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The paper describes a system for capturing and compressing video data in real time. It has principally been used for recording activity on the screen of computer workstations. Its main application has been in evaluating the useability of computer based products from a human factors point of view, as it allows the replay of screen displays with the same high fidelity as the original computer system. The displays are captured in digital form, and owing to the capacity and speed of digital storage media, it is necessary to apply significant data compression during the recording phase. The paper examines the degree of data compression necessary to be able to handle typical computer workstations. Some of the known techniques of data reduction are outlined, and a practical data compression system described.