize the signal delays introduced by the transmission of the different video color signals over different length wire pairs.

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Automatic Skew Compensation For Video Transmission over Unshielded Wire Pairs

This article sets forth a technique for automatically correcting for skew when video is being transmitted on 3 pairs of wires, such as in a CAT5 cable.  Each pair is used to transmit one color (red, green or blue) of an RGB video color signal.  The skew error results from the cable having multiple pairs, where in a long cable each pair is slightly different in length.  With video transmission, the time it takes the signal to arrive at the destination is directly proportional to the wire length.  If the three wire lengths are not equal, the colors will arrive at the monitor at different times.  This produces video distortion known as ‘separation of colors’ where each color arrives at a different time and is displayed on the monitor slightly offset form the others, thus producing blurry images with color tinted outlines.

To correct the problem we have to make the three colors arrive at the same time in the receiver.  In order to compensate for the skew, additional delay is provided in some of the twisted pairs in the cable to equalize the signal delays introduced by the transmission of the different video color signals.  That is, additional delay is inserted into a shorter length wire to make its delay match that of a longer wire. The delays vary between different cable manufacturers, and of course the skew delay is different depending on cable length. Since the CAT5 cable is originally designed to transmit Ethernet digital data (not video), it is manufactured with an intentional skew between different wire pairs. This is intentional to reduce crosstalk in the cable, but with video it produces the skew errors.  The introduction of signal delay into cable pairs can be provided by a number of mechanisms known in the art.  For example, adjustable delay elements can be provided in the receiver to align the arrival times of each video color signal at the outputs of such elements to one another.  Alternatively, such delay compensating mechanism can be provided at the transmitter for such signals. 

In a typical video distribution system, one may have a single video source being distributed to many receivers or one may have many video sources connected thru a switch to a single receiver: a user station.  The latter is the more interesting situation because, when the user selects a particular video channel, the receiver has to automatically adjust the skew delay for that particular channel. And, each channel will require a different delay setting since each is wired with a different length cable.  An example of such an application is a KVM (keyboard, video, mouse) switch system for managing servers in a data center.  In such an application, a user at a workstation remotely manages a number of servers.  The user selects a remote server and sends keyboard and mouse signals to the server, receiving the video that is displayed on the workstation monitor.  As a...