MORE OR LESS
Apr 1, 2003 12:00 PM, By Doug Eisengrein
Although the maiden voyage of “Phantom Power” skimmed the surface of compression, this column will dig deeper into the specifics. As I touched upon previously, compression is achieved through the manipulation of just a few common parameters: threshold, ratio, attack, release and make-up. I will dissect each of these along with a couple of the (slightly) less common parameters — look-ahead and knee control — and examine how they can be used most effectively.
CROSSING THAT LINE
Threshold is the most vital setting in compression. It is a measurement of decibels; it draws the line between what gets compressed and what does not. Lowering the threshold results in more signal compression; raising it results in less. Seem simple? This setting is volume-dependent. For example, if you want to fully compress a signal that is low in volume, you need to use a very low threshold setting. On the other hand, if you're dealing with a very loud source and want very mild compression, your threshold setting will be much higher. There are, of course, infinite possibilities in between. I often obtain my best results with threshold settings ranging between -6 and -12 dB. In many cases, I also tend to find an audible compression window. Within a certain threshold range, there may be drastic audible differences to the sound, but beyond a certain point, the difference seems all but inaudible. That situation, by the way, is a red flag that too much compression is probably being used.
GOING AROUND THE BEND
Ratio controls the amount (percentage) of signal compression that occurs beyond the threshold. A good ratio setting depends upon the source signal and the results you are shooting for. For example, to achieve a really fat-sounding, even-level kick drum in a 4/4 track, I lean toward a higher ratio setting, such as 4:1 or 5:1. To add just a little shimmer to a tight closed hat or a ride or to impart just a touch more depth and presence to a synth pad, I find middle to lower ratios more suitable. I don't always compress final mixes, but when I do, just a slight ratio like 1.5:1 or 2:1 does the trick. As with threshold, the audible window often applies to ratio, as well.
Working side-by-side with ratio is knee control. Although not all compressors include this feature, I find it indispensable — especially those that crossfade between hard and soft settings. A hard knee provides a tighter compressed sound with lots of punch. It works great on kick drums; closed hats; or any short, repetitive percussive elements. A soft knee setting produces a softer, more fluid sound and is well-suited to compressing entire mixes; drones; flowing bass lines; and resonant acoustic sounds, such as pianos or strings.
Compression and limiting are two sides of the same coin. The goal of compression is generally to raise the lower portions of a signal by gently reducing the dynamic range and then by raising the output; the goal of limiting is just as the name implies: to limit (lower) the loudest extremes of a signal, generally with less output gain. To make your compressor act as a limiter, you set a very high compression ratio, often 10:1 or higher. Some compressors sport ratios as high as infinity; at that point, the compressor becomes a brick wall limiter, meaning that no matter how hot the signal is on input, the output will not exceed the threshold setting. With most electronic-music production, however, I find that kind of limiting unnatural-sounding and rarely needed.
ON THE HORIZON
Just as threshold and ratio work together intimately, they also work closely with a parameter called look-ahead. Again, not all compressors boast this feature, but those that do can be keenly effective at compression and (especially) limiting. Without it, a compressor sometimes cannot anticipate the sharpest spikes in a signal, particularly if they are short-lived, like a sudden snare smack. Look-ahead allows a compressor to slightly buffer its output (according to a user-definable setting), allowing for proper spike adjustment and more fine-tuned compression overall.
Attack and release act like a light switch: They control when compression kicks on and off with regard to time (usually measured in milliseconds). They soften or sharpen the audible edges of compression in a way not dissimilar to attack and release in a synthesizer's envelope filter. By raising the attack time, you allow for more of the dynamics to “slip through” before full compression kicks in. Conversely, by raising the release time, it places more of a “hold” on the signal before compression turns off, thereby smoothing over the change. I tend to use smaller attack settings (10 to 30 ms). They usually provide the punch that I am seeking without producing an unnatural, choppy sound. More often, I dial in release, usually by slowly raising it until the resulting sound is very smooth. Attack and release controls are subtle creatures; they can provide a fine polish to the sound but are often audible only to a concentrated ear.
The final link in the close-knit compression chain is make-up (output gain). A good compressor's make-up can be set manually or run autodynamically. Autodynamic means that the compressor automatically adjusts the output level to match that of the dry input after compression has taken place. With a good compressor, this results in a natural and smooth sound while retaining the compression characteristics of fattening the tone. You may choose to set the output gain manually if you need extra gain (in the case of a low-level dry signal) or if you treat the compressor more as a limiter (usually in the case of a hot signal) when you want to actually tame the peaks and not just fatten the sound.
What become most clear when using compression are the inherent relationships between the various controls. Making a change with one often necessitates making a change with another. It's like driving a five-speed car: Ease off the gas, press down the clutch, change gears, ease off the clutch, press the gas again and so on. Like driving, compression is a skill that is learned only by performance, and, of course, once you get the knack, the terrain is never quite the same.
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