Recording Basics
Owner’s Manual
9
English
Techie Words? Don’t Be Afraid.
Signals — Level and Decibel
Assume the softest sound a human being can hear is at
a level of “1,” and the loudest sound a human being can
endure hearing is at a level of “1,000,000.” The
difference expressed in these numbers is huge, and
you have to use too many digits to express the level.
This is very inconvenient. So, we use a term or unit
called “decibel (dB),” defining the normal level
difference between the softest and loudest sounds
detectable by human hearing to be 120 dB.
A decibel is a relative value based on a reference level
of 0 dB. Audio devices usually treat audio as electrical
signals. There are various types of decibel
measurements: dBu, dBV, dBm, etc., but the most
popular one is dBu, which is based on 0.775V as the
reference level (0 dBu). The output level of a
microphone is very low –– about several millivolts (–60
dBu — –30 dBu). On the other hand, the maximum
output of a mixer can be 12 V (+24 dBu).
To create clear sounds, a signal should be input to a
mixer at an appropriate level. When you connect audio
devices to your mixer, be sure to match the nominal
input of an instrument to the output level of the mixer,
then adjust the input level with the mixer’s gain control.
Balanced or Unbalanced?
In most cases, shielded cables are used to transfer
signals between audio devices. Shielded cables are
divided into two types: balanced and unbalanced.
Balanced lines are very good at rejecting noise, and
they are the best choice for long cable runs or for
transferring very weak signals. Unbalanced cables are
usually used for line-level signals.
Microphone:
Use balanced lines.
Short line-level runs:
Unbalanced lines are fine.
Long line-level runs:
Use balanced lines.
We are constantly surrounded by random electromagnetic
radiation (noise), such as radio and TV signals as well as
spurious electromagnetic noise generated by power lines,
motors, electric appliances, computers, and other
sources. The longer the wire, the more noise it is likely to
pick up. To avoid noise, use the shortest cable possible.
How balanced lines work
A cable that features an XLR plug on each end is a
balanced line. It consists of hot (+), cold (–), and
ground conductors.
A device transmits an original hot (+) signal through the
hot conductor and the same, but inverted, signal
through the cool (–) conductor to the destination. The
receiving device inverts the inverted signal back to
normal and combines it with the original hot signal.
Any noise induced in the line will be exactly the same in
both conductors, and thus in phase. The trick is that the
phase of one signal (through the cool (–) conductor) is
reversed at the receiving end of the line so that the
desired audio signals become in-phase, and the
induced noise suddenly finds itself out of phase. The
out-of-phase noise signal is effectively canceled while
the audio signal is left intact.
Recording Basics
+ 20 dBu
0 dBu
0.775 V
-20 dBu
-40 dBu
-60 dBu
Professional audio gear, such as a
mixer or a power amplifier, features
line inputs and outputs with a
nominal level of +4 dBu.
Keyboards and other line instruments
have line inputs and outputs with a
nominal level of –10 dBu.
Microphone levels vary depending
on the sound source. Speech is at
about –30 dBu. Bird chirping is
lower than –50 dBu. Closely
recorded drums could reach 0 dBu.
input signal
gain
control
output signal
too high
appropriate
level
too low
Anatomy of a shielded cable
As shown in the illustration above, a shielded cable
consists of one hot (and cold conductor) wrapped in a
metal net (shield, or ground conductor). The ground
functions as a barrier against noise, protecting the
signal from noise that may be induced in the line.
hot
cold
shield
(ground)
jacket
balanced
unbalanced
