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Signal Processing Fundamentals: Dynamic Controllers

Compressors, expanders, noise gates and limiters - altering frequency content and amplitude level

By Dennis A. Bohn March 21, 2019

Image courtesy of Mary Theresa McLean

Dynamic controllers or processors represent a class of signal processing devices used to alter an audio signal based solely upon its frequency content and amplitude level, thus the term “dynamic” since the processing is completely program dependent.

The two most common dynamic effects are compressors and expanders, with limiters and noise gates (or just “gates”) being special cases of these.

The dynamic range of an audio passage is the ratio of the loudest (undistorted) signal to the quietest (just audible) signal, expressed in dB.

This article is provided by Rane Corporation.

Usually the maximum output signal is restricted by the size of the power supplies (you cannot swing more voltage than is available), while the minimum output signal is fixed by the noise floor (you cannot put out an audible signal less than the noise).

Professional-grade analog signal processing equipment can output maximum levels of +26 dBu, with the best noise floors being down around -94 dBu.

This gives a maximum dynamic range of 120 dB (equivalent to 20-bit digital audio)—pretty impressive number—but very difficult to work with.

Thus were born dynamic processors.

Compressors

Compressors are signal processing units used to reduce (compress) the dynamic range of the signal passing through them. The modern use for compressors is to turn down just the loudest signals dynamically.

For instance, an input dynamic range of 110 dB might pass through a compressor and exit with a new dynamic range of 70 dB. This clever bit of processing is normally done using a VCA (voltage controlled amplifier) whose gain is determined by a control voltage derived from the input signal.

Therefore, whenever the input signal exceeds the threshold point, the control voltage becomes proportional to the signal’s dynamic content. This lets the music peaks turn down the gain.

Before compressors, a human did this at the mixing board and we called it gain-riding. This person literally turned down the gain anytime it got too loud for the system to handle.

You need to reduce the dynamic range because extreme ranges of dynamic material are very difficult for sound systems to handle. If you turn it up as loud as you want for the average signals, then along comes these huge musical peaks, which are vital to the punch and drama of the music, yet are way too large for the power amps and loudspeakers to handle.

Either the power amps clip, or the loudspeakers bottom out (reach their travel limits), or both—and the system sounds terrible.

Or going the other way, if you set the system gain to prevent these overload occurrences, then when things get nice and quiet, and the vocals drop real low, nobody can hear a thing. It’s always something. So you buy a compressor.

Using it is quite simple: Set a threshold point, above which everything will be turned down a certain amount, and then select a ratio defining just how much a “certain amount” is. All audio below the threshold point is unaffected and all audio above this point is compressed by the ratio amount. The earlier example of reducing 110 dB to 70 dB requires a ratio setting of 1.6:1 (110/70 = 1.6).

The key to understanding compressors is to always think in terms of increasing level changes in dB above the threshold point. A compressor makes these increases smaller.

From our example, for every 1.6 dB increase above the threshold point the output only increases 1 dB. In this regard compressors make loud sounds quieter. If the sound gets louder by 1.6 dB and the output only increases by 1 dB, then the loud sound has been made quieter.

Some compressors include attack and release controls. The attack time is the amount of time that passes between the moment the input signal exceeds the threshold and the moment that the gain is actually reduced.

The release time is just the opposite—the amount of time that passes between the moment the input signal drops below the threshold and the moment that the gain is restored.

These controls are very difficult to set, and yet once set, rarely need changing. Because of this difficulty, and the terrible sounding consequences of wrong settings, Rane correctly presets these controls to cover a wide variety of music and speech—one less thing for you to worry about.


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