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MULTIMEDIA TECHNOLOGY SMM 3001

MULTIMEDIA TECHNOLOGY SMM 3001. DATA COMPRESSION. In this chapter. The basic principles for compressing data Data compression methods for text & numeric data How digital images & graphics are compressed for storage & transmission Compression methods for digital video & audio.

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MULTIMEDIA TECHNOLOGY SMM 3001

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  1. MULTIMEDIA TECHNOLOGYSMM 3001 DATA COMPRESSION

  2. In this chapter • The basic principles for compressing data • Data compression methods for text & numeric data • How digital images & graphics are compressed for storage & transmission • Compression methods for digital video & audio

  3. Data compression concepts • Compression techniques are used to replace a file with another that is smaller • Decompression techniques expands the compressed file to recover the original data -- either exactly or in facsimile • A pair of compression/decompression techniques that work together is called a codec for short

  4. Data compression concepts

  5. Types of codecs • Codecs that upon decompression always reproduce the original file exactly are called lossless codecs • Codecs that reproduce only an approximation of the original file upon decompression are called lossy codecs • Codecs that take approximately the same amount of time to compress and decompress a file are referred to as symmetric codecs • By contrast, codecs that feature simple fast decompression but significantly slower compression are called asymmetric codecs

  6. Compression encoding • Compression is an encoding process that filters the original file in several successive stages

  7. Codec Methods • Syntactic encoding (also called entropy encoding) methods attempt to reduce the redundancy of symbolic patterns in a file without any regard to the type of information represented • Semantic methods consider special properties of the type of information represented to reduce nonessential information in a file • Hybrid methods combine both syntactic and semantic methods

  8. Compressing Text and Numerical DataLossless Syntactic Methods • Run-Length Encoding (RLE) • looks for repeated sequences of symbols • widely used for fax (facsimile) transmissions • Huffman Codes • exploits frequency distribution of symbols in a source • adaptive Huffman coding builds its own frequency tables rather than use predefined statistics • Lempel-Ziv-Welch (LZW) compression • based on recognizing patterns of strings in original file • fast and yields good results (50% typical reduction)

  9. Compressing Images • GIF (Graphic Interchange Format) Codec • employs LZW method for lossless compression • TIFF (Tagged Image File Format) Codec • lossless syntactic method • JPEG (Joint Photographics Experts Group) Codec • umbrella term covering several lossy and lossless methods • baseline method is most commonly used one -- lossy method based on a hybrid method

  10. Compressing Video • Video compression employs both spatial and temporal compression • spatial techniques compress individual frames • temporal methods compress data in frames over time • QuickTime and AVI (Audio Video Interleaved) are two popular (and incompatible with each other) formats

  11. Compressing VideoSome additional methods • DVI (Digital Video Interactive) • Motion-JPEG • MPEG (Motion Pictures Experts Group) • The px64 Standard

  12. Temporal Compression in Video • Lossy strategies for eliminating redundancy of information between frames employ temporal compression -- referred to as interframe compression • Sequence of frames are considered together • key frames • difference frames • Used in QuickTime and DVI

  13. Temporal Compression in Video(cont’d) • MPEG and related codecs employ a more complex frame-referencing method • intrapictures (I pictures) • predicted pictures (P pictures) • bidirectional pictures (B pictures)

  14. Compressing Audio • Widely used method is ADPCM (Adaptive Differential Pulse Code Modulation) • ADPCM • lossy method • employs a differencing technique related to those used in video compression • used in DVI

  15. Summary • Compressing data means reducing the effective size of a data file for storage or transmission • Particular paired compression/decompression methods are called codecs • Codecs that cannot reproduce the original file exactly are called lossy methods; those that reproduce the original exactly are called lossless methods • Text and numbers usually require lossless methods • Images, video, and sound codecs are usually lossy

  16. Summary (cont’d) • Syntactic methods attempt to reduce the redundancy of symbolic patterns in a file without any regard to the type of information represented • Semantic methods exploit characteristics inherent in the type of information being represented • The use of codecs is not an exact science -- the effectiveness and suitability of any method will depend on the exact nature of the original file and the intended use for the compressed file

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