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Fast Compact NRZI Decoding Algorithm

IP.com Disclosure Number: IPCOM000052255D
Original Publication Date: 1981-May-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 2 page(s) / 40K

Publishing Venue

IBM

Related People

Peterson, JE: AUTHOR

Abstract

A method is provided for decoding a bit string representation of a NRZI encoded serial datastream in order to recover the original unencoded data.

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Fast Compact NRZI Decoding Algorithm

A method is provided for decoding a bit string representation of a NRZI encoded serial datastream in order to recover the original unencoded data.

The bit string representation is produced by sampling the NRZI encoded datastream once per clock interval (but out of phase with the clock) to produce a binary output that is packed into sequential bit positions in storage. This sampling technique is used to record the output of a data communications line or magnetic recording device in a digitized, but unprocessed, form for later analysis where both encoded and decoded versions of the datastream are desired.

The bit string is decoded in groups of n bits (referred to as bytes), where n is > 0. The binary value of each byte is used as an index into one of two translation tables to produce each decoded byte of output. Translation table A is used if the first bit of the current byte and the last bit of the previous byte have the same value (both are 1 or both are 0), and translation table B is used if they have opposite values. When starting the algorithm, the value of the first previous byte is arbitrary and the first bit of output is of no value.

Each table consists of 2/n-1/ entries of n bits each. Each table entry string contains the decoded representation of the index byte. The first bit of each table entry string in table A always has a value of 1, and the first bit in table B is always
0.

If the first bit of the byte to be trans...