404 WM-OM-E Rev I
owrt Overwrite
Definition The ratio of residual to original power of a low-frequency disk waveform
overwritten by a higher frequency waveform.
Description owrt measures the residual power of a low-frequency LF waveform after it
has been overwritten by a high-frequency HF waveform. The LF
waveform should be stored to memory (M1-M4). The HF waveform can
then be input to the DDA, and overwrite calculated where:
owrt = 20 log (V
r
/V
o
),
where V
r
is the residual V
rms
of the sine wave component of the HF
waveform at the LF base frequency after the HF waveform write, and V
o
is
the V
rms
of the sine wave component of the LF waveform at the LF base
frequency. The calculation is performed by the DDA making a
narrow-band power measurement (see nbpw parameter description) at
LF, for both the HF and LF waveforms, and subtracting the second result
from the first. A menu enables the choice of which waveform, HF or LF, is
assigned to which DDA channel or trace (1, 2, 3, 4, M1 to M4, or F1 to F8).
The menu button is used to set the input for HF or LF, while the input for
the selected waveform is set with the associated knob. The owrt results
are presented in dB. All averaging, including statistics and trend average,
is performed on linear units. Average results are converted to dB.
Note: In typical use, it is preferable to use nbpw to measure the LF
waveform, and then the residual LF in the HF separately, instead of the
owrt parameter. Overwrite is the difference between the nbpw readings in
dB. There are two reasons why this is preferable: 1) nbpw, with statistics
on, provides average power readings. With owrt the low frequency signal
is typically a stored single-shot acquisition due to the difficulty finding a
suitable trigger for time domain averaging of a head signal. 2) owrt
computes both nbpw results each time. If the LF is stored, this is not
necessary. So nbpw will take twice as many acquisitions as owrt and
achieve a more stable average result in the same amount of time.
Parameter Settings Selecting this parameter accesses a frequency setting dialog. This
frequency is used to calculate nbpw for both the HF and LF waveforms. If
a large number of digits is used, selection of the exact frequency may be
difficult. In this case, a number with fewer digits and less precision should
be chosen for the approximate frequency, then the precision should be
increased as desired and the exact value chosen.
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