Hardware Reference
In-Depth Information
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), you easily can trace the MFM encoding pattern to the end of the
byte. You can see that the 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 individual
bits. Starting from the left, the first group that matches the groups listed in 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 in MFM, 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 up to 40%. PRML replaces
the standard “detect one peak at a time” approach of traditional analog peak-detect, read/write
channels with digital signal processing.
As the data density of hard drives increases, the drive must necessarily record the flux reversals
closer together on the medium. This makes reading the data on the disk more difficult because the
adjacent magnetic peaks can begin to interfere with each other. PRML modifies the way the drive
reads the data from the disk. The controller analyzes the analog data stream it receives from the heads
by using digital signal sampling, processing, and detection algorithms (this is the partial response
element) and predicts the sequence of bits the data stream is most likely to represent (the maximum
likelihood element). PRML technology can take an analog waveform, which might be filled with
noise and stray signals, and produce an accurate reading from it.
This might not sound like a precise method of reading data that must be bit-perfect to be usable, but
the aggregate effect of the digital signal processing filters out the noise efficiently enough to enable
the drive to place the flux change pulses much more closely together on the platter, thus achieving
greater densities. Most drives with capacities of 2GB or above use PRML technology in their endec
circuits.
Capacity Measurements
In December 1998, the International Electrotechnical Commission (IEC)—the leading international
 
 
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