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Attack The
attack time is how long a compressor
takes to pull the gain down, once the
input signal has reached or exceeded the
Threshold level. With a fast attack
setting, the signal is controlled almost
immediately, whereas a slower attack
time will allow the start of a transient
or percussive sound to pass through
unchanged, before the compressor gets
its act together and does something
about it. Creating a deliberate
overshoot by setting an attack time of
several milliseconds is a much-used way
of enhancing the percussive
characteristics of instruments such as
guitars or drums. For
most musical uses, an initial attack
setting of between 1 and 20 mS is
typical. However, when treating sound
such as vocals, a fast attack time
generally gives the best results,
because it brings the level under
control very quickly, producing a more
natural sound. Release The
Release sets how long it takes for the
compressor's gain to come back up to
normal once the input signal has fallen
back below the Threshold. If the release
time is too fast, the signal level may
'pump'—in other words, you can hear
the level of the signal going up and
down. This is usually a bad thing, but
again, it has its creative uses,
especially in rock music. If the release
time is too long, the gain may not have
recovered by the time the next 'above
Threshold' sound occurs. A good starting
point for the release time is between
0.2 and 0.6 seconds. Auto
Attack/Release Some
models of compressor have an Auto mode,
which adjusts the attack and release
characteristics during operation to suit
the dynamics of the music being
processed. In the case of complex mixes
or vocals where the dynamics are
constantly changing, the Auto mode may
do a better job than fixed manual
settings. Peak/RMS operation: Every
compressor uses a circuit known as a
side chain, and the side chain's job in
life is to measure how big the signal
is, so that it knows when it needs
compressing. This information is then
used to control the gain circuit, which
may be based around a Voltage-controlled
Amplifier (VCA), a Field Effect
Transistor (FET) or even a valve. The
compressor will behave differently,
depending on whether the side chain
responds to average signal levels or to
absolute signal peaks. An
RMS level detector works rather like the
human ear, which pays less attention to
short duration, loud sounds than to
longer sounds of the same level. Though
RMS offers the closest approximation to
the way in which our ears respond to
sound, many American engineers prefer to
work with Peak, possibly because it
provides a greater degree of control.
And though RMS provides a very
natural-sounding dynamic control, short
signal peaks will get through unnoticed,
even if a fast attack time is set, which
means the engineer has less control over
the absolute peak signal levels. This
can be a problem when making digital
recordings, as clipping is to be avoided
at all costs. The difference between
Peak and RMS sensing tends to show up
most on music that contains percussive
sounds, where the Peak type of
compressor will more accurately track
the peak levels of the individual drum
beats.
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way to look at it is to say that the
greater the difference between a
signal's peak and average level, the
more apparent the difference between RMS
and peak compression/limiting will be.
On a sustained pad sound with no peaks,
there should be no appreciable
difference. Peak sensing can sometimes
sound over-controlled, unless the amount
of compression used is slight. It's
really down to personal choice, and all
judgements should be based on listening
tests.
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A
compressor's side chain follows the
envelope of the signal being fed into
it, but if the attack and release times
are set to their fastest positions, it
is likely that the compressor will
attempt to respond not to the envelope
of the input signal but to individual
cycles of the input waveform. This is
particularly significant when the input
signal is from a bass instrument, as the
individual cycles are relatively long,
compared to higher frequencies. If
compression of the individual waveform
cycles is allowed to occur, very bad
distortion is audible, as the waveform
itself gets reshaped by the compression
process. We could simply increase the
release time of the compressor so that
it becomes too slow to react to
individual cycles, but sometimes it's
useful to be able to set a very fast
release time.
A
better option is to use the Hold time
control, if you have one. Hold
introduces a slight delay before the
release phase is initiated, which
prevents the envelope shaper from going
into release mode until the Hold time
has elapsed. If the Hold time is set
longer than the duration of a single
cycle of the lowest audible frequency,
the compressor will be forced to wait
long enough for the next cycle to come
along, thus avoiding distortion. A Hold
time of 50ms will prevent this
distortion mechanism causing problems
down to 20Hz. If your compressor doesn't
have a separate Hold time control, it
may still have a built-in, preset amount
of Hold time. A 50ms hold time isn't
going to adversely affect any other
aspect of the compressor's operation,
and leaves the user with one less
control to worry about.
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