Geek Speak: Compress to Impress

August 13, 2014
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IF SOMEWHERE THERE EXISTED A LIST OF EFFECTS RANKED by fun, sexiness, and potential for general grin-inducing mayhem, compression would likely lurk somewhere near “ground lift” deep in the nether regions of that list. Narrowing a bass’s dynamic range is simply not as immediately thrilling as the squelch of a juicy envelope filter or the buzzsaw wail of a good fuzz pedal. Yet, if the list were reordered by actual utility, not just temporary excitement, then compression would be No. 1 with a bullet. It’s undoubtedly the most frequently used effect in the studio, as essential to modern recording as the microphone. And while it may not enjoy the same popularity in most bass players’ live rigs, it’s still a key component of many a front-of-house sound system.

Teletronix LA-2A
dbx 160x
UREI 1176
Given the all-encompassing presence of compression in music making, and taking account of its special role for bass, getting a good handle on the technology and how it can help you sound better is time well spent.

Compressors and their more aggressive cousins, limiters, are amplifiers designed to reduce a signal’s dynamic range. Dynamic range is the span, measured in decibels, between the loudest and quietest portions of an audio signal. Compressors make loud passages quieter and the quiet bits louder, and they allowsignificant control over the intensity and character of this process. The point of it all is that a signal with a narrower dynamic range will sound punchier, more dynamically even, and easier to place in the mix. All good things bass-wise.

The average compressor features a standardized set of controls, with more advanced models offering additional parameters and routing options for even more control. Let’s investigate the standard feature-set:

Threshold Compressors reduce the gain of an audio signal when it goes above a certain volume threshold, commonly set in decibels. If the threshold control is set low, more of the signal will be subject to the compression effect. If it’s set high, the compressor will only affect the loudest portions of a signal.

Ratio The degree to which a compressor actually compresses is most often expressed as a ratio. For example, a 6:1 ratio means that if an input signal is 6dB over the threshold, the compressor’s output will be 1dB over the threshold. In tandem with the threshold control, the ratio control can be thought of as the overall intensity of the effect. Low compression ratios are gentle, while extremely high ratios, sometimes expressed as ∞:1, prevent the signal from ever exceeding the threshold. At these highest ratio settings, a compressor becomes a limiter.

Attack A full-featured compressor includes a control that governs how quickly the compressor begins to affect the input signal. The attack is the time between the input signal triggering the compression effect and the input signal getting reduced by an amount set by the ratio control. Depending on the nature of the input signal, a super-fast attack time may cause an unnatural-sounding, hard-edged reduction in gain. Governing the attack time allows a player or engineer to dial in a smooth-sounding transition into the compressed phase of the signal.

Release The release phase occurs after a signal begins to decay beneath the threshold. It describes the time it takes for the uncompressed or “unity” gain to return to the signal. Depending on the compressor’s design, the release may interact with the attack as the signal continues to go above and below the threshold.

Makeup (or output) gain While the common perception of compression is that it imparts a greater sense of overall loudness to a signal, it actually necessarily results in a lower-gain output than the input. A compressor must provide a means of compensating for this reduction to achieve the desired output level. Makeup gain can either match or exceed the input level.

Soft and hard knees While not commonly found on bass-specific compressors, robust studio units allow a user control of the compressor “knee,” the break in the response curve of the compressor as the signal crosses the threshold. A soft knee gently increases the compression ratio until it reaches the level set by the user. A hard knee more rapidly applies the full compression ratio to a signal, for a more sudden and dramatic compression effect.

What makes compressors more than a little fascinating are the myriad ways they work their special magic. There are four main types of compressors, and each has a unique sonic signature.

A tube compressor utilizes an array of vacuum tubes to alter gain, with the compression effect coming from a tube configured in a “variable-mu” state, in which the grid-to-cathode voltage varies to alter gain. This old-fashioned method usually results in slow attack and release times and fairly gentle ratios. All those tubes (and the accompanying input and output transformers) impart a distinctly “tube-y” sound, meaning warmth, subtle distortion, and a buttery response that’s hard to duplicate with other methods. The all-time classic variable-mu compressor is the Fairchild 670.

Optical compressors take an entirely different route. They may or may not include tubes for the gain stages, but the compression effect is achieved through the use of light-sensitive resistor and a small lamp, pointed directly at the resistor. As the input signal level increases, the lamp shines brighter, producing greater resistance in the light-sensitive component. An optical compressor’s special quality is due to the unique response times of the light and resistor. The lamp takes time to illuminate and darken, and the sensitivity of the resistor also varies according to the intensity and frequency of the illumination. The resulting sound is often characterized as smooth and transparent. The most famous optical compressor is the Teletronix LA-2A, now produced by Universal Audio.

Voltage-controlled amplifier compressors (VCA) use a uniquely configured solid-state amplifier circuit to govern gain according to a control voltage. In a VCA compressor, the control voltage is the input signal itself. VCA compressors are generally considered the cleanest and most transparent of compressor types. A particularly beloved example is the dbx 160.

Another common solid-state design utilizes Field Effect Transistors (FETs), semiconducting devices that emulate the response curve and distortion characteristics of tubes. FET compressors are reliable and sound relatively bright and clean. The classic example is the UREI 1176, now also produced by Universal Audio.

On bass, compressors can do wonders for slap tone, make fingerstyle passages sound smooth and even, and in the hands of a good engineer, really help carve out a bass’s ideal presence in a mix. Live, they can be invaluable in concert with other effects, especially those that seem to occasionally explode with unruly transients. Whether your encounters with compression come in the studio or onstage, take time to understand the critical impact they can have on tone. It’s a sure sign of sonic savvy.

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