Section 17.8. Summary


17.8. Summary

A number of algorithms effectively compress voice, still images, and moving images. A raw voice signal is converted to a binary-encoded form known as pulse code modulation (PCM). In this process, sampling and quantization both present some sort of loss. Compression schemes used for the preparation of an image include the Joint Photographic Experts Group (JPEG), a compression standard of still images. JPEG consists of three processes: discrete cosine transform (DCT), quantization , and compression . In particular, DCT converts a snapshot of a real image into a matrix of corresponding values, and the quantization phase converts the values generated by DCT to simple numbers .

A motion image , or video, is the rapid display of still images to fool the human eye. Standards differ on the number of still images that make a video clip. The common standard that defines video compression is the Moving Pictures Expert Group (MPEG). MPEG-1 layer 3 (MP3) is a technology for compressing audio for networking and producing CD-quality sound.

Compression has limits, as presented by Shannon. Shannon's theorem expresses the probability of a typical sequence of length n with entropy H X (x) to be equal to 2 - nH X (x) . Although some compression processes entail loss of data, others do not, such as Huffman or run-length encoding techniques.

The next chapter looks at multimedia networking, voice over IP (VIP), and streaming video.



Computer and Communication Networks
Computer and Communication Networks (paperback)
ISBN: 0131389106
EAN: 2147483647
Year: 2007
Pages: 211
Authors: Nader F. Mir

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