Handbook for Sound Engineers

VI Meters and Devices 1001

The correction factor of 15.74 dB is added to the meter

reading of +1 dBm for a true level reading of

+16.74 dBm. Typical correction factors are shown in

Table 26-1.

If a VU meter is connected across a line impedance
different from that for which it was originally calibrated,
the voltage supplied to the meter will either be lower or
higher than the original calibration; therefore, the meter
would indicate incorrectly. Two circuits are shown in Fig.
26-6, one a 600: circuit and the other a 16: circuit.
Both are dissipating the same amount of power; yet the
voltage for the 600: circuit is 0.775 V, and for the 16:
circuit it is 0.127 V. As can be seen, if a VU meter is

connected across the 16: circuit, it will not deflect the
same amount as for the 600: circuit, although the same
amount of power is flowing in each circuit. To arrive at

the correct power level in the 16: circuit, a correction
factor must be applied to the meter indication.

26.2.5 Voltages at Various Impedances

If the line voltage for a given level at 600: is known,

voltages for other line impedances may be calculated

using

(26-5)

where,

Vx is the unknown voltage,

V is the voltage for 600: ,

Z is the new impedance.

As an example, assume voltage Vx is required for a

line impedance of 150: at a level of +4 dBm. Refer-

ring to Fig. 26-7, the voltage for a level of +4 dBm at

600 : is 1.23 V. The new voltage may now be calcu-

lated using

Voltages for a line impedance of 600: for levels

between 0 and +50 dBm may be taken from Fig. 26-7.

Voltage across 600: can be calculated from dBm with

the following equation

(26-6)

Table 26-1. Correction Factors in dBm to Be Applied
to a VU Meter When Connected Across an
Impedance Other Than 600:

Line Impedance—: Meter Cal 600 :—dB

10,000 12.22

(^5000) 9.21
(^2500) 6.20
(^1000) 2.22
600 0.000
500 +0.791
250 +3.800
200 +4.770
150 +6.020
125 +6.810
100 +7.780
50 +10.790
30 +13.010
16 +15.740
15 +16.020
8 +18.750
4 +21.760
dBcorr 10 600
16
= log---------
= 10 u1.574
=15.74 dB
Figure 26-6. Voltage across lines of different impedance
but with the same power in milliwatts.
P = 0.001 W 0.775 V 600 Ω
P = 0.001 W 0.127 V 16 Ω
600 Ω line
16 Ω line
Vx V Z
600
---------=
Vx 123 150
600
---------=
=0.615V
V 0.6 10
dBm
----------- 10
= u