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HOME
RECORDING |
June
15, 2001 ISSUE |
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Gating
& Expansion |
BY
KEN LANYON |
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A
while back, I wrote an article on the use of compression and how
it can improve your tracking and mixes. As you will recall,
compression reduces various volume peaks by a certain ratio when
the peaks cross a certain threshold volume. In essence, it
makes the loud sounds softer and the soft sounds louder.
This is helpful to keep the volume of a certain track consistent,
as well as to bring out and spread the tonality of the instrument
or voice. |
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Another
tool for controlling dynamics is a noise gate. A noise gate
comes in very handy when you have a loud noise floor, some
constant hissing or buzzing, or bleed from another instrument on
the same track. Sometimes the compression you added to even
out the volume of soft and loud sounds has an unwanted side effect
of bringing up the noise or instrument bleeding in a track.
A noise gate comes in very handy to drive the noise and bleed back
down to normal levels. |
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What
a gate does is let the loudest sounds through while suppressing
the others. Imagine a door opening and closing, allowing
only certain signals to get past and shutting out others.
The loudest sounds will cause the gate to fully open, and then
shut again by an amount that you can set. As with
compressors, noise gates also have threshold,
attack, and release controls, as well as a hold time. |
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threshold is the point at which the gate opens. Like the
others, you need to lower this to the point where it opens for the
sound you want to hear but is also NOT triggered to open by other
sounds. For example, imagine a live drum kit has been
recorded and you want to clean up the tracks. You have soloed the
snare track and without gating, can hear the kick drum in
it. You will want to gate this so that it opens only when
the crack of the snare occurs and not for the kick at all.
Depending on how hard the player hits the kick, this shouldn’t
be a problem because the volume of the kick will be lower due to
its inherent distance from the snare mic. There is a chance
however, that the high hat may bleed into your snare signal at a
higher volume than the kick, and may cause problems when you are
trying to set your threshold to open only for the snare hits. There is a certain amount of tolerance you need to have when
addressing gating issues like this. |
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also helps to manipulate the attack, hold, and release
knobs. Attack is how quickly the gate opens and lets the
sound through once it has crossed the threshold. The hold is
how long the gate is open after the signal goes below the
threshold. Finally, the release is how fast the gate closes
after the hold time is over. It is here that you have to
think about the attack and decays of the type of instrument you
are gating so you know where to start setting the knobs.
Once you understand that, for example, a gate normally needs a
quick attack for a snare, you can easily listen and hear how a
fast attack verses a slow one affects the sound. Of course,
music is all about what sounds good to you, so really try to play
around. |
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The
last parameter you have control of is the range. Range is
how much the gate closes after the sound has gone under the
threshold. Think of this as how low the volume goes.
The gate can close all the way, letting no sound through, or just
be set to a lower volume. It is typical to set the gate to
close all the way, but again, creating music is all about YOUR
tastes! |
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You will find that
in many types of music, gating is often applied on sources from
analog tape because some signals may be recorded so hot that bleed
into the next track occurs. This would be things like a kick
drum or snare. It is even possible for a very hot SMPTE
signal to bleed into the adjacent track. Therefore, it is
wise to set the gate to only open when necessary so the offending
noise is not heard through the track. Going back to the
example of the kick drum bleeding into the snare, it is nice to
have the kick gated out so that you can now EQ the snare to your
tastes without affecting the overall kick sound. If you didn’t
gate the snare track, then any EQ you used on the snare would
affect the kick on the snare track, and would mix with the
original kick track to make a sound that may not be true to what
you are wanting to hear. The bottom line is that gating is
the best way of isolating your tracks from extraneous noise once
they are recorded.
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Another
cool thing that you can do with a gate is to set it off on one
track triggered by another tracks signal. Imagine that
you have a guitar track that has some long sustaining chords on
it. You can pass that tracks signal through the gate like normal,
but instead of the guitars triggering the gate, you can plug a
copy of the kick drums signal into the "Key Input" on
the back. The kick drums rhythm is now opening the gate,
creating a cool new rhythm on the guitar track that is in time
with the kick. This technique is usually called keying the gate. |
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Another related
device is called an expander. Where the gate will shut off
the channel ("close the door") when a signal drops
below the threshold, the expander will turn down the signal by a
ratio. With a typical expansion ratio of 1:2, the unit will
turn down a signal 2 dB if the signal is 1 dB below the
threshold. If the signal drops 2 dB below the threshold, the
expander will turn it down 4 dB, etc. A gate is really an
expander with a very high ratio. The illustration below
shows the difference between the two units. |
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Expansion Compared To
Gating |
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Other controls on
the expander, like attack time and release time, work just like
the controls on the gate. Expanders may also have other
controls, like range, normally found on gates. Better
expanders will have ratio controls where you can use anything
from a mild expansion ratio of 1:2 to a gating ratio of
1:100. |
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Probably the biggest
use for expanders is as a "soft gate" for control of
leakage or to drive down noises that are too loud after using
compression. You may find instances where it is hard to set
the threshold on the gate so that all of the unwanted sounds are
below the threshold and all of the wanted signal is above the
threshold. In this situation the gate will start to chop off
some of the usable signal and in this case an expander would work
better. Another example where an expander will work
better then a gate is on a track that "fades in" but has
unwanted noise or bleed. |
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Hopefully you can
now see why noise gates and expanders can be as helpful as
compressors in any home studio. |
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Copyright ©
2001, by Ken Lanyon, ALL RIGHTS RESERVED |
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Illustration
Copyright © 1998 by Robert Dennis, ALL RIGHTS RESERVED |
Published in Recording
Engineer's Quarterly and Alexander magazines with permission |
USE OF THIS ARTICLE SUBJECT TO
USER AGREEMENT |
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