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Using Compression and Compressors for Audio Recording
Written by Michael Cooper - © 2005, PRIMEDIA Business Magazines & Media Inc. Reprinted with Permission.
Of all the processes used in modern music production, compression is perhaps the least understood. One reason is compression’s sonic results are often subtle and thus hard to hear—especially for budding engineers. Another hurdle is presented by the various and differing compressor control parameters; those, too, are typically subtle in their individual sonic effects, and they work together interactively, further complicating the stew. Then there’s the confusion that lies in the bewildering array of product types and models the engineer must choose from before even reaching for a control knob. For example, for a given application, should you select a VCA-based compressor or one controlled by an opto-electrical element? A solid-state or tube design (or a hybrid of the two)? Analog or digital compression? A hardware compressor or one that is software based? And so on.

With so many variables, it’s no wonder compressors and compression remain a mystery for many users. Yet, if you want to master the arts of recording and mixing, learning compression’s intricacies is imperative. After all, the production processes for most of today’s popular music forms—with the notable exceptions of classical and some jazz—rely heavily on compression. Simply put, if you’re not compressing properly, you’re not getting the best sounds possible.

This article will guide you through the maze of compressor options and explain practical compression applications in plain English. I’ll start with the basics of compression, citing examples of various production techniques and the theories behind them. I’ll also tell you which features to look for in a compressor and why they’re important. Finally, I will survey specific types and designs of compressors, describe some models, and offer opinions about which models do the best jobs on which instruments.

Improvement Plan
Compression falls under the broader category of dynamics processing. The term “dynamics” refers to changes in loudness level, so dynamic range is the difference between the softest and loudest sounds that a source produces, or that a track contains. A dynamics processor’s purpose is simply to increase or decrease a signal’s dynamic range, which alters how the levels fluctuate within that range. Types of dynamics processors include gates, expanders, limiters, levelers, and compressors.

A compressor is a type of dynamics processor that “squeezes” a signal’s dynamic range—that is, it reduces the difference in volume, or level, between the loudest and softest parts of a performance. The process of reducing volume is called gain reduction. Properly applied, gain reduction makes a performance sound more consistent from beginning to end. For that reason, compression is a great remedy for a performance in which the levels fluctuate too widely.

By reducing dynamic range, a compressor also allows for the processed signal’s overall level to be raised—that is, become “hotter”—resulting in increased loudness without pushing the signal’s loudest parts into distortion. Bringing up the overall level has the additional benefit of making lower-level sounds louder than they were before compression. The result is that subtle nuances such as mouth sounds and ghosted notes—as well as burps, string buzzes, and snare rattles—are louder, clearer, and easier to hear.

Of course, you may not want to make burps, string buzzes, and other incidental performance sounds more audible. Therefore, apply compression only when musically appropriate—when the end result will sound better than what you started with.

You can always add compression after a track is recorded (during mixdown), but sometimes it is desirable to use compression during the recording process. That approach has several potential benefits. For one, a compressor makes it easier to capture usable tracks when recording an instrument with a wide dynamic range. Moreover, solving level-fluctuation problems during tracking frees you from having to solve them at mixdown. That, in turn, leaves more time and brain power—not to mention gear—for focusing on the mix’s creative aspects.

For those recording to any digital medium, using a compressor during tracking ensures that sounds are encoded at a higher level. Because more bits are used, better bit resolution results. Furthermore, by putting a lid on peaks, the compressor also helps avoid digital clipping on extraloud notes. For those recording to analog tape, compressing during tracking allows the signal level to be raised higher above the noise floor, which results in an improved signal-to-noise ratio.

Tricks of the Trade
In addition to problem solving—smoothing out rough performances, improving digital resolution and signal-to-noise ratio, avoiding digital clipping, and the like—you can also employ compressors in numerous creative applications. For example, a compressor can dramatically change the envelope of a sound in much the same way an envelope generator works in a synthesizer. That and other compression tricks can give a vicious attack to a lackluster snare drum, add crunchy edge and sustain to a mild-mannered electric guitar, make a lead vocal sound so urgent that listeners will dial 911, or pump up an entire mix until the band sounds like it’s exploding out of the speakers.

In simplest terms, think of a compressor as an automatic volume controller. Indeed, before compressors were invented, engineers typically had to “ride gain” on a channel to maintain consistent volume levels. (Then again, many engineers still ride gain, even when using compressors.) However, a compressor controls levels with a speed and accuracy that is impossible to achieve manually—sort of like a magic genie adjusting the track’s fader with lightning-fast reflexes. The compressor’s control settings determine when and how much that fader moves.

Depending on how its controls are set, a compressor reduces either transient peaks—the short-lived, attack portions of a sound—or the average-level portions of the sound, and sometimes both. Examples of transient peaks include the stick strike on a drum head and guitar-string plucks. A sound’s average-level portions include a snare drum shell’s ringing and the sustain of a guitar note after it is plucked. Certain instruments—a wood block, for instance—produce mostly transients and very little sustain. Others, such as vocals and organs, typically produce mild transients that barely peak above their average levels.

