Audio Engineering

Measurement 67

2.19.3 Calibrating a VI Instrument

The instrument should be calibrated to read a true level of zero VU when an input of
a 1000-Hz steady-state sine wave signal of 0 dBm (0.001 W) is connected to it. For
example, typical calibration is when the instrument indicates  4, the attenuator value
is 4, and it is connected across a 600 circuit. Levels read on a VI instrument when the
source is the aforementioned sine wave signal should be stated as dBm levels.

2.19.3.1 Reading a VI Instrument on Program Material

Because of the ballistic properties of VI instruments, they exhibit what has been called
“ instrument lag. ” On short-duration peak levels, they will “ lag ” by approximately 10 dB.
Stated another way, if we read a true VU level of  8 VU on a speech signal, then the
level in dBm becomes  18 dBm. This means that the associated amplifi cation equipment,
when fed a true VU level of  8 VU, must have a steady-state sine wave capability of
 18 dBm to avoid overload.

2.19.3.2 Rule

Levels stated in VU are assumed to be program material, and levels stated in dBm are
assumed to be steady-state sine wave.

Circuit impedance (Ω)

30

25

20

15

10

5

0

dB correct

ion factor

1 2 5 10 20 50 100 200 500 1K

True VU  Instrument indication  attenuator

setting 10log (600/Zact)

Figure 2.16 : Relationship between circuit impedance and dB correction value.