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Triple Density MFM Coding

IP.com Disclosure Number: IPCOM000090493D
Original Publication Date: 1969-May-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 3 page(s) / 54K

Publishing Venue

IBM

Related People

Tang, DT: AUTHOR

Abstract

By observing certain run-length constraints, a type of modified frequency modulation MFM coding is achieved for enabling data to be recorded at three times the density possible with standard double~frequency DF coding.

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Triple Density MFM Coding

By observing certain run-length constraints, a type of modified frequency modulation MFM coding is achieved for enabling data to be recorded at three times the density possible with standard double~frequency DF coding.

DF encoding involves the insertion of a 1 clock bit between each pair of data bits in an input sequence to insure that 1 bits occur often enough in the resulting sequence to satisfy the self-clocking requirement: of the data handling system. The density at which such encoded data can be recorded is limited by the minimum physical separation that must exist between 1's to avoid intersymbol interference. MFM encoding avoids placing 1's in adjoining positions. Conventional MFM encoding involves the use of both 1 and 0 clock bits. A 1 clock bit is inserted between each pair of 0's in the input data sequence. A 0 clock bit is inserted between every other pair of data bits. The resulting MFM sequence is a run-length-limited, or dk-limited, sequence in which d, the minimum number of 0's between succeeding 1's in the sequence, has a value of 1 and k, the maximum number of consecutive 0's in the sequence, has a value of
3. MFM sequences having run-length limitations of d=1 and k=3 can be recorded at twice the density of standard DF-encoded sequences without causing intersymbol interference. They also provide 1 pulses at the required intervals for clocking.

MFM-encoded sequences in which d=2 and k=5 are recorded at three times the density of DF-encoded sequences, while providing adequate clocking. The information density in this case is 2.4 times that of the corresponding DF-encoding. The desired 2-5 dk, or run-length, limitations are provided by an encoding method which converts every two bits of the original data sequence into a five-bit code word in accordance with the t...