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Taming Your Audio Signal with Compression

By Kendall Scott

Compression is an indispensable tool for controlling audio signal levels, whether you are a studio engineer, videographer, field-recording tech, home enthusiast, or performing musician. Since the advent of audio-tape recording, there has been a never-ending quest to maintain constant signal levels to achieve optimal audio performance. Let's take a closer look at compression and how you can make it work for you.

Avalon VT-747SP

What is it?

A compressor/limiter is designed to attenuate an audio signal as it approaches a predetermined threshold level. Some vintage units like the Teletronix LA-2A or the newer DBX 160AD feature few knobs, but are quick, easy to use, and have a unique sonic signature born of their excellent designs. Others, such as the Avalon VT-747SP or Focusrite Compounder, offer many control options for extreme versatility.

There are two main functions that a compressor/limiter can fulfill.

1) Managing signal levels

A compressor monitors the level of your audio signals much the same way an audio engineer maintains consistent recording levels while capturing a live performance. The piano, for example, is an instrument capable of delivering an extremely wide dynamic range, from very quiet passages to thundering crescendos. As the pianist plays louder, it is the engineer's job to decrease the signal level to prevent the recording equipment from overloading. Conversely, when the pianist returns to a more modest volume, the engineer must increase the signal to its earlier level to maintain a consistent recording level.

But adjusting your input level manually for the duration of such a recording would take a fair amount of skill, and an almost inhumanly quick hand, to catch every unexpected loud volume peak as it occurs.

A compressor can make your life a great deal easier by making these gain changes for you—by decreasing your audio signal level during loud passages, quickly and accurately.

Applying compression to an audio signal can help keep these stronger dynamics under control with minimal sonic alteration of the original signal. Unlike adding reverb or delay to sweeten your recording, a compressor, properly set, is almost inaudible while doing its job. In fact, if all you are trying to do is control your amplitude levels, then the more transparent the process, the better for your recording.

2) As an effect

By using stronger settings, it is also possible to use compression as an audible effect. This can add punch and personality to your recordings. As mentioned previously, a large number of audio sound sources have a dynamic range that can exceed the limits of many recording devices.

Compressors take signals with a very wide dynamic range and compress them into a narrower dynamic range to help you attain a better match between your sound source and audio recording chain. Heavy compression squeezes the same amount of input signal through a tighter aperture, which can have several results:

A) More perceived source signal level at lower volumes

B) Increase in sustain of some instruments, especially guitar, piano, and bass

C) Strong, smooth, dynamically consistent tone that exhibits a certain sonic "weight," like a radio announcer's voice

D) More reactive settings can produce maximum punch on instruments like drums, bass, and guitar

Strong compressor settings, which can be heard every day in the broadcast world, are clearly audible and have a unique flavor. The thick, rich voice of a radio announcer is not only dependent upon the quality of the source voice, but also on equalization and heavy doses of compression. If you have ever watched programming on television and have been jolted by the higher volume of the interstitial commercial, then you have experienced the effects of compression. By cramming as much signal as possible into a narrower dynamic space, announcers, commercials, and musical material are guaranteed to have a strong sonic footprint when vying against competing neighboring stations.

Making the Connection

Even though the compressor can be used as an effect, it is important to remember that 100% of your audio signal must pass through it in order for it to do its job effectively. A delay or reverb unit is usually connected to the effects send of a mixer, which then allows you to adjust its desired percentage. A compressor should be placed directly in line with the signal so that you are controlling its percentage from the unit itself.

This can be done by connecting directly to the unit or, better still, by making use of the insert connections on your mixer. An insert jack gives you an access point to the signal after it has been properly preamplified at the mixer's input, but before it is sent through the EQ, effects, and other sections of the mixer for other modification. Insert cables are Y-cables that feature a single stereo (tip-ring-sleeve) plug on one end and two mono (tip-sleeve) plugs at the ends of the Y. The single stereo plug connects to the insert jack of your mixer, while the Y ends connect to the input and output of your compressor. The jack and cable are wired in such a way as to send the signal from the mixer to the compressor on one side of the cable, and from the compressor back to the mixer, with one simple connection. The sound can then continue down the channel strip through the EQ, effects, fader, and other controls.

As with anything, there are exceptions to this rule that dictate other specialized uses. You may wish to manually split the source signal into several different frequency bands and compress only the lows, for example, to introduce a form of multi-band compression. (More on this later.)

Controls and Concepts

The key to using a compressor/limiter properly is in learning what each control does on its own, and also understanding how they react with one another. Higher ratio settings may compel you to add more output, for example. Let's have a look.

Threshold Ratio

Threshold: This control lets you choose the decibel level at which the unit will begin attenuating your signal. Lower settings will cause the unit to work on even the quietest signals, while higher settings let louder signals pass unmodified until the selected threshold is reached.

Ratio: The ratio control dictates how much compression you are applying to the signal and it is marked in decibels with settings such as 1:1, 2:1, 4:1, etc. A setting of 1:1 represents no applied compression. A setting of 2:1, for example, means that for every 2dB the signal goes above the chosen threshold, only 1dB will be allowed to pass.