The number of controls on compressors varies greatly, depending on design, cost, and other factors. Units that employ voltage-control amplifiers (VCAs), for example, typically have at least five controls: threshold, ratio, attack time, release time, and output level. Full-featured VCA models may offer more than twice that many controls, whereas some expensive opto-electrical compressors may provide only two control knobs.

Note that units with fewer controls are not necessarily less capable; rather, they typically provide automatic control of parameters such as attack and release time, or they “gang” two parameters (threshold and ratio, for example) on to one knob. I’ll discuss those types of compressors in more detail later. First, I’ll analyze the five controls common to most VCA-based compressors.

High Five
Threshold is the level at which compression kicks in and starts to reduce the signal’s level, or gain; the threshold control lets you set that level. With threshold at 0 dB, for example, all signals at or above 0 dB get compressed, while those that fall below 0 dB are unaffected. Therefore, to control peaks, set the threshold to a level below the level of the peaks but above the average level of the signal. That way, peaks that exceed the threshold get attenuated while the average levels pass unaffected through the unit. Clearly, a proper threshold setting is critical to a compressor’s performance: if the threshold is set too high, the unit will not process any of the signal; if the threshold is set too low, the unit will react to—that is, attenuate—every portion of the signal.

Ratio expresses the difference between signal increases (volume) at the compressor’s input and increases at its output; the number on the left refers to input and the right to output. Therefore, the ratio control determines how much the signal will be attenuated once it exceeds the threshold. For example, a 2:1 ratio will let a signal increase in level only 1 dB for every 2 dB it exceeds the threshold (see Fig. 1). Likewise, if the signal exceeds the threshold by 6 dB at a 2:1 ratio, the compressor attenuates the signal by 3 dB, a net gain increase of only 3 dB. In that case, the compressor’s gain-reduction meter (if it has one) will show 3 dB of gain reduction.

Typically, different instruments and performances call for different compression ratios. For example, to compress a ballad’s near-perfect vocal track, a mild 2:1 ratio would probably suffice; at that ratio, and with the appropriate threshold dialed in, the compressor tightens up the performance enough to ensure quiet phrases are not lost in the mix and higher levels are not overbearing. At the other extreme, a bass guitar track that alternates between mellow finger-pad technique and aggressive pop ’n’ slap can easily have a huge dynamic range. To yield consistent levels from that type of performance, a higher ratio such as 10:1 may be in order.

Note that threshold and ratio work together to affect a signal’s output level. The lower the ratio, the less control the compressor has on the signal; the lower the threshold, the lower the signal level subject to compression. The relationship between the two controls affords flexibility and sonic variation. There are, for example, two different-sounding ways to get the same amount of gain reduction out of a compressor—low threshold and low ratio or high threshold and high ratio.

Attack time is how long it takes—measured in milliseconds (ms) or microseconds (µ)—for the compressor to kick in once the signal exceeds the threshold. A slow attack time lets inherently fast transient signals pass threshold before compressing the rest of the signal; a fast attack catches transients, but may diminish high-frequency content.

One thing worth noting is that manufacturers sometimes measure attack times differently. Some specify attack time as the time it takes for the compressor to react after the threshold is exceeded, and others specify attack time as how long it takes for the compressor to reach, say, 67 or 90 percent of the maximum gain-reduction level it will ultimately achieve. Fortunately, the exact definition is of little importance, as typically attack time is set by ear. Depending on what kind of effect you’re going for, simply decrease the attack time until unruly peaks are tamed or increase it until average levels are lowered and desirable peaks get through unscathed. If you’re having trouble hearing your settings’ effect, watching a downstream peak-level meter (that is, one that monitors the levels after the process—the compressor’s output-level meter, for example) will let you visually confirm what portion of the sound is attenuated.

Release time is how long—measured in seconds or hundredths of a second—it takes for the compressor to return the signal to unity gain (its unprocessed state) after the signal falls back below threshold. That is, once the release time passes, the compressor lets the signal pass through unaffected. In general, slower release times result in a more natural sound.

In general, set fast attack and release times when you want the compressor to do its job and get out of the way quickly—for instance, when you want to put a lid on transient guitar plucks but allow the ringing notes to pass through unaffected. Conversely, a moderate attack time coupled with a long release is perfect for those David Gilmour–esque guitar solos in which you want notes to sustain forever. At two seconds or longer, the extended release time causes the compressor to slowly restore compressed levels to their original (higher) gain, just as the sustained notes start to naturally die off, which counteracts the decay and makes the tails of the notes louder.

A compressor’s last control stage is its output level. That control is also known as make-up gain because it is used to make up for the gain reduction caused by the compressor. The usual approach is to increase the processed signal’s output level so it matches the unprocessed signal’s level. That creates unity gain between the two signals, which makes it easier to compare them using the bypass switch and ensures appropriate levels when recording or mixing. This is an excerpt from the following article: The Big Squeeze.

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