Compressed Digital Video

When compression is introduced into a video transport system, a drastic reduction in bandwidth can be implemented.  A digital composite signal requires 144 Mbps and an HD-SDI signal requires 1.485 Gbps.  When considering a system that will transport many channels of digital video, an enormous amount of bandwidth is required.  A compression system removes redundant or repetitive information from the digital data stream.  A compression or transmission encoding scheme will take advantage of limitations in the human eye.  The human eye has lower sensitivity or resolution to color detail.  Many compression or encoding schemes take this into account and compress or omit certain color details.

There are two basic types of compression systems, Lossless and Lossy. A lossless compression system does not degrade the video or audio quality.  The receiver unit recovers the original uncompressed information.  A lossless compression system strictly removes repetitive information from the data stream.  Most video content has repetitive information from one video frame to the next.  For example, the background image may not change from frame to frame. Therefore there is no need to resend this information if it stays the same.    Unfortunately a lossless compression schemes does not offer significant bandwidth savings.  A compression rate of 3 to 4 times is needed pected.

A lossy compression scheme can achieve very high levels of compression but at the cost of image or signal quality.  A lossy compression algorithm removes detail from the original image.  Once the information has been removed, it cannot be reconstructed.   There are many compression and encoding schemes used in video transport.  The 4:2:2, 4:1:1 and 4:1:0 encoding schemes mentioned earlier are a technique used to reduce bandwidth.  Since the human eye has less sensitivity or less resolution for color, these encoding schemes cut bandwidth for the color information.  The human eye has more resolution horizontally than vertically.  When taking this into account, most video formats have a higher horizontal resolution than vertical.

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