|Dynamic processing is one of the most effective
tools that a recording engineer has to make sure all of those
important parts that have been recorded will actually be heard in
the final product. The term "Dynamic Range" refers to the level
difference between the loudest musical passage and the softest musical passage.
devices change this level difference.
|TYPES OF PROCESSING
means that any signal that tries to exceed a set "threshold" level will not be
allowed to increase as much. In other words the compressor turns down the maximum
passages so they won't be as loud. With the threshold control set to
maximum, no passages
will be turned down; as the threshold control is brought down some peaks of the music will
exceed the threshold and be turned down. Compression is used to even out the volume of the
music so that all passages are more even.
|The term "expand" means that any signal the falls bellow the
threshold level will be made to go lower in level than it would without expansion. In
other words expansion is just the opposite of compression. One use of expansion is to
"undo" the over-use of compression. Another use would be to make low level
sounds (like mechanical sounds of instruments) less noticeable.
Figure 16a - Effect Of Compression &
|The term '"1imit" means that the signal will not be allowed to
exceed the threshold level at all. Limiters are used to prevent distortion by setting the
threshold control just below the distortion level. The limiter then prevents any peak from
getting to the level where it would distort.
Figure 16b - Limiting Compared To
|The term "gate" refers to turning off the signal when it falls
below the threshold level. A range control is included so that signal can just be turned
down rather than completely off. Gates are used to get rid of noise and leakage when the
instrument is not playing. This is done by setting the threshold level lower than the
level of the signal but higher than the level of the noise or leakage.
Figure 16c - Gating Compared to
|DYNAMICS PROCESSING CONTROLS
|The threshold control adjusts the level where the dynamic processor will
start to work. In a compressor or limiter when the loud passages at the input
the threshold level set, the unit will turn down these loud passages. In an expander or
gate, the unit will turn down any incoming signals that are below the
threshold level. The threshold is
usually adjustable by a control marked "threshold." Reducing the threshold
level means that more peaks of the signal will trigger the compressor to turn down the
gain; increasing the threshold level means that an expander or
gate will turn down more low-level signals.
control determines how much the signals that are being compressed or expanded will be
|If a compressor has a 2:1 ratio, the compressor will turn down the gain
so that if the input signal is 2dB above the threshold level, the output increases only
one dB. If the input signal is 4 dB above the threshold, the gain will be turned down so
that the output only rises 2 dB above the threshold (a 2 to 1 ratio). At a 4:1
ratio the input signal has to be 4 dB above the threshold for the output to increase 1 dB.
When the ratio control is set to 10:1 or more, the compressor is called a limiter because
the unit is effectively preventing the peak levels from increasing any significant degree
above the threshold level.
|Expanders also have ratios. An expansion ratio of 1:2 means that if the
input signal drops 1 dB below the threshold level and the gain will be reduced so that the
output is 2 dB below the threshold level. If an expander has a very high expansion
ratio, it becomes a gate.
|Gain Reduction Meters
|You can tell "how much" compression or expansion you are using
by reading the gain reduction meter. The meter will read how many dB that the signal
is being turned down by the compression or expansion. With the gain reduction meter
reading "-6" the output is being turned down dB or to half the level on that
Figure 18a -
Gain Reduction During Compression
Figure 18b -
Gain Reduction During Expansion
|Attack & Release Times -
The attack time is how fast the dynamics processor will react to a
signal crossing the threshold level, going up. In a compressor it is the time it
takes the compressor to reduce gain on a high-level passage. On an expander, it is
the time that the expander takes to restore full gain after the audio level comes up after
a low level passage.
|The release time is how fast the dynamics processor will react to a
signal crossing the threshold level, going down. In a compressor, it is the time it
takes the unit to restore gain after the high-level passage is over with. In an
expander, it is the time the expander takes to turn down a low level passage (below the
|Very generally speaking, fast attack times are good. Release
times should be adjusted for the frequency and how percussive the signal is.
Compressors for bass signals must not be set to a very short release time or the gain will
be changed within one cycle, causing distortion. Generally speaking, the fastest
release time that sounds natural is the best for both compression and
|Makeup Gain (Output
Level) - The makeup gain is a level control at the output of the compressor.
The compressor reduces signal strength of the louder passages. After this
compression, the peak level of the signal is less. This makeup gain control allows
the compressed signal to be turned up so that its peaks are as high as they were before
compression. This action of "making up" gain increases the lower level
passages of the signal.
|Knee - How abruptly or gradually the compression ratio
is applied to the signal when the signal is near the set threshold level. A
Knee means that the full compression ratio is applied to any signal above the
threshold level. A Soft Knee means that a mild compression ratio is applied
to signals approaching the threshold level and that the ratio is increased as the signal
rises above the threshold level; the full compression ratio is applied to signals well
above the threshold level. The soft knee makes the gain reduction less obvious - sort of
like turning down the volume of a stereo slowly rather than abruptly. Soft knee is
used in applications where you are using a compressor to even out volume changes in an
instrument. DBX, a well-known brand of compressors, trademarked their soft knee
function with the name Overeasy.
|The graphs to the right are the traditional way of showing the
input/output characteristics of hard knee and soft knee compressors, and the graphs
resemble "knees" (hence the name). The above graph shows how the
output signal would be different in these two types of compressors.
sidechain is the control circuit of a compressor. Usually the compressor's
sidechain simply receives the input signal into the compressor [and most explanations of
how a compressor works assume that the input signal also goes to the
|On many compressors you can insert a
into the sidechain by plugging into the "sidechain" or "key" input.
(on some compressors you also have to push a button labeled "sidechain" or
"key"). When a different signal is put into the sidechain, the
compressor operates according to the signal fed to the sidechain input rather than to the
main input. In other words the main signal (going into the regular input and output)
is changed by the level variations of the signal plugged into the sidechain input.
Detecting - There are two ways you could define the
"level" of a signal. One way would be to use the
absolute peak level of the waveform; the second way would be to
use the "average" level of the waveform. In
percussive instruments, such as drums, there is a huge difference
between the peak level and the average level. In other
instruments, such as a flute or violin, the two levels are much
closer. Electronic circuits overload according to the
"peak" level but your ear responds to the
"average" level. If you are using a compressor to
help prevent overload (say of a digital recording) you would want
the compressor to respond to the peak level. If you
are using a compressor to make an instrument more even in volume,
you would want the compressor to respond to the
"average" level. the term RMS is an
acronym for the mathematical process of determining a signal's
effective average level [the "M" stands for
"mean" - another word for average]. Many
compressors allow the user to chose between average and peak
|Auto Release - Auto Release
is variable release time feature in compressors and helps make the
signal's volume even to the ear. With auto release
activated, the compressor's release time is longer for peaks going
well above the threshold and shorter for peaks going slightly
above the threshold.
|Range - The Range Control is a expander or
gate control. It sets the maximum amount of dB that the
expander or gate will reduce gain on lower level passages.
|Breathing - Breathing is hearing the gain
restore after a compressor works, with you hearing the noise or
leakage increase between notes. Generally breathing comes
from the release time being too long when hefty amounts of gain
reduction are being used.