Handbook for Sound Engineers

1000 Chapter 26

26.2.2 Reference Levels

In the early days of broadcasting and recording, both
10 mW and 12.5 mW into a 500: line were used as a
reference level. However, later this was changed to
6 mW. In May 1939 the present standard of 1 mW into a
600 : line was adopted. This reference level was
selected as a level that would conform to the telephone
company's standards of limiting the signal level on a
transmission line to a value that would produce a
minimum of crosstalk and still provide a satisfactory
signal-to-noise ratio (SNR). The 1 mW reference level
is a unit quantity and is readily applicable to the decimal
system, being related to the watt by the factor 10^3.

Zero level is a reference power level of 1 mW of
power into a 600: load. This is equivalent to a voltage
of 0.775 V.

26.2.3 VU Meter Impedance

The VU meter and its attenuator impress a 7500:
impedance onto a circuit. The VU meter system consists
of an indicator movement, a variable attenuator, and a
series resistor of 3600:, Fig. 26-5. Meter manufac-
turers supply only the meter movement; the external
circuitry is added later. A 200 μA D’Arsonval meter
movement with an internal resistance of 3900: and a
full-wave, copper-oxide or selenium rectifier are
contained within the meter case. The attenuator is vari-
able in steps of 2 dB, presents a constant impedance of
3900 : to the meter movement, and prevents the ballis-
tics of the meter from being affected when the attenu-
ator setting is changed.

Standard VU meters are designed to read 0 VU, or
100%, with 1.228 V (+4 dBm) applied to the instru-
ment. If the meter is used with the attenuator but
without the 3600: series resistor and is connected
across a 600: load in which 1 mW of power is
flowing, the movement will be deflected to the 100%
calibration point. This method is not recommended
since the impedance looking back into the meter is only
3900 : and loads the 600: circuit. It is the usual prac-

tice to keep the impedance of bridging devices at a ratio

of 10:1 or greater.

Increasing the input impedance of the VU meter

from 3900:to 7500: creates a 4 dB loss across the

3600 : resistor. If a signal of 1 mW (0.775 V) is

impressed across the input terminals of the circuit in

Fig. 26-6, it will not deflect the meter to the 0 VU cali-

bration but only to the 4 VU (or decibel) mark, or

approximately 65%. This means that if the meter is to

be deflected to the 100% point, the input signal must be

increased to a +4 dBm. This is the reason why 1 mW of

power will be indicated at the 4 dB calibration mark.

Attenuators used with VU meters start at a +4 dBm.

The bridging loss caused by the VU meter being

inserted into the circuit is the drop in signal level caused

by the absorption of power by the meter circuit. As a

rule, the power absorbed is quite small and may be

ignored. However, at high powers, it may become

important. Bridging loss may be calculated by the

equation

(26-2)

where,

BR is the VU meter input impedance,

Z is the line impedance.

A 7500: VU meter has a bridging loss of 0.34 dB.

26.2.4 VU Impedance Level Correction

VU meters are calibrated for 1 mW of power across a

600 : load as 4 VU, therefore when a VU meter is

connected across any other impedance, a correction

must be added to the indicated reading to give a proper

VU reading. The equation for the level correction is

(26-3)

where,

dBcorr is the decibel amount added to the VU reading,

Z 2 is the impedance for which the meter is calibrated,

Z 1 is the impedance of the circuit bridged.

A typical example of applying a correction factor is

as follows: a VU meter calibrated for a line impedance

of 600: is bridged across a 16: loudspeaker line and

indicates a level of +1 dBm. The true VU would be

.(26-4)

The correction factor from Eq. 26-2 is

Figure 26-5. A 7500 : VU meter, calibrated for 1 mW
reference level or 0.775 V across 600 :.

3600 7

600 7

0.775 V 7

R 1 R 1

R 2

3900 7

Rectifier

meter

3900 7

internal

R (^3) resistance
3900 7attenuator
dBloss 20
2 BR+Z
2 BR
= log-------------------
dBcorr 10
Z 2
Z 1
= log-----
VU 1 dBm+= correction factor