RackPack: Preference Mic Pre
11
VU Meter
The VU meter displays the output level of the preamp. The gauge indi-
cates levels from -20 dB to +5 dB. If necessary you can lower the sen-
sitivity by 10 dB so that the gauge goes up to +15 dB output level (see
“-10 dB” below and “About Leveling” on the previous page).
The VU meter is custom made to meet SPL specifications and assures
a balanced optical perception thanks to it’s optimized ballistics.
An especially interesting feature is the option to switch between two
display modes: VU mode and PPM mode. The VU mode (VU=Volume
Unit) displays average levels, thus provides information on the over-
all loudness. The PPM mode (PPM=Peak Program Meter) displays the
peak levels.
The integration time of the display complies with the BBC require-
ments. In VU mode the rise time up to 0 dB is approx. 300 ms. In PPM
mode the rise time up to 0 dB is about 2 ms and the release time down
to -20 dB is at a “slow” 1,5 seconds. This calibration ensures to display
even short peaks up to about +6 dB since the needle does not have to
travel the entire distance of the gauge every time.
PPM
The PPM button allows to switch the VU metering characteristics from
VU display mode (button is deactivated and not illuminated) to PPM
display mode (button is active and illuminated). A/D converter dis-
play also make use of the PPM mode. Monitoring peak levels is most
important to avoid overloading the converter and to prevent audible
distortion. Peak levels are always above the average levels. It may
make sense in most cases to also press the -10 dB button to prevent
the needle from getting stuck on the right side of the gauge.
-10dB
With this button you can change the sensitivity of the VU display. If
you press the -10 dB button (button is illuminated), the sensitivity of
the display is lowered by 10 dB. With the needle for example at 0dB,
a value of +10 dB is displayed. Now you can read values of up to +15 dB.
You always have to add these 10 dB to the displayed values, for
example if you compare these values with those of an A/D converter.
Remember that A/D converters show peak levels, not average levels.
For comparison press the PPM button to activate the PPM mode.
Control Elements
VU Meter, PPM, -10 dB
20
SIG
OVL
10 7 5
3
1 0
3
5
0
30
50
70
100
%
50
70
100
VU
11.
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