998 Chapter 26
The VU meter is a device whose standard has
remained the same since 1961. The meter consists of a
200 mAdc D’Arsonval movement fed from a full-wave,
copper-oxide rectifier mounted within the meter case.
VU meters are calibrated in reference to 1 mW of power
into a 600: load. A typical moving coil VU meter is
shown in Fig. 26-1.
In the 1920s and 30s copper-oxide rectifier power-
level meters were inaccurate and not satisfactory for
program monitoring. The development of an entirely
new meter was jointly undertaken by the Bell Telephone
Laboratories, Columbia Broadcasting System (CBS),
and the National Broadcasting Company (NBC). The
results of this research were not only the development
of a new type VI meter but also the standardization of a
new reference level of 1 mW, a unit that was adopted by
the electronics industry in May 1939. The current stan-
dard is ANSI C16.5-1961, formerly the Acoustical
Society of America (ASA) C16.5-1961.
The characteristics of the dBm VU meter are as
follows:
- General. The meter consists of a dc meter movement
with a full-wave, copper-oxide rectifier unit
(mounted in the instrument case) and responds
approximately to the root-mean-square (rms) value of
the impressed voltage. This value will vary somewhat
depending on the waveforms and the percentage of
harmonics present in the signal. - Instrument Scale. The face of the instrument may
have either of the two scale cards shown in Fig. 26-2.
Each card has two scales: a VU scale ranging from
20 to +3 VU and a percent-modulation scale
ranging from 0 to 100%, with 100% coinciding with
the 0 point on the VU scale. The normal point for
reading volume levels is at 0 VU or 100%, which are
located to the right of the center at about 71% of the
full-scale arc.- Dynamic Characteristics. With the instrument
connected across a 600: external resistance, the
sudden application of a sine-wave voltage, sufficient
to give a steady-state deflection at the 0 VU or 100
scale point, shall cause the pointer to overshoot not
less than 1% or more than 1.5% (0.15 dB). The
pointer shall reach 99 on the percent scale in 0.3 s. - Response Versus Frequency. The instrument sensi-
tivity shall not depart from that at 1 kHz by more
than 0.2 dB between 35 Hz and 10 kHz, or more than
0.5 dB, between 25 Hz and 16 kHz. - Impedance. For bridging across a line, the volume
indicator, including the instrument and proper series
resistor (3600:), shall have an impedance of 7500:
when measured with a sinusoidal voltage sufficient to
deflect the meter to 0 VU or the 100% scale point. - Sensitivity. The application of a sinusoidal potential
of 1.228 V (4 dB above 1 mW in a 600: line) to the
instrument in series with the proper resistance
(3600:) will cause a deflection to the 0 VU or 100%
point. - Harmonic Distortion. The harmonic distortion
introduced in a 600: circuit, caused by bridging the
volume indicator across it, is less than 0.3%, under
the worst possible condition (no loss in the variable
attenuator). - Overload. The instrument must be capable of with-
standing, without injury or effect on the calibration,
overload peaks of ten times the voltage equivalent to
Figure 26-1. Moving coil VU meter. Courtesy Simpson
Electric.
Figure 26-2. VU meter scales.A. Recording and test equipment.B. Broadcast monitoring.