Knee-jerk Reaction

The Aphex Systems 661 tube compressor/limiter features ultratransparent, VCA-based gain control and both manual and automatic operation.

In addition to the controls and parameters already discussed, several more-subtle parameters and design features often figure prominently into a compressor’s performance or sound. One such parameter is the knee, which is related to the compressor’s threshold control. The knee determines how quickly and smoothly the compressor will transition from no action to the full ratio of gain reduction set on the unit once the signal passes threshold. Generally, a compressor’s knee is hard or soft, though some units—the Aphex 661 tube compressor/limiter ($749), for example—provide switchable hard- and soft-knee compression.

In hard-knee compression, the unit processes the audio signal at the selected ratio once the input signal passes the threshold. Although useful for applications such as peak limiting and de-essing (discussed later), a hard knee can sound abrupt, especially with higher ratios.

A soft-knee compressor, or one set to soft-knee compression, begins to compress as the signal approaches the threshold level and gradually increases the ratio until the signal attains threshold, at which point it equals the selected ratio value. The gentler, logarithmic increase of soft-knee processing tends to sound more transparent (less noticeable) than hard-knee compression, and thus is usually preferable for most vocals and instruments.

In addition to manual controls for attack and release times, some compressors offer an automatic mode, called auto mode, that does some of the tweaking for you. That is often referred to as program-dependent or adaptive processing. In auto mode, the compressor’s detector circuitry analyzes the program content (the audio-input signal) and dynamically adjusts the attack and release times accordingly. For example, if a guitarist starts picking harder, the unit automatically decreases and therefore quickens its attack time to catch the increased peaks. On the other hand, an increase in average levels typically prompts longer release times to avoid pumping while the compressor returns to unity gain.

Auto mode’s main benefit is it precludes the need to tweak attack and release settings on performances in which the dynamics change radically. It also lets you set up quickly yet still get good results when the pressure is on. The downside is you lose some control over the sound. For example, you may like those peaks when the guitarist picks harder—in which case you probably would not want to use auto mode.

Some compressors—such as the MindPrint T-Comp Stereo Tube Compressor ($1,099)—offer a semiautomatic mode of operation. As the name suggests, semiautomatic mode lets the attack and release settings exert some influence on the adaptive processing.

Opto-electrical compressors may or may not offer an auto mode; however, even without one, those units provide something akin to automatic processing in that attack and release times—manually set or not—fluctuate based on program content. That is due to the inherent nature of opto-electrical compressors, which in general are slower and less exacting than VCA-based designs. Because the attack and release controls on optical compressors provide only approximate response times, many manufacturers simply put “fast” and “slow” on either side of the knob, rather than hash marks indicating exact times. (More on optical compressors in a bit.)

BACK | NEXT

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