416 WM-OM-E Rev I
The PRS data must correspond to the transitions on the media. This means it
must be written in direct write mode; it must not be scrambled or encoded.
NLTS calculated by correlation techniques and by the fifth harmonic elimination
techniques do correlate, but they are not identical. Fifth harmonic elimination uses
a pattern including only dibits and widely spaced transitions. Of course, a dibit is
the worst case for NLTS. The pseudorandom pattern has some dibits, some
tribits, some transitions 1 apart, etc. The reading that any method based on a PRS
comes up with is affected by all of the pattern. This tends to make the correlation
method come up with a somewhat lower value than fifth harmonic. That is the
better value to look at when determining write precomp, since the PRS is a lot like
real data.
The fifth harmonic method is not sensitive to amplitude asymmetry. Its drawbacks
are
• If the fifth harmonic is small, noise will tend to inflate the value read: for 20
dB SNR it may not be possible to read below 10% nlts
• Amplitude loss due to partial erasure will contribute to the fifth harmonic
about as much as actual NLTS.
The correlation method is less sensitive to amplitude loss due to partial erasure:
one reference cites experiments showing that 25% nlts and 25% amplitude loss
only inflates the nlts reading to 30%.
However, it is more sensitive to PW50/T ratio (it is most accurate at high ratio,
approaching 3.0). Partial erasure and hard/easy transition shift (due to DC erase)
creates separate correlation echo peaks which, depending on the pattern used,
may be close to the adjacent transition nlts echo and interfere. It is important to
pick a good PRS.
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