By David Salomon
In 1829, Louis Braille, a tender organist in a Paris church, blind given that age three, invented the well known code for the blind, nonetheless in universal use this day worldwide and named after him. Braille himself transformed his code in 1834, and there were a number of changes on account that. although, the fundamental layout of this code, the place every one personality is represented by way of a gaggle of three x 2 dots, has remained intact. The dots are embossed on thick paper and every may be raised or flat (i. e. , current or absent). each one dot is consequently such as one little bit of info. hence, the Braille code (Figure 1) is a 6-bit code and will for this reason signify sixty four symbols (the code of six flat dots shows a clean space). Braille's fans prolonged the functions of his code in different methods. One im portant extension is contractions. those are letters that, after they stand on my own, suggest phrases. for instance, the letter "b" status by myself (or with punctuation) skill the notice "but," the letter "e" status by myself capacity "every," and "p" potential "people. " one other extension is short-form phrases. those are combos of 2 or extra codes that suggest a whole note (short-form phrases might comprise contractions). for instance, "ab" capacity "about," "rcv" capability "receive," and "(the)mvs" capability "themselves. " (The "the" in parentheses is a contraction, dots 2-3-4-6. ) determine 2 exhibits a few examples of those specific codes.
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Extra info for A Guide to Data Compression Methods
10: Huffman Codes for Equal Probabilities. 13a shows a set of five symbols with their probabilities and a typical Huffman tree. 55· 1 bits to the average code size. 08 bits to the code size. 7 bits per symbol. 45 + 1 = 1. 7. This provides a way to calculate the average code size of a set of Huffman codes without any multiplications. Simply add the values of all the internal nodes of the tree. 11 illustrates why this works. 11: Composition of Nodes. 03 + .. 05 + ... 03 + .. 0 + ... 03 + .. = ad-3 + ...
The property illustrated here exists for any tree where a node contains the sum of its children. Statistics are no substitute for judgment. 5 Adaptive Huffman Coding The Huffman method assumes that the frequencies of occurrence of all the symbob of the alphabet are known to the compressor. In practice, the frequencies are seldom, if ever, known in advance. One solution is for the compressor to read the original data twice. The first time it just calculates the frequencies. The second time it compresses the data.
20). Next come run lengths of 2-7 white pixels, which were assigned slightly longer codes. Most run lengths were rare and were assigned long, 12-bit codes. Thus, the Group 3 standard uses a combination of RLE and Huffman coding. It is interesting to notice that the run length of 1664 white pels was assigned the short code 011000. 2 inches wide (~ 208 mm). A blank scan line produces 1,664 consecutive white pels, making this run length very common. 19: The Eight CCITT Training Documents. Since run lengths can be long, the Huffman algorithm was modified.