Audio Engineering

64 Chapter 2

movement in conjunction with a copper-oxide bridge-type rectifi er. The impedance of
the instrument and rectifi er is 3900 Ω. To minimize its effect when placed across a
600-Ω line, it is “ built out ” an additional 3600 Ω to a total value of 7500 Ω. The addition
of this build-out resistance causes a 4-dB loss between the circuit being measured and the
instrument. Therefore when a properly installed VI instrument is fed with 0 dBm across a
600 line, the meter would actually read  4 VU on its scale. (When the attenuator setting
is added, the total reading is indeed 0 VU.)

Presently, no major U.S. manufacturer offers for sale a standard volume indicator that
complies with the applicable standard (C16.5). The standard requires that an attenuator
be supplied with the instrument and none of the manufacturers do so. What they are
doing requires some attention. The instruments (usually high-impedance bridge types) are
calibrated so as to act as if the attenuator were present. When the meter reads 0 VU (on a
sine wave for calibration purposes), the true level is  4 dBm. This means a voltage of 1.23 V
across 600 Ω will cause the instrument to read an apparent 0 VU. Note that when reading
sine wave levels, the label used is “ dBm. ” When measuring program levels, the label used is
“ VU. ” The VU value is always the instrument indication plus the attenuator value.

Two different types of scales are available for VI meters ( Figure 2.14 ). Scale A is a VU
scale (recording studio use), and scale B is a modulation scale (broadcast use). On complex
waveforms (speech and music), the readings observed and the peak levels present are about
10 dB apart. This means that with a mixer amplifi er having a sine wave output capability of

600 Ω

600 Ω

3900 Ω

3900 Ω

300 Ω 7500 Ω

3600 Ω

3900 Ω 3900 Ω 3900 Ω

Line Load Attenuator

3900 Ω constant impedance

Meter

Figure 2.13 : Volume indicator instrument circuit.