Hardware Reference
In-Depth Information
upto 40%.PRML replaces the standard “detect one peak at a time” approach oftradition-
al 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 re-
versals closer together on the medium. This makes reading the data on the disk more dif-
ficult 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 ana-
log data stream it receives from the heads by using digital signal sampling, processing,
anddetectionalgorithms(thisisthepartialresponseelement)andpredictsthesequenceof
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 sig-
nals, and produce an accurate reading from it.
Thismightnotsoundlikeaveryprecisemethodofreadingdatathatmustbebit-perfectto
be usable, but the aggregate effect of the digital signal processing filters out the noise ef-
ficiently enough to enable the drive to place the flux change pulses much more closely to-
gether 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 in-
ternationalorganizationforworldwidestandardizationinelectrotechnology—approvedas
an IEC International Standard the names and symbols for prefixes for binary multiples
for use in the fields of data processing and data transmission. Prior to this, a lot of con-
fusion had existed as to whether a megabyte stood for 1 million bytes (10 6 ) or 1,048,576
bytes (2 20 ). Even so, these new prefixes have yet to be widely adopted and confusion still
reigns. The industry-standard abbreviations for the units used to measure the capacity of
magnetic (and other) drives are shown in Table 8.4 .
Table 8.4 Standard Abbreviations and Meanings
 
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