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Appendix 2

SunVideo and the XIL Programming Library

The following sequence of statements (taken from the Sunvideo documentation) show the steps taken by a simple XIL program to produce the video element of a video conference. It should help to clarify the sequence of events in the capture, compression and display of video images. It also helps to illustrate the connection between the SunVideo card and XIL. Nevertheless, it should be emphasised that the sequence shown here is specific to the XIL SunVideo environment. Other video cards may have different features; for example, some may support decompression while others may not support compression at all..

About the SunVideo card

Figure A1.1 depicts a workstation scenario. The workstation possesses a local CPU plus memory together with a SunVideo system which has four subsystems, Video Capture, Compression Engine, Frame Storage and the SBUS-Video Interface.

Figure A1.1 A SUN Workstation Scenario

The Video Capture subsystem accepts video in either NTSC (National Television System Committee) or PAL (Phase Alternation Line) formats from video cameras, VCRs and videodisks. It performs the video signal processing and digitizes the analogue input and then passes the digital data to the compression engine. The performance of the capture subsystem depends on the size of the image and whether it is an 8bit or 24bit colour image.

The Compression Engine is based on a CL4000 chip. Currently, this chip compresses the digital data, using either the CellB or JPEG or MPEG technique but it should be possible to use other compression algorithms since the required compression algorithm is downloaded to the compression engine by the XIL application.

The Frame Storage buffer is used by the CL4000 compression engine to store the immediate results of the compression operations. XIL programs can control the compression performance by changing the size of the buffer used.

The compressed video data is then transferred to the network via the SBus Video Interface which connects the SunVideo card and the SBus of the workstation.

For more details on Sun Video, there is a manual 'SunVideo 1.0 User's Guide'

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