Hardware Reference
In-Depth Information
The FM encoding example shown in
Figure 8.10
is easy to explain. Each bit cell has two
transition cells: one forthe clock information and one forthe data. All the clock transition
cells contain flux transitions, and the data transition cells contain a flux transition only if
the data is a 1 bit. No transition is present when the data is a 0 bit. Starting from the left,
the first data bit is 0, which decodes as a flux transition pattern of TN. The next bit is a 1,
which decodes as TT. The next bit is 0, which decodes as TN, and so on.
The MFM encoding scheme also has clock and data transition cells for each data bit to be
recorded. As you can see, however, the clock transition cells carry a flux transition only
when a 0 bit is stored after another 0 bit. Starting from the left, the first bit is a 0, and the
preceding bit is unknown (assume 0), so the flux transition pattern is TN for that bit. The
next bit is a 1, which always decodes to a transition-cell pattern of NT. The next bit is 0,
which was preceded by 1, so the pattern stored is NN. By using
Table 8.2
(shown earlier),
youeasilycantracetheMFMencodingpatterntotheendofthebyte.Youcanseethatthe
minimum and maximum numbers of transition cells between any two flux transitions are
one and three, respectively, which explains why MFM encoding can also be called RLL
1,3.
The RLL 2,7 pattern is more difficult to see because it encodes groups of bits rather than
individualbits.Startingfromtheleft,thefirstgroupthatmatchesthegroupslistedin
Table
8.3
is the first three bits, 010. These bits are translated into a flux transition pattern of
TNNTNN. The next two bits, 11, are translated as a group to TNNN; and the final group,
000 bits, is translated to NNNTNN to complete the byte. As you can see in this example,
no additional bits are needed to finish the last group.
Notice that the minimum and maximum numbers of empty transition cells between any
two flux transitions in this example are two and six, although a different example could
show a maximum of seven empty transition cells. This is where the RLL 2,7 designation
comes from.Because even fewer transitions are recorded than inMFM,the clock rate can
be increased to three times that of FM or 1.5 times that of MFM, thus storing more data
in the same space. Notice, however, that the resulting write waveform itself looks exactly
like a typical FM or MFM waveform in terms of the number and separation of the flux
transitions for a given physical portion of the disk. In other words, the physical minimum
and maximum distances between any two flux transitions remain the same in all three of
these encoding scheme examples.
Partial-Response, Maximum-Likelihood Decoders
Another feature often used in modern hard disk drives involves the disk read circuitry.
Read channel circuits using Partial-Response, Maximum-Likelihood (PRML) technology
enable disk drive manufacturers to increase the amount of data stored on a disk platter by
