Modified Frequency Modulation - MMFM

MMFM, (Modified Modified Frequency Modulation), also abbreviated M²FM, or M2FM, is similar to MFM, but suppresses additional clock bits, producing a longer maximum run length (a (1,4) RLL code). In particular, a clock pulse is only inserted between a pair of adjacent 0 bits if the first bit of the pair did not have a clock pulse inserted before it. In the example below, clock bits that would have been present in MFM are noted in bold:

Data: 1 1 0 1 0 0 1 0 0 0 1 0 0 0 0 1 0 0 0 0 0 1 Clock: 0 0 0 0 0 1 0 0 1 0 0 0 1 0 1 0 0 1 0 1 0 0 Encoded: 01010001001001001000010010001001001000100001

In this system, sync marks are made by inserting additional clock pulses between adjacent zero bits (following the MFM rule) where they would normally be omitted. In particular, the data bit pattern "10001" has a clock pulse inserted in the middle, where it would normally be omitted:

Data: 1 0 0 0 0 1 Normal: 0 1 0 1 0 Sync: 0 1 1 1 0