relative level of distortion is displayed numerically, in dB and percent while the bar graph
shows the absolute level in dBu. Unlike most instruments the LA102 uses a multi-stage
bandstop-plus-highpass filter to reject the fundamental, and no nulling process is
involved. Very low distortion readings require a source of precise frequency, as provided
by the LA101 which is quartz synthesised, but the unusually wide notch permits readings
down to -60dB with a frequency tolerance of ±2%, giving complete immunity from the
effects of wow and flutter or speed drift when measuring off tape Auto-nulling, tracking
filters are often too slow to give accurate results.
The input frequency is constantly monitored and the filter is switched if it enters the valid
zone for a different one. Operation is only valid at six points: the filters do not track, they
just switch. Automatic operation only operates for tone levels above -29dB, but the
filters can be manually locked by selecting the option explicitly (section 2.7), and
unlocked by pressing
again (or by pressing twice). Re-normalisation of
the level occurs every time the filter changes and also if the other channel is selected with
the
key. If in doubt press to re-normalise.
Distortion option 6 (40Hz) has limited bandwidth to reduce noise and quantising effects
from the LA101, a response to the tenth harmonic (400Hz) is considered adequate here.
Distortion option 7 is experimental only. Option 8 is a 1kHz notch filter, like option 3,
but it passes frequencies below 1kHz. Similarly options 9 and 10 provide a 6.3kHz and a
10kHz notch filter and they too pass frequencies below the notch frequency.
There is growing popularity for distortion measurements using the CCIR468-4 weighting
filter and quasi-peak rectifier which, it is claimed, give better correlation with subjective
assessment and this is provided on distortion options 11-15. When CCIR weighted
distortion is being measured the automatic frequency selector will choose options 11-15
and
can be used to toggle quickly between options 1-5 and options 11-15.
IMPORTANT: Distortion measurement differs from the other measurements in storing
a reference level automatically and calculating a relative reading. As with all relative
readings only the numerical display is relative; the bar graph ALWAYS shows the
absolute level in dBu or dBV. This is partly for ease of implementation, but it can also be
useful. Remember to press
again (or press twice) for correct relative
readings if the signal level changes, as the level is only stored once.
2.14 Wow and Flutter Measurement Wow & flutter W&F Measurement
Wow and flutter is measured by replaying a tape that has been recorded with a 3kHz or
3.15kHz tone (at 0VU) and pressing
(fig. 2.13). Option 1 gives a weighted quasi-
peak measurement to the universally accepted IEC386 standard, and option 2 gives the
corresponding unweighted measurement. As with other measurements this one is
autoranging, and settling may take a few seconds. The result can be read on the bar graph
or digital readout (fig 2.13) in dB, a perfectly valid form of measurement which we
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2. Manual Operation
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