Computerworld - As long as bandwidth is expensive and people are impatient, data compression is here to stay. Simply put, compression methods crunch data - text, graphics, audio or video - into a computer-decipherable shorthand that's 10% to 99% of its original size. The data takes up less storage space and requires less bandwidth to be transmitted over the Internet. In addition, many methods can squeeze multiple files into a single file called an archive.
There are two basic methods for compressing data: lossy and lossless. Lossless techniques compress data without destroying or losing anything during the process. When the original document is decompressed, it's bit-for-bit identical to the original. Lossy techniques let the file be compressed even smaller, but some data is lost forever.
 | | | Compression Standards
Joint Photographic Experts Group (JPEG): A still-image, lossy compression method that uses discrete cosine transform equations to compress images at a ratio of up to 20-to-1, without noticeable loss. Lempel-Ziv-Welch: This algorithm, used in many compression formats, including graphics interchange format and Tag Image File Format, takes each input sequence of bits of a given length and creates an entry in a lookup table, along with a shorter code. Lookup entries are part of the compressed file, enabling the decoding program to rebuild the table. Moving Pictures Experts Group (MPEG): A lossy compression method for video. MPEG-1 is used for CD-ROMs and video CDs. MPEG-2 compresses video for regular and high-definition television. MPEG Audio Layer 3 (MP3): An audio compression technology that's part of MPEG-1 that uses perceptual audio coding to compress CD-quality audio by a factor of 12. Fractal: A lossy compression method for color images that's well suited for natural objects, with compression ratios up to 100-to-1. PKZip: A popular lossless compression shareware program from PKWare Inc. The program uses an algorithm and a data library to encode or archive multiple date files. PKUnzip decompresses the files to their original states. Wavelet: This form of lossy compression uses a mathematical function that can compress images to a greater extent than other methods - sometimes to only one-fourth the size of a similar image compressed with JPEG. Windows Media Technology: This Microsoft Corp. product delivers better sound quality than MP3 for same-size files, as well as near-DVD-quality video. - Lee Copeland and Russell Kay
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"Lossy compression makes a trade-off: You give up accuracy for higher compression," says Steve Hoffenberg, director of product management at Sound Vision Inc., a digital imaging firm in Framingham, Mass. "If you're compressing bank records, you want to be sure they're identical before and after. But with an image or audio or video files, it's generally not crucial to restore every bit of data."
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