operates on the channel with the greatest level although manual range changing still
works normally.
Difference between the two channels
Fig. 2.9 Dual Level & Phase Bars Fig. 2.10 Noise Measurement
2.10 Noise Measurement Noise measurement A weighted noise CCIR weighting filter CCIR468-3 Mute
Noise is normally measured using the widely accepted CCIR468-4 weighted
measurement (identical to CCIR468-1 to 3 but with different tolerances) which is
provided on noise option 1. Pressing
on the LA102 will measure noise using the
last noise option (see fig. 2.10, section 2.7 and table 2.6). The display shows the absolute
noise level in dBu (relative to 0.775V) unless a ‘test level’ has been set (section 2.6) in
which case the numerical reading is relative to test level (but the bar graph always shows
the absolute level). Try silencing the LA101 by pressing
and a reading of around
-97dB should be obtained, this being the residual input noise of the LA102. Press
again to restore the signal. The slowness of autoranging when changing to a higher range
results from the slow quasi-peak rectifier response. Option 2 gives unweighted
measurement to the CCIR468-4 standard and most other standards are incorporated (see
appendix G for graphs of CCIR and ‘A’ weighting curves).
Option 3 gives rms noise measurement (with 22Hz-22kHz bandwidth), equivalent to level
option 2 but, like all noise measurements, configured for low signal levels (an extra 20dB
of gain is selected, so the maximum level that can be measured is +8dBu on noise
compared with +28dBu on level). Options 6 and 7 provide CCIR weighted noise
measurement, using the average reading meter and the rms meter respectively. Option 8
gives ‘A’ weighted noise measurement, and option 9 gives CCIR ARM-2k (‘A’ weighted,
average reading meter, normalised to 2kHz). Option 10 provides a wide band,
2Hz-100kHz noise measurement using the quasi-peak rectifier. Options 12 and 13 are
similar to options 2 and 3 but use a 400Hz high pass filter, and options 14 and 15 provide
CCIR unweighted and weighted measurements using the PPM rectifier.
22
2. Manual Operation
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