In simplest terms, think of a compressor as an automatic volume controller. Indeed, before compressors were invented, engineers typically had to “ride gain” on a channel to maintain consistent volume levels. (Then again, many engineers still ride gain, even when using compressors.) However, a compressor controls levels with a speed and accuracy that is impossible to achieve manually—sort of like a magic genie adjusting the track’s fader with lightning-fast reflexes. The compressor’s control settings determine when and how much that fader moves.

Depending on how its controls are set, a compressor reduces either transient peaks—the short-lived, attack portions of a sound—or the average-level portions of the sound, and sometimes both. Examples of transient peaks include the stick strike on a drum head and guitar-string plucks. A sound’s average-level portions include a snare drum shell’s ringing and the sustain of a guitar note after it is plucked. Certain instruments—a wood block, for instance—produce mostly transients and very little sustain. Others, such as vocals and organs, typically produce mild transients that barely peak above their average levels.

The number of controls on compressors varies greatly, depending on design, cost, and other factors. Units that employ voltage-control amplifiers (VCAs), for example, typically have at least five controls: threshold, ratio, attack time, release time, and output level. Full-featured VCA models may offer more than twice that many controls, whereas some expensive opto-electrical compressors may provide only two control knobs.

Note that units with fewer controls are not necessarily less capable; rather, they typically provide automatic control of parameters such as attack and release time, or they “gang” two parameters (threshold and ratio, for example) on to one knob. I’ll discuss those types of compressors in more detail later. First, I’ll analyze the five controls common to most VCA-based compressors.

High Five

FIG. 1: A compressor set to a 2:1 ratio with a threshold of 0 dB produces an equal increase in output level respective to input level below the threshold, assuming that make-up gain is kept at unity. Above the threshold, output level rises only 1 dB for every 2 dB increase in input level. The compression curve shown is hard knee.

Threshold is the level at which compression kicks in and starts to reduce the signal’s level, or gain; the threshold control lets you set that level. With threshold at 0 dB, for example, all signals at or above 0 dB get compressed, while those that fall below 0 dB are unaffected. Therefore, to control peaks, set the threshold to a level below the level of the peaks but above the average level of the signal. That way, peaks that exceed the threshold get attenuated while the average levels pass unaffected through the unit. Clearly, a proper threshold setting is critical to a compressor’s performance: if the threshold is set too high, the unit will not process any of the signal; if the threshold is set too low, the unit will react to—that is, attenuate—every portion of the signal.

Ratio expresses the difference between signal increases (volume) at the compressor’s input and increases at its output; the number on the left refers to input and the right to output. Therefore, the ratio control determines how much the signal will be attenuated once it exceeds the threshold. For example, a 2:1 ratio will let a signal increase in level only 1 dB for every 2 dB it exceeds the threshold (see Fig. 1). Likewise, if the signal exceeds the threshold by 6 dB at a 2:1 ratio, the compressor attenuates the signal by 3 dB, a net gain increase of only 3 dB. In that case, the compressor’s gain-reduction meter (if it has one) will show 3 dB of gain reduction.

BACK | NEXT

Reprinted with permission from Magazine, February, 2001
© 2001, Intertec Publishing, A Primedia Company All Rights Reserved