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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 April 11, 2011

This article is provided by Rane Corporation.

(Click here to read part 1, focusing on crossovers, and click here to read part 2, addressing equalizers.

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.

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 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.

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memefilter says

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

Technical error.  Dynamic *range* is not the ratio of loudest to quietest signals, but rather the *difference* between them.

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