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DYNAMIC CONTROLLERS
Dynamic controllers or processors represent a
class of signal processing devices used to alter an audio signal
based solely upon its frequency content and amplitude
level, thus the term “dynamic” since the processing
is completely program dependent. The two most common dynamic effects
are compressors and expanders, with limiters
and noise gates (or just “gates”) being special
cases of these.
The dynamic range of an audio passage is the ratio of the
loudest (undistorted) signal to the quietest (just audible) signal,
expressed in dB. Usually the maximum output signal is restricted
by the size of the power supplies (you cannot swing more voltage
than is available), while the minimum output signal is fixed by
the noise floor (you cannot put out an audible signal less than
the noise).
Professional-grade analog signal processing equipment can output
maximum levels of +26 dBu, with the best noise floors being down
around –94 dBu. This gives a maximum dynamic range
of 120 dB (equivalent to 20-bit digital audio) pretty impressive
number but very difficult to work with. Thus were born dynamic processors.
Compressors
Compressors are signal processing units used to reduce
(compress) the dynamic range of the signal passing through
them. The modern use for compressors is to turn down just the loudest
signals dynamically. For instance, an input dynamic range
of 110 dB might pass through a compressor and exit with a new dynamic
range of 70 dB.
This clever bit of processing is normally done using a VCA (voltage
controlled amplifier) whose gain is determined by a control voltage
derived from the input signal. Therefore, whenever the input signal
exceeds the threshold point, the control voltage becomes proportional
to the signal’s dynamic content. This lets the music peaks
turn down the gain.
Before compressors, a human did this at the mixing board and we
called
it gain-riding. This person literally turned down the gain
anytime it got too loud for the system to handle. You need to reduce
the dynamic range because extreme ranges of dynamic material are
very difficult for sound
systems to handle.
If you turn it up as loud as you want for the average signals, then
along comes these huge musical peaks, which are vital to the punch
and drama of the music, yet are way too large for the power
amps and loudspeakers to handle. Either the power amps clip, or
the loudspeakers bottom out (reach their travel limits),
or both — and the system sounds terrible.
Or going the other way, if you set the system gain to prevent these
overload occurrences, then when things get nice and quiet, and the
vocals drop real low, nobody can hear a thing. It’s always
something. So you buy a compressor. Using it is quite simple:
Set a threshold point, above which everything will be turned down
a certain amount, and then select a ratio defining just how much
a “certain amount” is.
All audio below the threshold point is unaffected and all All signal
hanges less than this setting essentially produce zero output, i.e.,
the output is muted. audio above this point is compressed by the
ratio amount. The earlier example of reducing 110 dB to 70 dB requires
a ratio setting of 1.6:1 (110/70 = 1.6).
The key to understanding compressors is to always think in terms
of increasing level in dB above the
threshold point. A compressor makes these increases smaller.
From our example, for every 1.6 dB increase above the threshold
point the output only increases 1 dB. In this regard compressors
make loud sounds quieter. If the sound gets louder by 1.6 dB
and the output only increases by 1 dB, then the loud sound has been
made quieter. Some compressors include attack and release
controls.
The attack time is the amount of time that passes between
the moment the input signal exceeds the threshold and the moment
that the gain is actually reduced. The release time is
just the opposite — the amount of time that passes between
the moment the input signal drops below the threshold and the moment
that the gain is restored.
These controls are very difficult to set, and yet once set, rarely
need changing. Because of this difficulty, and the terrible sounding
consequences of wrong settings, Rane correctly presets these controls
to cover a wide variety of music and speech one less thing for you
to worry about.
System overload is not the only place we find compressors. Another
popular use is in the making of sound. For example when
used in conjunction with microphones and musical instrument pick-ups,
compressors help determine the final timbre (tone) by selectively
compressing specific frequencies and waveforms.
Common examples are “fattening” drum sounds, increasing
guitar sustain, vocal “smoothing,” and “bringing
up” specific sounds out of the mix, etc. It is quite amazing
what a little compression can do. Check your owner’s manual
for more tips.
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