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Technical Requirements for Encoding and Decoding
You might think that since there are two magnetic polarities, N-S and S-N, they could be used nicely to represent a "one" and a "zero" respectively, to allow easy encoding of digital information. Simple! Well, that would be nice, but as with most things in real life, it usually doesn't work that way. :^) There are three key reasons why it is not possible to do this simple 1-to-1 encoding:
Therefore, in order to encode data on the hard disk so that we'll be able to read it back reliably, we need to take the issues above into account. We must encode using flux reversals, not absolute fields. We must keep the number of consecutive fields of same polarity to a minimum. And to keep track of which bit is where, some sort of clock synchronization must be added to the encoding sequence. Considering the highway example above, this is somewhat analogous to adding markers or milestones along the road.
In addition to the requirements we just examined, there's another design limit that must be taken into account: the magnetization limits of the media itself. Each linear inch of space on a track can only store so many flux reversals. This is one of the limitations in recording density, the number of bits that can be stored on the platter surface. Since we need to use some flux reversals to provide clock synchronization, these are not available for data. A prime goal of data encoding methods is therefore to decrease the number of flux reversals used for clocking relative to the number used for real data.
The earliest encoding methods were relatively primitive and wasted a lot of flux reversals on clock information. Over time, storage engineers discovered progressively better methods that used fewer flux reversals to encode the same amount of information. This allowed the data to effectively be packed tighter into the same amount of space. It's important to understand the distinction of what density means in this context. Hardware technology strives to allow more bits to be stored in the same area by allowing more flux reversals per linear inch of track. Encoding methods strive to allow more bits to be stored by allowing more bits to be encoded (on average) per flux reversal.
Note: There are in fact many
data encoding and decoding methods. I only examine here the most common ones used for