VI Meters and Devices 99726.1 GeneralTo operate a sound recording or reproducing system
properly, some method for determining the signal levels
in different parts of the system to avoid overloading,
noise, and distortion is required. This is the purpose of
the volume indicator (VI) meter. A VI meter is a meter
used to measure levels of audio-frequency signals. The
term volume indicator is generally associated with
meters calibrated in decibels. Until recently, volume
unit (VU) meters were devices to measure power with
respect to 1 mW of power across a 600: line. Today
VU measurements are made with respect to many
different bases.
VU meters were first used by the telephone
company. They were used to measure the level of the
signal being sent down the line. The lines were open
wire pair of AWG #6 wire spaced 12 inches apart,
which translated to a characteristic impedance of 600:
as determined with the equation(26-1)where,
D is the spacing of the two wires,
d is the diameter of the wire.Today, most amplifying devices have a high-imped-
ance input and a low-impedance output as specified by a
1978 I.E.C. standard requiring the output impedance of a
device to be less than 50: and the input impedance to be
greater than 10 k:. Since very little power is transferred
between 50: and 10 k: , it makes more sense to make
measurements as voltage gain rather than power gain.
It is important to know what kind of measurement
reference is being used. The following are some of the
common references:
dBm. The original definition of the dB. It is power level
in dB referenced to 0 dB or 1 mW and a 600:load.dBW. Power referenced to 1 watt.dBf. Power referenced to 1 femtowatt (1 × 10^15 W).dBV. Voltage referenced to 1 Vrms. dBV is not affected
by impedance.
10 dBV. A voltage reference level used by many
consumer products and is equal to 0.316 Vrms.
dBu. Voltage referenced to 0.775 VrmsIt is not
affected by impedance. The u stands for unterminated.+4 dBu. The pro-audio voltage reference level of
1.23 Vrms.dB FS. Digital audio reference level equal to full scale,
which is the maximum peak voltage level before digital
clipping of a data converter. Full-scale value varies with
each design.dBA. An unofficial method of stating loudness
measurements using the “A” weighted curve on a sound
level meter.dBC. An unofficial method of stating loudness
measurements using the “C” weighted curve on a sound
level meter.dB-SPL. Sound pressure level referenced to
0.0002μbar where 1μbar = 1 dyne/cm^2 or the threshold
of hearing.dBr. An arbitrary reference level that must be speci-
fied. It can be used for many different references as long
as it is specified.DIN Scale. The DIN scale as used in Germany and
Austria uses +6 dBu as the reference level for the 0 dB
mark. This is equivalent to 1.55 Vrms.26.2 Standard VU MetersA volume unit (VU) meter is a special form of VI meter
used for monitoring broadcast, recording circuits and
sound reinforcement systems. Such meters employ
special ballistics that average out complex waveforms
to properly indicate program material that varies simul-
taneously in both amplitude and frequency. For complex
waveforms, such as speech, a VU meter reads between
the average and the peak values of the complex wave.
No simple relationship exists between volume measured
in VU and the power of a complex waveform. The indi-
cated reading will depend on the particular wave shape
at the moment. For sine-wave measurements, a change
of one VU is numerically equal to a change of 1 dB.
VU meters are designed to have a dynamic charac-
teristic that approximates the response of the human ear.
When a speech waveform is applied to a VU meter, the
movement will indicate peaks and valleys in the signal.
The average of the three highest peaks in 10 s (disre-
garding occasional extremes) is taken to be the indica-
tion of the meter movement.
Many meters marked as VU meters are not actually
such meters, since they do not have the special ballistics
and characteristics of the standard VU meter.Z 276 2 D
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