Video Compression

This document deals with the issues of video compression. The algorithm, which is used by the MPEG standards, will be elucidated upon in order to explain video compression. Only visual compression will be discussed (no audio compression). References and links to further readings will be provided in the text. What is compression? Compression is a reversible conversion (encoding) of data that contains fewer bits. This allows a more efficient storage and transmission of the data. The inverse process is called decompression (decoding). Software and hardware that can encode and decode are called decoders. Both combined form a codec and should not be confused with the terms data container or compression algorithms. Lossless compression allows a 100% recovery of the original data. It is usually used for text or executable files, where a loss of information is a major damage. These compression algorithms often use statistical information to reduce redundancies. Huffman-Coding [1] and Run Length Encoding [2] are two popular examples allowing high compression ratios depending on the data. Using lossy compression does not allow an exact recovery of the original data. Nevertheless it can be used for data, which is not very sensitive to losses and which contains a lot of redundancies, such as images, video or sound. Lossy compression allows higher compression ratios than lossless compression. A simple calculation shows that an uncompressed video produces an enormous amount of data: a resolution of 720x576 pixels (PAL), with a refresh rate of 25 fps and 8-bit colour depth, would require the following bandwidth: 720 x 576 x 25 x 8 + 2 x (360 x 576 x 25 x 8) = 1.66 Mb/s (luminance + chrominance) For High Definition Television (HDTV): 1920 x 1080 x 60 x 8 + 2 x (960 x 1080 x 60 x 8) = 1.99 Gb/s Even with powerful computer systems (storage, processor power, network bandwidth), such data amount cause extreme high computational demands for managing the data. Fortunately, digital video contains a great deal of redundancy. Thus it is suitable for compression, which can reduce these problems significantly. Especially lossy compression techniques deliver high compression ratios for video data. However, one must keep in mind that there is always a trade-off between data size (therefore computational time) and quality. The higher the compression ratio, the lower the size and the lower the quality. The encoding and decoding process itself also needs computational Uncompressed Image …