Freq Show
The misconception that splitband compression is the same as frequency-conscious compression is common. A splitband compressor splits the audio signal into two or more frequency bands so each band can be processed by its own independent compressor circuitry (each with its own controls). That lets you compress, for example, a guitar’s bass frequencies differently from the highs.
A compressor that offers—or is set up to provide—frequency-conscious compression is still a full-band device acting on the entire signal. The difference between it and normal compression is simply that the detector is set to be called into action by the prevalence of specific, user-selected frequencies. Frequency-conscious compression has dozens of useful applications, but space limitations dictate I save that vast subject for a future article.

Down and Dirty
Now I’ll discuss various types of compressors on the market and which designs and models are best for different recording and mixing applications. I’ll start with analog compressors, which can be subdivided into four categories based on the type of gain-control element they use: opto-electrical, Variable-Mu, FET, or VCA. Each design has benefits and drawbacks.

To increase the usefulness of this article, I tested a sampling of compressors chosen to represent the various design and feature sets available. Bear in mind that this sampling is not meant to be comprehensive, nor is it intended to overlook or denigrate units not mentioned here.

An opto-electrical compressor (opto, for short), uses a special opto-electrical cell in its sidechain that consists of either an electro-luminescent panel—basically a night-light—or an LED that shines on a light-dependent resistor (LDR). In simple terms, the light panel, or LED, shines with increasing intensity on the LDR as the audio-input signal gets louder, and the LDR causes a corresponding increase in compression of the audio-input signal. Because the LDR has an inherent memory effect, it releases slower when the light is brighter or has been shining for a while. In practical terms, that means heavy or near-continuous compression results in longer release times.

Vintage-style optos generally have only two control knobs, typically labeled gain reduction and gain. Turning up the gain-reduction knob feeds more signal to the opto cell, effectively lowering the threshold and causing more compression. The gain knob sets post-compression output level, or make-up gain.

As mentioned earlier, opto elements have a natural lag time in their attack response; indeed, all the time constants are inherently adaptive. Optos also have, by nature, a soft knee. For those reasons, opto compressors tend to have a natural-sounding attack and release. The downside is they are usually not quick enough to catch fast transients, so substantial overshoot is not uncommon. Typical applications for opto compressors include vocals, bass, and electric guitar. However, one thing I’ve learned is not to try to pigeonhole equipment based on general design characteristics.

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