In practice, recording to magnetic media results in magnetized regions whose shape may vary a little depending on what adjacent regions look like. The shape variations are minimized by keeping the intensity of magnetized regions as low as is possible, but still these variations limit the density with which data can be written. The signal from older read heads measures changes in magnetization, not the magnetization per se, but that has very little impact on the storage technologies.
Recording technologies must allow data to be written and recovered from transition data that is not very strong and may have moved a little from its nominal location from a medium that may be moving at a rate that is only approximately known. Doing this without losing track of timing requires fairly frequent polarity transitions. An attempt to simply record binary data would run into trouble with long strings of 0s or 1s where it would become difficult to tell exactly how many contiguous bits are present. In practice, it is difficult to handle more than about 8 consecutive "bits" of the same polarity reliably.
The first technology used for disk recording simply followed each data bit with a clock bit that switched polarity with every clock pulse. This guaranteed at least one transition in any arbitrary four contiguous recording intervals. That's called Frequency Modulation - FM. It is not a very efficient way to store data due to the large number of clock bits.
The first improved modulation technique was Modified FM (MFM). There are a lot of ways to describe MFM -- all confusing. Perhaps the simplest is to regard it as a signal where the clock has been removed, ones have a transition in mid bit. Zeros do not. Polarity is reversed between consecutive zeros. This results in a maximum of four intervals between polarity changes and a minimum of two. Because the minimum is two instead of the one in FM, the number of recording intervals can be doubled without changing the minimum recorded domain size. MFM does require the ability to distinguish between 1, 1.5, and 2 time intervals, but that is feasible. MFM allows twice as many bits to be stored in the same space as FM. Floppy drives use MFM modulation.
Another way to regard MFM is as being a (1,3)RLL code. Run Length Limited (RLL) codes have a minimum code length, a maximum length and define a set of bit patterns that will meet the minimum and maximum constrains no matter how the codes are combined. e.g. for (1,3)RLL 100, 110, 101 and 010 patterns might be permitted. 000, 001, 011 and 111 would then have to be excluded because they can cause a run length of more than 3 identical bits when combined with a legitimate code. e.g. 100 followed by 001 would have 4 consecutive zeros. The legitimate codes are mapped to binary values by the disk controller. (1,3)RLL allows four values to be encoded in three bit intervals.
Hard disks typically use (1,7)RLL or (2,7)RLL. PC hard drives use (2,7)RLL. (2,7)RLL yields a 33% improvement in storage density relative to MFM.
PRML uses a different approach to improving storage density. To permit greater data density, recorded data amplitude is reduced and bits are packed together more closely. The digital signal is then recovered using digital signal processing techniques on the analog data stream from the drive. PRML achieves a 30-40% improvement in storage density over RLL modulation without PRML. EPRML modifies the algorithms used in PRML to achieve additional improvements claimed to be 20-70%.
Overall EPRML offers 5 to 10 times the recording density of FM exclusive of any improvements in read/write heads, recording media, and other hardware.
Return To Index Copyright 1994-2002 by Donald Kenney.