The audio compressor,
is a pretty useful item, and one which you need to add to your system at
some point if you are recording any type of audio, especially vocals.
The audio compressor automatically adjusts and maintains the
signal levels as they go to H/Disk or Tape to be recorded.
This compression evens out the distance between loud
& quiet parts, by crushing the audio if it gets too loud, and
raising the audio in the quiet sections. Compressors also can radically
beef up a synth bass sound for example, making it sound tight 'n'
punchy. So all in all it's something to get although you should probably
get a Delay & Reverb unit first.
There are different types of compressor, but these are some of the
basic controls you'll find on a unit....
- INPUT........To set the level in.
- THRESHOLD........This sets how high the signal must reach before
the compressor kicks in.
- RATIO.......This sets how much compression is applied in ratio to
the Db rise in signal level above the Threshold.
- ATTACK.......This sets how fast the compressor kicks in once the
Threshold has been breached.
- DECAY........This sets how fast the compressor lets go, once the
input signal has dropped back below the threshold.
- LINK ......Links the two sides for stereo operation.
- OUTPUT......Sets the output signal level.
Ok.... Lets take a look at the different types of
compressor & stuff relating to compression.....
HARD KNEE / SOFT KNEE
If you use a normal compressor, nothing occurs until the threshold is
breached...But when that happens, the compression cuts in......
Lets say you have set a RATIO of 4:1......Once the
threshold is passed, the compressor allows only 1db of signal level
increase at the output, for every 4 db in input signal level rise above
the threshold setting......
On a Hard Knee compressor, this full amount of
compression (as set by the Ratio) is applied in full, as soon as the
input level rises above the threshold.....This is a standard type of
Soft Knee compressors work differently......they apply compression
gradually as the signal approaches the threshold level....
As the input signal gets within about 10db of the
threshold level, the Soft Knee compressor starts to gently apply
compression, but with a very low Ratio, which increases proportionately
as the Input level gets nearer to the Threshold setting...... so that by
the time the Input level actually reaches the Threshold level, the
compressor is applying its gain reduction at the full level as set by
the Ratio Control......
Soft Knee compressors are thus more subtle, as they
don't wait, & then suddenly apply the full level of compression at
the Threshold breach point .....because they apply the compression
gradually, they are more subtle in use, and thus more suitable for
compressing whole mixes, or gentler sounds that hover around the
Some units such as the Alesis compressor, allow you to
switch between a Hard & Soft Knee function.....
PEAK & RMS
Some compressors, such as the Alesis 3060, allow you to
switch between PEAK...and RMS operation. Basically, a compressor listens
to the input signal through the "SIDE-CHAIN" circuit, and then
tells the VCA (voltage control amplifier) to apply compression when
needed according to the settings made....The compressor will respond
differently depending on whether it is monitoring the input signal in
either Peak or RMS mode...
The PEAK setting makes the compressor crush any signal rising above the
threshold, no matter how fast the transient.....This is an ideal mode to
use for something like digital recording, where you need to absolutely
stop any signals from overloading the input, because digital cannot be
"saturated" in the way tape can, and you get terrible digital
distortion......Peak compression however is not very smooth or natural
sounding, & can produce very un-natural noticeable results unless
you use a low compression ratio .....However, it can work well on fast
attacking sounds like drums, working fast to maintain a more even level
for each drum hit.
The RMS mode setting is a more natural sounding mode, and responds
similarly to the human ear...(Oh yes...ears do have compression
!!)....... RMS mode doesn't bother too much about quick short peaks that
might cut through above the average signal level....even if you set a
fast attack time.....RMS mode works on a wider average than PEAK mode,
thus allowing some fast transients through, but closing down more when
continuous loud peaks appear.
As mentioned above, the compressor monitors the MAIN
incoming signal through a side chain curcuit......The Side-Chain Socket
provides an alternative input for a different control signal than that
which is going into the main input....or provides the ability to patch
into the side-chain something like an Eq unit so as the compressor
responds only to frequencies boosted by the Eq.......This Eq does not
effect the actual input signal...only the controls of the unit.....the
controls then adjust the actual Main Input Signal.
The "Stereo Link" switch on a compressor
allows the two sides of the unit to be linked together for processing
stereo signals .....Imagine trying it without this function .....A loud
signal occurs at the left input, and suddenly the volume of the left
side signal is reduced....this will really screw up a stereo sound...
Basically the Link control forces both sides of the unit
to work together, based on the average of the two input signals, or
whichever is the loudest at any time......Also the control of both sides
is placed with ONE of the two sets of controls .....different
manufacturers use different methods to do this....some use just a
straight "One side controls both".....others also average the
two sides settings, or hand control to whichever of the two sides has
the highest settings.
