VI Meters and Devices 999a reading of 0 VU or 100% for 0.50 s and a contin-
uous overload of five times that voltage.26.2.1 Meter Ballistics
Meter ballistics are the mechanical and electrical char-
acteristics built into the meter movement. A given char-
acteristic may be obtained by shaping the pole pieces
and counterweighting the pointer mechanism. Shunts
are sometimes used across the meter terminals, but this
use will reduce the sensitivity of the movement.
The ballistics characteristics of a typical old-style VI
meter or voltmeter and a standard VU meter, when a
1000 Hz signal is applied for a period of 1 s, are shown
in Fig. 26-3. Note the VU meter comes to a steady state
at the end of 0.30 s, while the VI meter continues to
oscillate showing peaks and valleys over a period of 1 s.
An ac voltmeter would be even worse than the old style
VI meter as it would never settle down and would
constantly overshoot. This clearly indicates why the
ballistics of the VU meter are desirable for monitoring
program material containing complex waveforms.
A VU meter reads the rms value of the waveform.
On a sine wave, the rms VU indicator of the peak is
only 3 dB above the reading; however, on voice or
music, the peak may be 10–12 dB above the VU
reading. This difference is called the crest factor and is
illustrated in Fig. 26-4.
Because of the meter ballistics, a VU meter indicates
somewhere between the average and the peak values.
Program material is of a complex and transient nature;
therefore, the VU meter reading is considerably under
the instantaneous peak program level. This means that
8–14 dB peaks present in the program material are not
indicated by the meter because the meter movement
cannot follow small instantaneous peaks. Even if they
could be seen, it would be too late to reduce the level.
Therefore, the meter must either be set or caused to
indicate in a manner that will not overload the system in
which it is operating.
Since VU meters do not include the true peak values
of program material (complex waveforms), it is quite
easy to overload a recording system. To protect against
these unseen peaks, a lead or margin of safety is
inserted in the VU meter circuit.
To insert a lead into a VU meter circuit, the VU
meter is connected across a bridging bus with a
sine-wave level of +14 dBm. A 400 Hz or 1000 Hz
signal is sent into the input of the recording console.
The mixer control is set to its normal operating range,
and the signal level is adjusted to bring the bus level to
+14 dBm (the VU meter reads 100% or 0 dBm).
Remove the input signal and return the VU meter
attenuator to its +6 dBm position. This inserts an 8 dB
lead or margin of safety in the VU meter by making it
8 dB more sensitive. Thus, it protects the system against
unseen peaks up to 8 dB. The program material is now
mixed in the usual manner. Some recording activities,
because of the heavy peaks and overloads encountered
in some types of music, use a 10–12 dB lead in the VU
meter.
Radio transmitters are adjusted in a similar manner.
However, in this instance, the percent modulation indi-
cated by the VU meter indicates the percent modulation
of the radio transmitter.Figure 26-3. Comparison of the original VI meter and the
present VU meter ballistics when a 1000 Hz signal is
applied for 1 s.Peak factor
10
15 dB
greater than
sine wave
0.001 sVU meterVoltmeter180
160
140
120
100
80
60
40
20
0% scale deflection0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
Time sFigure 26-4. Crest factor caused by the peak of music or
voice being greater than rms.Peak
rmsSine wavePeak
Crest factorVoicerms3