Owner`s manual

84
Decibels
Audio signal levels are generally expressed in units called “decibels” which are
abbreviated as “dB”. This is a “logarithmic” scale where each doubling of signal
level is represented by an increase of 6dB. Therefore a signal of 6dB is twice as
big as a 0dB signal and a signal of 12dB is four times as big as a 0dB signal.
Since digital audio signals are represented by binary data, each bit of audio
information represents 6dB. A 16-bit number can represent a total range of 96dB
and a 24-bit number can represent a total range of 144dB (6 times the number of
bits). It’s much easier to say that one signal is 72dB less than another instead of
saying it is 1/4096 the size of the other one. It also more accurately represents the
way we hear sounds, since the smaller signal in the above example will still be
audible and not appear to be only 1/4096 as loud when we listen to it.
Just as there are different types of degrees used to represent temperature
(Fahrenheit, Celsius, etc), there are different types of decibels used to represent the
level of analog audio signals. The most common are dBu and dBV decibels. Both
of these represent voltage levels and still double for every increase of 6dB. It is
only the reference point, or 0dB level that is different. A 0dBV signal has a
voltage level of 1.0 volts. A 0dBu signal has a voltage level of .775 volts. Since
.775 is approximately 2dB less than 1.0, converting dBV levels into dBu levels is
as simple as subtracting 2dB (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 0dBm representing 1 milliwatt.
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.