Some units include an "AUTO" mode switch....This switch
adjusts the Attack & Release during use to automatically suit the
dynamics of the audio that is being processed.....This is a good mode to
use if you're doing stereo compression on a mix, where there are
constantly changing dynamics.....In these cases it will probably be more
effective to use this mode than trying to set the unit up
manually.....Auto mode is also useful for things like acoustic guitars,
or bass, which is often difficult to compress without getting pumping
||2:1 - 8:1
||4:1 - 10:1
||- 5-15 db
||5:1 - 10:1
||8:1 - 10:1
|Kik & Snare
||5:1 - 10:1
||4:1 - 12:1
||2:1 - 6:1
These are by no means fixed rules you must adhere to...but rather
good starting points ....Don't be afraid to experiment ...often, a
horrendously compressed piece of audio can give a very dramatic effect
.......Compression as with other FX, is very much a listening
choice.....Try starting with these figures, & then adjust things
until it sounds right .....Always check the sound in context of the mix
as well ....Often people only check the compression in solo ......Things
can sound quite different once the sound is in place in the mix.
COMPRESSOR FX & TIPS
COMPRESSOR FX & TIPS
GETTING MORE ATTACK FROM YOUR SOUNDS
Fairly common one this, you probably know it, but if you don't it's
pretty simple ......Getting a snappy emphasis in this way works with
most sounds, but obviously has greater effect the more hard the attack
of the sound is....
Set-up your compressor on a sound like a snare drum for example
.......Simply set a slower attack ...somewhere in the region of 1-5
ms.......This allows the initial fast attack of the snare to bust
through before the compressor kicks in to crush the sound...
This little technique also works great on bass synth
sounds with a fast attack...
Ok....This is a cool one for all you Pirate radio owners or Bedroom
DJ's.......You know when your listening to the radio, and the music's
pumping' away....Then the DJ says something over the music,and as soon
as he does, the backing drops in volume a little to make way for the
voice......Then when they stop chatting, the music just comes up again
to it's normal level.......Well this is known as ducking....and it works
As I mentioned in the Compressor section, the unit
listens to the incoming signal through a "Side chain" curcuit......The
Side chain gives instructions to the actual compressor by the settings
you make in the front panel controls......as soon as the Side chain
hears the signal go over the Threshold level, it tells the Amplifier (VCA)
at the input to turn down the level....
OK......lets suppose that the music backing is coming
through the compressor, but.....the DJ's voice is being fed into the
Sidechain........Ah...yes indeed, as soon as the compressor
"hears" the DJ's/presenters voice, it turns down the incoming
signal .......which is the music backing.......Here's how:
YOUR COMPRESSOR MUST HAVE SIDE-CHAIN SOCKETS ON THE BACK
The side chain socket allows an alternative input signal to override the
main input signal for controlling the compressor...NOT....as the signal
to be processed.
OK ......first with a mono music signal.....
Bring the backing music into the Main Input...
Bring a send from the DJ's voice into the Side-chain Socket.....
Set the compressor to fast attack, short/auto release, & ratio about
3 or 4 to 1...in soft knee mode (if you have option)...
Set the music running, and get the DJ to chat ...repeating phrases of a
few seconds, & leaving spaces of a few seconds..
Adjust the compression ratio ...until you get a low music level you like
that lets the DJ be easily heard when he speaks...
Adjust the attack & release, so as the music reduces fast enough,
& returns to level at a speed you like (usually quite fast)...
Run some tests with the DJ chatting fast & furious ...leaving quick
gaps here & there .....tweak the settings until it sounds just
Run your set !!.......
For a stereo backing signal, just do as above, but put the music thru
the Left & Right channels of the compressor, and set it to
"Stereo Link" mode....Then adjust the settings on the control
side .......Check out which side takes control in Stereo mode, &
patch the DJ's voice in to that sides Side-chain input.
Argggh..!!... sibilance....all that "SSSS-ing" overload
coming off the mic' when the singer pronounces "S"
sounds..........Well, you just need to do some De-essing....again by
utilizing the Side-chain....
Set up exactly as above in the "Ducking" section.......but
send the Vocal you are processing, through into the Main Input on the
Check the Side chain socket which will be a stereo 1/4 inch jack
socket......find out which is the send & return (tip or
ring).....make up a stereo 1/4 inch to 2 X mono 1/4 inch lead....
Patch a Parametric equalizer or Graphic eq into the side-chain.....just
take a line FROM the Side-chain socket to the EQ INPUT....then return
the signal FROM the EQ, back to the side-chain socket via the tip or
Now the Equalizer is patched into the Side-chain ....remember, the side
chain controls the Amp on the compressor........
Now setup the EQ to TUNE INTO the problem high frequency ....USUALLY IN
THE 5 - 10K REGION.....& give about 10 db of Boost...
OK....now the compressor, is tuned via the Side-chain to compress MORE
heavily signals of the range that the EQ is tuned to...
Set the ratio etc to crush the Sibilant phrases without sounding
unatural...and off you go......
Ah.....set up this one, & you'll be in heaven as you get crisp
vocals, with no SSS-ing...If you don't have an outboard eq...you can use
even a little graphic guitar footpedal...or even a spare mixer channel
to patch into the side-chain....remember...the Eq is only to CONTROL the
frequencies that the compressor will crush.....The actual vocal does not
pass through the Eq.....so don't worry about noise problems from the Eq
You often hear compressors referred to as
"Limiters".......Well limiting, is basically where the unit
has a fixed as opposed to ratio based compression, above which no signal
can pass ....period !
In order to get this effect ....simply set the
compression ratio to Infinity..(all the way clockwise)...This now tells
the compressor........"Look ...above the db level set by the
threshold, let nothing pass.".......and there you have it