[BC] VU Meter

[Gil & Judy Gillivan]

Does this help??

 

A unit used for the scale of meters ("VU meters," but in standards referred
to as "volume indicators") used to monitor audio power levels in recording
and broadcast studios, so that the signal can be weakened or amplified to
bring it within the optimal range for recording or broadcast. Abbreviated,
VU. The term "volume unit" is rarely heard; readings are usually described
as, for example, "minus 4 vee you".

The VU meter was developed in the late 1930s as a joint project of Bell
Labs, CBS and NBC. Its response was intended to approximate the response of
the human ear to complex, changing waveforms, and its readings lie somewhere
between the peak and average values. The specifications cover the meter's
ballistics (how fast and far the needle moves when various signals are
applied) as well as frequency response. In May 1939, a reference power level
of 1 milliwatt into 600 ohms was adopted, and readings on the VU scale are
roughly log to the base 10 of the power ratio referred to that reference
level. For a pure sine wave, 0 VU = 1 dBm
<http://www.sizes.com/units/decibel.htm#dBm> , but the value of the "volume
unit" for real world audio signals is embodied in the unique characteristics
of the meter itself.  Further, in practice, engineers found it necessary to
add an isolating resistor, which made 0 VU = +4 dBm (again, for a pure sine
wave).

Many inexpensive meters, though marked in VU, do not meet the standard. In
Europe, the VU meter has generally been scorned in favor of peak reading
meters. A VU meter is not suitable for monitoring during digital recording;
peak-reading meters are used instead.

H. A. Chinn, D. K. Gannett, and R. M. Morris.
A new standard volume indicator and reference level.
Proceedings of the Institute of Radio Engineers, volume 28, pages 1-17
(January 1940).

Page 15: "3. Dynamic Characteristics
"If a 100-cycle voltage of such amplitude as to give a steady reading of 100
on the voltage scale is suddenly applied, the pointer should reach 99 in 0.3
second and should then overswing the 100 point by at least 1.0 and not more
than 1.5 per cent.
...
"5. Calibration
"The reading of the volume indicator (complete assembly as shown
schematically in Fig. 18) shall be 0 vu when it is connected to a 600-ohm
resistance in which is flowing 1 milliwatt of sine-wave power at 1000 cycles
per second, or n vu when the calibrating power is n decibels above 1
milliwatt."