Hardware Reference
In-Depth Information
codes the pulse train back into the original binary data. Over the years, several schemes
for encoding data in this manner have been developed; some are better or more efficient
than others, which you see later in this section.
Other descriptions of the data-encoding process might be much simpler, but they omit the
factsthatmakesomeoftheissuesrelatedtoharddrivereliabilitysocritical—namely,tim-
ing. Engineers and designers are constantly pushing the envelope to stuff more and more
bits of information into the limited quantity of magnetic flux reversals per inch. What
they'vecomeupwith,essentially,isadesigninwhichthebitsofinformationaredecoded
notonlyfromthepresenceorabsenceoffluxreversals,butfromthetimingbetweenthem.
Themoreaccuratelytheycantimethereversals,themoreinformationthatcanbeencoded
(and subsequently decoded) from that timing information.
In any form of binary signaling, the use of timing is significant. When a read or write
waveform is interpreted, the timing of each voltage transition event is critical. Timing is
what defines a particular bit or transition cell—that is, the time window within which the
driveiseitherwritingorreadingatransition.Ifthetimingisoff,agivenvoltagetransition
might be recognized at the wrong time as being in a different cell, which would throw the
conversion or encoding off, resulting in bits being missed, added, or misinterpreted. To
ensure that the timing is precise, the transmitting and receiving devices must be in perfect
synchronization. Forexample, ifrecordinga0isdonebyplacing notransition onthedisk
for a given time period or cell, imagine recording ten 0 bits in a row—you would have a
long period of time (ten cells) with no activity, no transitions at all.
Imagine now that the clock on the encoder was slightly off time while reading data as
compared to when it was originally written. If it were fast, the encoder might think that
duringthislongstretchof10cellswithnotransitions,onlyninecellshadactuallyelapsed.
Orifitwereslow,itmightthinkthat11cellshadelapsedinstead.Ineithercase,thiswould
result in a read error, meaning the bits that were originally written would not be read as
beingthesame.Topreventtimingerrorsindriveencoding/decoding,perfectsynchroniza-
tion isnecessary between the reading andwriting processes. This synchronization often is
accomplishedbyaddingaseparatetimingsignal,calleda clock signal ,tothetransmission
betweenthetwodevices.Theclockanddatasignalsalsocanbecombinedandtransmitted
as a single signal. Most magnetic data-encoding schemes use this type of combination of
clock and data signals.
Adding a clock signal to the data ensures that the communicating devices can accurately
interpret the individual bit cells. Each bit cell is boundedbytwo other cells containing the
clocktransitions.Becauseclockinformationissentalongwiththedata,theclocksremain
in sync, even if the medium contains a long string of identical 0 bits. Unfortunately, the
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