Well, as promised in my last tirade about the use of compressors in

mixing live audio, I will attempt and describe the most common set of

adjustments available on your average audio compressor/limiter. Though,

some compressor designers  throw-in a few more controls to make it even

more interesting (ie “knee” adjustment, side-chain filters, etc.), the following

are the most common.

Threshold -  this control determines the signal threshold (or

ceiling level)  at which the compressor  begins to  affect the audio

passing through it.  If the signal coming through the compressor is

below this “threshold or ceiling” setting, the compressor has no affect

on the audio passing through it.  It is only when a signal reaches the

level of the Threshold setting (set by the user) that the compressor

automatically begins to affect the audio.

To determine to proper Threshold level, it is necessary to watch

for the Threshold lights to begin lighting up during the course of an

average amount of source material to determine the peaks in the audio

signal  that exceed the established Threshold settings. Based on the

observed incoming signal, you can adjust the threshold control so the

compressor begins to compress at the desired peaks in the dynamics. As

the Threshold is adjusted, the compression level meter will start to

show when the source material is in compression (and how many decibels

of compression are being induced).

Attack Time — This control determines how quickly the compressor

responds to transients or peaks in the  audio levels that exceed the

threshold setting.  Set to fast attack times, the compressor quickly

responds to audio that has triggered the Threshold.  Slower attack

time settings allow the compressor to respond at a gentler,  slower rate.

The attack rate is usually stated in units of milliseconds.

Slow attack time settings tend to exaggerate the sound source

transients (giving the sound more punch, edge, presence). While fast

attack times tend to soften  or dull the incoming transients (while

reducing overall dynamic range.  There are an infinite number of

possible attack settings , and much experimentation is needed to

determine the best attack settings for the job at hand.

Release Time -This control is the inverse of Attack Time, as it

determines how quickly the compressor recovers (or releases control)

from a signal that has exceeded  the Threshold setting.  Fast release

times tend to make a compressor pump or breathe, due to the rapid return

to an uncompressed state.  Slower release times are less noticeable (and more

desirable) in most cases.

Slow release times can give a signal a sustained presence (which is

a great way to lengthen the sustain of acoustic, elec., and bass guitars.

Fast release times are sometimes desirable when trying to hard-limit

very short-duration sounds that are peaky  and pesky . (see

hard-limiting below).

Compression Ratio – This one is a little bit more involved, and a

little harder to understand (but we’ll give it a shot).  This control

determines to what degree the audio is reduced when the incoming signal

exceeds the threshold setting, and it is expressed as a Ratio (ie 1:1,

2:1, 3:1, 4:1, etc.) When a ratio of 10:1 or greater is chosen,

the effect is considered to be “Limiting” instead of compression, as

this high of a ratio is putting a hard-cap on the dynamics (essentially,

starting to square-off the waveform).  Limiting is used when the desired

effect is to not allow the dynamics to go beyond a fixed level.

Compression, on the other hand, still allows for dynamics to pass, but

in a less-restrictive way than hard-limiting. If the  compressor ratio

adjustment is set at a 2:1 ratio, this means that if the input gain

rises two decibels above the established threshold, the compressor will

reduce the incoming level by only 1 decibel. If the ratio is set to 4:1

ratio, this means that if the incoming level increases by 4 decibels the

compressor will only allow for a 1 decibel increase in output.  Once the

ratio gets as high as 10:1  the  incoming level must rise 10 decibels in

order for the compressor to increase the output by 1 decibel (again, any

ratio above 10:1 is considered hard-limiting).  For most live

applications, I find that  the more conservative ratios are most

desirable for most applications (2:1, 3:1, 4:1).  Also, I think the

less-is-more model is usually best when it comes to the amount of

compression used (with the exception of extremely-dynamic instruments,

such as drums , lead guitars, and the occasional overly-dynamic singer .

Also, more extreme compressor settings can be applied when the desired

result is more of an obvious effect , rather than just a subtle, automatic gain-

ridiing.  Remember, the higher the compression ratio, the more severe

the affect on the incoming audio (and the greater chance of noticeable

artifacts – ( ie more squashed sounding, duller, etc.)

Make-up Gain — This control is used to bring the overall level of

the compressed signal up (without affecting the dynamics). It’s just a

simple and quick way to make-up for the loss of overall-level due to the

affects of compression.

This entry was posted on Wednesday, November 18th, 2009 at 10:52 am and is filed under Gearheads. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.