L15-coding-2009 - Information Coding Trac D. Tran ECE...

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1 Information Coding Information Coding Trac D. Tran ECE Department The Johns Hopkins University Baltimore, MD 21218 Outline Outline Fixed-length codes Definition Properties and applications Examples ASCII Variable-length codes Definition Properties and applications Examples Morse code Shannon-Fano code Huffman code
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2 Fixed Fixed -Length Codes Length Codes Properties Use the same number of bits to represent all possible symbols produced by the source Simplify the decoding process Examples American Standard Code for Information Interchange (ASCII) code Bar codes One used by the US Postal Service Universal Product Code (UPC) on products in stores Credit card codes ASCII Code ASCII Code ASCII is used to encode and communicate alphanumeric characters for plain text 128 common characters: lower-case and upper-case letters, numbers, punctuation marks… 7 bits per character First 32 are control characters (for example, for printer control) Since a byte is a common structured unit of computers, it is common to use 8 bits per character – there are an additional 128 special symbols Example 00110111 00110011 00110001 00101110 00110000 00110010 00110101 55 51 49 46 48 50 53 7 3 1 . 0 2 5 Character Dec. index Bin. code
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3 ASCII Table ASCII Table Variable Variable -Length Codes Length Codes Main problem with fixed-length codes: inefficiency
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This note was uploaded on 02/02/2010 for the course ENGINEERIN 520.101 taught by Professor Tracdtran during the Winter '06 term at Johns Hopkins.

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L15-coding-2009 - Information Coding Trac D. Tran ECE...

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