100
Earlier tube-based audio equipment used standardized input and output impedances
of 600 ohms, so a 0dBm 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 0dBu is identical to a level of 0dBm.
Digital signals, after they are recorded, no longer directly represent any physical
quantity such as voltage or power and 0dB 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 have 0dB at the
top and range downward from there. A signal that is 30dB below full scale would
simply be referred to as a –30dB signal.
Nominal Signal Levels and Headroom
Today’s equipment is generally referred to as +4 dBu equipment (professional) or
–10dBV 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 –10 dBV 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”. Your Echo card provides approximately 14dB of headroom
allowing an 18dBu signal to be recorded.
Unbalanced and Balanced Inputs and Outputs
An unbalanced signal, commonly used for guitars and consumer electronics,
contains two components, a ground signal and a “hot” or active signal. The ground
is the barrel of a ¼” connector and the shell of an “RCA” style connector.
A balanced signal contains two active signals instead of one in addition to the
ground. These are referred to as the “plus” and “minus” signals. A balanced input
amplifier amplifies the difference between these two signals. Any extraneous
noise picked up from power lines or other sources will appear equally on both the
plus and minus inputs. This is called “common mode” noise since it is common to
