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different. A 0 dBV signal has a voltage level of 1.0 volts. A 0 dBu signal
has a voltage level of .775 volts. Since .775 is approximately 2 dB less than
1.0, converting dBV levels into dBu levels is as simple as subtracting 2 dB
(2.21 to be exact).
Signals are also occasionally represented with units of dBm. This is an
older unit that measures power instead of voltage levels with 0 dBm
representing 1 milliwatt. Earlier tube-based audio equipment used
standardized input and output impedances of 600 ohms, so a 0 dBm signal
was produced with a voltage of .775 volts. Since most of today’s equipment
uses impedances other than 600 ohms, it is more useful to represent signals
by voltages rather than power and the dBu unit was introduced. A signal
level of 0 dBu is identical to a level of 0 dBm.
Digital signals, after they are recorded, no longer directly represent any
physical quantity such as voltage or power and 0 dB is generally used to
represent a “full-scale” or maximum signal level. All other signal levels are
lower and are expressed as negative decibels. Most meters on digital
equipment (Mona included) have 0 dB at the top and range downward from
there. A signal that is 30 dB below full scale would simply be referred to as
a –30 dB signal.
Nominal Signal Levels and Headroom
Today’s equipment is generally referred to as +4 dBu equipment
(professional) or –10 dBV equipment (consumer). These levels are the
typical or “nominal” signal levels you can expect to see with professional
(studio) equipment such as mixers or with consumer equipment such as
home stereos and CD players. A +4 dBu signal has a voltage level of 1.23
volts and a –10dBV signal has a voltage level of .316 volts.
The above nominal levels represent typical or average levels that are often
exceeded when recording loud signals such as drum beats. The difference
between the nominal level and the loudest signal that can be recorded
without clipping is called “headroom”. Mona provides approximately 18
dB of headroom allowing a 22dBu signal (9.8 volts or 28 volts peak to
peak) to be recorded. Of course, increasing the gain with the trim knob will
allow manually setting the headroom for smaller signals.