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<!DOCTYPE HTML PUBLIC "html.dtd"><HTML><HEAD><TITLE>The Data Compression Book-:Glossary</TITLE><META NAME="ROBOTS" CONTENT="NOINDEX, NOFOLLOW"><SCRIPT><!--function displayWindow(url, width, height) { var Win = window.open(url,"displayWindow",'width=' + width +',height=' + height + ',resizable=1,scrollbars=yes');}//--></SCRIPT></HEAD><BODY BGCOLOR="#FFFFFF" VLINK="#DD0000" TEXT="#000000" LINK="#DD0000" ALINK="#FF0000"><TD WIDTH="540" VALIGN="TOP"><!-- <CENTER><TABLE><TR><TD><FORM METHOD="GET" ACTION="http://search.itknowledge.com/excite/cgi-bin/AT-foldocsearch.cgi"><INPUT NAME="search" SIZE="20" VALUE=""><BR><CENTER><INPUT NAME="searchButton" TYPE="submit" VALUE="Glossary Search"></CENTER><INPUT NAME="source" TYPE="hidden" VALUE="local" CHECKED> <INPUT NAME="bltext" TYPE="hidden" VALUE="Back to Search"><INPUT NAME="sp" TYPE="hidden" VALUE="sp"></FORM></TD><TD><IMG SRC="http://www.itknowledge.com/images/dotclear.gif" WIDTH="15" HEIGHT="1"></TD><TD><FORM METHOD="POST" ACTION="http://search.itknowledge.com/excite/cgi-bin/AT-subscriptionsearch.cgi"><INPUT NAME="search" SIZE="20" VALUE=""><BR><CENTER><INPUT NAME="searchButton" TYPE="submit" VALUE=" Book Search "></CENTER><INPUT NAME="source" TYPE="hidden" VALUE="local" CHECKED> <INPUT NAME="backlink" TYPE="hidden" VALUE="http://search.itknowledge.com:80/excite/AT-subscriptionquery.html"><INPUT NAME="bltext" TYPE="hidden" VALUE="Back to Search"><INPUT NAME="sp" TYPE="hidden" VALUE="sp"></FORM></TD></TR></TABLE></CENTER> --><!-- ISBN=1558514341//--><!-- TITLE=The Data Compression Book-//--><!-- AUTHOR=Mark Nelson//--><!-- PUBLISHER=IDG Books Worldwide, Inc.//--><!-- IMPRINT=M & T Books//--><!-- CHAPTER=0//--><!-- PAGES=527-536//--><!-- UNASSIGNED1//--><!-- UNASSIGNED2//--><CENTER><TABLE BORDER><TR><TD><A HREF="ewtoc.html">Table of Contents</A></TD></TR></TABLE></CENTER><P><BR></P><H2><A NAME="Heading1"></A><FONT COLOR="#000077">Glossary</FONT></H2><DL><DT><B>Adaptive compression, Adaptive modeling</B><DD>Data compression techniques that use a model can either use a fixed model for the entire stream they are processing, or modify the model as the stream is processed. Techniques that modify the model as it is processed are said to use adaptive modeling. An example of an adaptive compression technique would be LZW compression.<DT><B>ADPCM</B><DD>Adaptive Differential Pulse Code Modulation. Standard PCM encoding is a common technique for encoding audio data. Telephone conversations and audio CDs both use conventional PCM. PCM samples a waveform at uniform steps and encodes the level of the waveform. DPCM is Differential Pulse Code Modulation. DPCM doesn’t encode the level, it instead encodes the difference from the last sample. ADPCM takes that a step further, and modifies the coding of the difference depending on the state of the waveform. PCM encoding in telephone systems uses 64 K bits per second. ADPCM can reduce that rate to 32 or even 16 K bits per second with relatively little reduction in voice quality.<DT><B>Alphabet</B><DD>An Alphabet is the set of all of the possible symbols that may appear in a message. For example, when compressing ASCII text files, the Alphabet would probably consist of characters 0x00 through 0x7f.<DT><B>Archive</B><DD>An archive is a volume or file containing one or more files that may or may not have been compressed. An archive is typically used as a convenient way to store or transport files. Programs such as ARC and PKZip compress files before placing them into archives.<DT><B>Arithmetic coding</B><DD>Traditional coding techniques such as ASCII or Huffman coding encode symbols into unique patterns of bits. Arithmetic coding instead takes an entire text and encodes it as a single floating point number less than 1 and greater than or equal to 0. Arithmetic coding can more efficiently encode texts by eliminating the quantization effects of other coding techniques.<DT><B>ARC, MS-DOS program</B><DD>ARC is a commercial archiving program created by System Enhancement Associates, of Wayne, N.J. ARC was one of the earliest compression/archive utilities to achieve wide popularity in the desktop computing world, beginning in the mid-1980s.<DT><B>ARJ, MS-DOS program</B><DD>ARJ is a commercial archiving program created by Robert Jung. ARJ is free of charge for individual use, but commercial users must pay a license fee. ARJ is also supplied with ANSI C source code for extracting files from ARJ archives that may be distributed without restrictions.<DT><B>Block Coding</B><DD>Compression of images is frequently done by coding smaller blocks of the image independent of one another. For example, the JPEG algorithm uses an 8-by-8 block size when compressing graphics.<DT><B>CCITT</B><DD>CCITT is the International Telegraph and Telephone Consultative Committee. This standards organization is responsible for the sanctioning of many compression and transmission methods in use today, including several PCM and ADPCM techniques, FAX transmission, and the evolving JPEG and MPEG standards.<DT><B>Codes, (en)coding</B><DD>Symbols that are to be stored or manipulated by a computer are converted to codes. This process is referred to as coding. ASCII and EBCDIC are two of the most common methods of coding written text. Data compression can occur if more efficient methods of coding, such as Huffman coding, are used.<DT><B>COMPACT, UNIX program</B><DD>COMPACT is a UNIX program that used Dynamic Huffman coding to compress files. It generally fell out of use in favor of the COMPRESS program.<DT><B>COMPRESS, UNIX program</B><DD>COMPRESS is a UNIX program that uses an LZW implementation to compress files. COMPRESS has found widespread use in the UNIX community, and is available in the public domain. It has recently been thought that COMPRESS may infringe on a Unisys patent, which may curtail its use and distribution.<DT><B>Compression ratios</B><DD>Compression ratios are used to describe the difference between a file and a compressed copy of itself. There are several different ways of expressing this number. One common method is a ratio between input and output, as in “a 4:1 compression ratio.” Another popular method is to express the difference between the files as a percentage ranging from 0% to 100% (or greater, if the compression failed to actually reduce the size of the file). Some people invert this scale, using 100% as the “best” compression ratio. Occasionally, you still see the ratio of compressed to plain files expressed as “bits per byte.”⌨️ 快捷键说明
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