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Re: Write Signal on Disk II Question
glen herrmannsfeldt wrote:
Bryan Parkoff wrote:
(snip)
QA is ideal to show digital bit stream. Write Signal is analog
bit stream. It controls the current in the coil of the read/write
head. Also, it has second head like as erase head.. The high/low
state of the Write Signal determines the direction of the magnetic
field set up in the read/write head. Please explain more about the
field reversal and the lack of field reversal why Write Signal is
required to toggle while QA is always set for all 8 digital bits.
When writing the current in the write coil is toggled. On reading,
a current pulse is generated when the head goes over a transition.
The important characteristics are the minimum distance between
transitions and the ratio of the maximum to the minimum distance.
Because of bit shifting and disk speed variations the ratio can't
get so large and accurately detect which bit cell a transition
belongs in.
Exactly.
Reliable reading depends on accurate clock recovery from the data
stream, and the unavoidable speed variations of mechanical transport
require that "phase" corrections be made continuously.
Since timing information is carried only by transitions, the density
of transitions must be kept high enough for reliable clock recovery.
In addition, the analog head signal processor (MC3470?) has an AGC
function which adjusts gain continuously to maximize the reliability
of transition detection in the presence of nonuniform media. The
time constant of this AGC is fast enough to compensate for localized
media problems, but that also means that it will, within a few bit
times, turn up the gain sufficiently to misdetect noise as transitions.
The AGC is kept at a proper level by a high density of real transitions.
-michael
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