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Dynamic Processors:
Compressors, Limiters, Expanders & Gates
All these devices are designed to alter either the dynamic range of a signal, its beginning or end (attack, release), or the ambient sounds around a signal. All sounds have a dynamic range, which is the span between the loudest and softest parts of the signal, and an envelope, which is a way of describing the beginning, middle, and end of a sound. These characteristics can be used to describe a single transient, an individual sound, an entire track, or the whole mix; and these processors can be used in any of these situations.

Dynamic range is usually expressed in dB. Envelope is expressed as Attack, Sustain, and Release. Put simply the purpose of these units are:

• Compressor -- to "shrink" the loudness range of a signal; to "squeeze" it. It lowers the ratio between the loudest and softest components of a signal. A great example is working with a lead vocalist. Typically a singer's level varies quite a bit, both from actual output of the voice, different phrasing and consonants, and variations in mic distance. Properly used a compressor will bring the different levels closer together, giving a much more controlled performance that is easier to place in the mix. This is true of both studio and live performance. In contracting it keeps levels under control and keeps both quiet sounds from disappearing and loud sounds from "scaring" the audience or driving the sound system into distortion. It can easily be used to control an entire mix, perfect for small AV or church sound systems.
  
• Limiter -- is similar to a compressor, but instead of lowering the loudest levels it prevents them from exceeding a predetermined maximum. This is used frequently as protection for PA systems, for both musical performance and installed systems.
  
• Expander -- the opposite of a compressor, it exaggerates the ratio between soft and loud. You might use one to "push" breathing sounds, or fingers sliding on an acoustic guitar further into the background.
  
• Gate -- a "hard" expander. It typically cuts off signals below a certain level, to prevent background noises from entering a mix. A common application is to gate drums, to prevent cymbals from "leaking" into a tom-tom track.

To achieve this, variable gain is accomplished typically in two different ways:

• In most units a VCA, a Voltage Controlled Amplifier is used. A VCA is ideal for most limiting, gating, and expansion. Depending on the circuit design and the type of VCA, it can sound very good with excellent specs, or suffer from distortion and time problems if the wrong VCA is used or if the circuit is poor. Typically low cost compressors have some problems.
  
• Some will use optical circuitry which has a very different sound and is usually used in more expensive processors. The "vintage" units were almost always based on this design. They use a photocell with a light source, which can be a lamp or LED (in modern designs) and the gain is varied based on the amount of light received by the photocell. These are usually true compressors in which low level gain is increased and high level gain is reduced. The advantage they have is a little more transient response with a more aggressive attack. A VCA will "roll off" an attack more than an optical design.
  

Definitions:

• Threshold -- the signal level at which the device begins working. Some units will have a fixed threshold and you vary the Input level to achieve the same effect.
  
• Ratio -- the amount of compression or expansion taking place after the threshold is reached. On a compressor, a setting of 4:1 means that for every 4 dB of signal exceeding the threshold, there will only be a 1 dB increase at the output.
  
• Attack -- the speed at which the circuit reacts to the incoming signal
• Hold -- in gates, to determine the length of time the gate remains "on"
  
• Release -- the speed at which the circuit allows the signal to resume normal levels
  
• Sidechain -- Being able to access the sidechain of a dynamics processor allows you to control the detector circuitry of a compressor or gate, but not the signal itself. An example by inserting an EQ into the sidechain of a gate, you can cause the gate to be triggered by the frequency content of a signal and not just its level, useful in gating a snare drum to get rid of a high hat.
  
• Auto, or Program Dependent Attack & Release -- instead of manually setting these, an AUTO mode allows them to be manipulated by the signal itself, quicker than you can change it.

Applications?

Almost everywhere. In the studio, with a live band, in church sound systems, background music in restaurants, DJ systems, broadcast mixes, film & video post, etc., etc. Practically everywhere audio is used, a dynamics processor can help out.

In the studio: controlling vocal levels, gating breath noises, increasing the sustain of a guitar, smoothing a bass line, getting rid of "Marshall amp buzz", lengthening cymbal crashes, getting better drum tracks by gating, and much more.

Live PA: controlling levels, improving intelligibility of vocals, protecting the speaker system, fattening the bass sounds, etc. As mentioned earlier, one of the best ways to improve the sound of a small portable PA is to put a good stereo compressor, like the Blue Max, on the stereo output of the mixer, before the amp, or to insert it between the stereo out and the amp input on a powered mixer.

Sound contracting: similar to live PA. Controlling levels and system protection.

DJ systems: Put one on the DJ mic to improve level and intelligibility and put a stereo unit on the main mix for control and protection.

Broadcast: smoothing variations in music and announcers. Giving the on air talent that "radio" sound.
. . . almost everywhere!

John Saviano