What is MFM ?
| Modified Frequency Modulation |
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The first encoding system for recording digital data on magnetic media, was frequency modulation, of course abbreviated FM. (This has nothing whatever to do with FM radio, of course, except for a similarity in the concept of how the data is encoded.) This is a simple scheme, where a one is recorded as two consecutive flux reversals, and a zero is recorded as a flux reversal followed by no flux reversal. This can also be thought of as follows: a one is a reversal to represent the clock followed by a reversal to represent the "one", and a zero is a reversal to represent the clock followed by "no reversal" to represent the "zero". The name "frequency modulation" can be seen in the patterns that are created if you look at the encoding pattern of a stream of ones or zeros. If we designate "R" to represent a flux reversal and "N" to represent no flux reversal, a byte of zeroes would be encoded as "RNRNRNRNRNRNRNRN", while a byte of all ones would be "RRRRRRRRRRRRRRRR". As you can see, the ones have double the frequency of reversals compared to the zeros; hence frequency modulation. The problem with FM is that it is wasteful: each bit requires two flux reversal positions, resulting in a theoretical overhead of 100% compared to the ideal case (one reversal per bit). FM is obsolete and is no longer used. In fact, it was obsolete before the PC was really invented; it was originally used in floppy disks of older machines. The replacement for FM is modified frequency modulation, or MFM. MFM improves on FM by reducing the number of flux reversals inserted just for the clock. Instead of inserting a clock reversal before each bit, one is inserted only between consecutive zeroes. This means far fewer reversals are needed on average per bit. This allows the clock frequency to be doubled, allowing for approximately double the storage capacity of FM. MFM encoding was used on the earliest hard disks, and also on floppy disks. In fact, MFM is still the standard that is used for floppy disks today. For hard disks it was replaced by the more efficient RLL and its variants. Presumably this did not happen for floppy disks because the need for more efficiency was not nearly so great, compared to the need for backward compatibility. |
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