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GAIN STRUCTURE
The most fundamental basic of mixing is to ob-
tain a good 'gain structure'. Gain is the amount
of amplification applied to an input signal, and
is expressed in dB, a logarithmic ratio. There-
fore zero gain (also called 'unity') means no in-
crease or decrease in the level of a signal as it
passes through an amplifier or 'gain stage'. Any-
thing above zero makes the signar 'louder' and
anything below reduces the level (also called
'attenuation'). Attenuation of minus infinity (- )
means a total reduction in signal level, ie, no
signal comes out the other end.
Most professional mixers have an input gain or
trim control, which is the first circuitry the signal
passes through. The aim of this is to bring the
signal up to the optimum level for the mixer to
work with. This is the most crucial part of the
signal chain; it typically has more gain available
than the other stages and any mistakes here will
taint the signal through the whole mixer.
All amplifiers add some noise to the signal as it
passes through, but by keeping the gain high,
the sound to noise ratio (S/N) is generally
increased. However, at the other end of the scale
there is the problem of overload or 'clipping'. An
amplifier only has limited power.
If you push it too far and try to amplify a signal
too much, the peaks of the signal will be clipped
off when they pass the amp's limit.
This results in distortion and is audiable as a
harsh, unpleasant 'breaking-up' sound.
Mixers generally have some aid to setting up a
channel, such as a peak light or metering like the
MM122. This allows you to observe the peaks
and adjust the gain to avoid clipping. When all
the channels have been set in this way, the other
big bonus is that they are all conveniently at
roughly the same level to start mixing with.
A mixer has several levels of gain: input trim, EQ,
channel fader and main fader, not to mention any
gain due to insert processors and the AUX sends,
returns and effects. The key is to keep them be-
low clipping, but high enough to maximize the S/
N. It is also beneficial in the real world to have
enough space to crank it up should you need to.
NATURE OF NOISE
Noise is typified by a hiss and is present to some
degree in all amplified music. It is caused by ran-
dom thermal movement of atoms in the electrical
components. To eradicate it requires tempera-
tures approaching absolute zero (-273 degrees
Celsius), but careful circuit design and gain stag-
ing can minimize it at room temperature.
The shorter the path of a signal, and the fewer
the gain stages, the less noise the signal will be
picked up.
Another type of audio problem is RF (radio
frequency) interference. This is caused by the
circuitry and wiring of electronic devices picking
up radio signals which are amplified through the
gain stages. One way to minimize this is by keep-
ing signal levels high so that any interference
picked up is much quieter than the signal. Shield-
ing the wires with a foil layer also helps, and steel
casings like the MM122's are very effective at
preventing RF from entering the mixer. RF is a
big problem with longer cable runs, and so bal-
anced lines, which electrically reject interference,
are commonly used for these.
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