Is it dBm or dBu?
Many manufacturers’ specifications show a value in dBm when the correct unit is
dBu. This special section describes the difference between the two, and when each
should be used.
In the early years ofbroadcasting and professional audio, most equipment was
connected for maximum power transfer from one device to another. This required
matching the output impedance ofone device to the input impedance ofthe next
device. Due to the transformer and vacuum tube technology of the day, the
standard input and output impedances were
600
Ω
. The dBm unit, a measurement ofpower relative to 1 milliwatt, was used to
measure the power transmitted from one device to another.
However, actual power meters are (and always have been) very rare in audio.
Most measurements are made with voltmeters. The power can be calculated from
a voltmeter measurement ifthe impedance is known (by calculating the voltage
squared divided by the impedance).
Since the early technology almost always used 600
Ω
impedances, early
measurement devices assumed this impedance. Although they did not measure
power, as long as they assumed that the voltmeter was connected across a 600
Ω
load they could correctly calculate the power in dBm. Since the voltage across a
600
Ω
resistor is 0.7764 Volts when it is dissipating one milliwatt, most early meters
were calibrated to read 0 dBm when this voltage was applied. But they were not
directly measuring the power; ifthe circuit impedance was not 600
Ω
, the power
calculation, and therefore the dBm measurement, was not correct.
Modern audio equipment generally has a very low output impedance and a very
high input impedance. This transfers a negligible amount of power but the
maximum amount ofvoltage. The dBm, being a power unit, is not appropriate for
measurements on these devices. A more appropriate unit is the dBu, which is a
unit of voltage. In fact, it is defined as the voltage in decibels relative to 0.7764
Volts, which is the voltage across a 600
Ω
resistor dissipating one milliwatt.
Since the old dBm-measuring instruments measured voltage, not power, they only
correctly read dBm when the circuit impedance is 600
Ω
. However, since they
measure voltage, they correctly read dBu no matter what the circuit impedance.
Those old meters calibrated in dBm are actually dBu meters.
(continued on following page)
4
Oper
ati
o
n
Units : Amplitude units
Operation
Portable One Plus Access User's Manual
4-73
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