You may also notice a small infinity symbol at the far end of this range, denoting a maximum compression setting or limiting.

There are times when you may simply want to create a ceiling for your audio levels without affecting them otherwise. Limiting may be thought of as compression with an infinitely-high ratio, meaning that any signal that approaches the chosen threshold can never go past it. This is great for preventing overload peaks on your main mix, or for attenuating a handful of louder notes that may otherwise overload your recording device.

Attack

Attack: The attack time is the speed at which compression will kick in after the signal crosses the chosen threshold. Lower settings or faster times will create a snappier response, while higher or longer settings will give a more relaxed feel. Instruments with a very quick attack, like drums and bass, usually have a lot of energy right at the start of their sound. A fast attack will catch these transients and help keep your signal within safe levels.

Release: The release time controls the time that the compressor will continue to work after the signal drops below the chosen threshold. Shorter settings will release the signal immediately, while longer settings hang on to it, creating an effect of increased sustain, especially for guitars, as mentioned earlier.

Pumping/Breathing: Different instruments have varied amplitude envelopes. Some, like drums, are short and fast while others have a more gentle attack with more sustain, such as a violin or human voice. By setting the attack and release times to reflect the nature of the program material, you will be able to achieve smooth, transparent results. If inaccurate settings are used on shorter, sharp sounds like drums, higher compression settings will produce an audible pumping or breathing effect as the compressor tries to catch each individual drum hit or bass note unsuccessfully. This is usually considered undesirable, but can be used constructively and fine-tuned as an effect to alter the envelope of drums and other instruments.

Output or "Makeup" Gain: This control allows you to bring your total signal output back up to a more useable level if it has been decreased by higher compression settings. As you lower the threshold and increase the compression ratio, your total signal output will, no doubt, decrease. The output knob will let you tailor your final output to agree more with your requirements and sensibilities.

Limiter: A separate limiter section will usually override any other settings and provide a straightforward preset peak reduction structure.

Expander/Gate: Many compression units include an Expander/Gate circuit to increase their versatility. An expander has the opposite function of a compressor— it expands the dynamic range of your signal, making softer sections seem quieter and louder moments even more so. Most modern units feature "downward expansion" to offer you a form gain reduction that brings low-level signals even lower. An extreme setting of expansion called "gating" allows the incoming signal to pass only if it reaches a certain level, as set by the threshold control. The release time will dictate the time envelope of the "gate." This setting can be used to reduce noise by lowering the signal to 0dB when the noise floor is reached, or it can be used as an effect that abruptly opens to let the signal pass and then closes to dead silence after it passes the threshold (think in terms of Phil Collins' drums on the song "In the Air Tonight").

OverEasy/Hard and Soft Knee: The attack dictates how the compressor will behave when the signal reaches the threshold. A hard knee setting (normal) represents a sudden application of compression right at the onset of the threshold, and is ideal for strong compression effects. A soft knee or OverEasy setting will produce a smoother transition to the onset of compression, which can be great for an easier feel and more latitude.

Give it a try

At first it may be a bit difficult to get the results that you want from your compressor. All of the controls are so interactive that it will take some trial and error to get the hang of it. Below are some starting points for a few popular sources, but you must remember that these are not hard and fast settings, since every input source will be different.

Vocal Setting
Local Vocal Setting
Acoustic Guitar Setting
Electric Guitar Setting
Electric Bass Setting
Kick and Snare Settings
Mix Setting
General Setting

What else can it do?

On the rear panel of some compressors you may find an input labeled "side-chain" which is a direct connection to the unit's detection circuitry. What useful things can you do with this input?

Ducking: The technique of reducing the level of one signal with the increased output level of another is called "ducking," and it is used widely in broadcast and voice-over applications. Let's suppose that a radio station is running its music through a compressor, and that the DJ's microphone is connected to the side-chain of the compressor. When the DJ speaks, the level of the music will automatically be reduced so that he can make his announcements. When he stops speaking, the compressor will return the music to its normal level.

Ducking

De-essing: By pairing a compressor with an equalizer you can reduce the sibilance, or overly-accentuated vocal s, sh, ch, z, and j sounds in your source material. Sibilance occurs in a frequency range of anywhere from 4 to 8 kHz, and can be difficult to attack with EQ alone. By feeding a 4-8 kHz boosted copy of the vocals into the side-chain input of the compressor, you will cause it to respond when these offending frequencies are detected.

De-essing

Multi-Band Compression: There are times when it would be a wiser choice to compress only a certain range of the frequency spectrum, and the tool for this job is the multi-band compressor. If you were to run an entire song mix through a compressor you would notice that because low frequency energy is inherently much stronger than high frequency energy, it would react to the thump of the bass drum, bass guitar, and any other bass-heavy source. If you wanted to compress the low end separately to give it a tighter feel but leave the highs free to breathe, then a multi-band unit will let you adjust your compression levels for several frequency ranges simultaneously.

Multi-Band Compression

Plug-ins

Software compressor/limiters are a convenient and cost-effective way to maintain your computer tracks. There is an incredible range available that covers everything from vintage recreations to powerful mastering suites. They will open directly in your audio application without the need for external patching. In most cases, by purchasing one you will be able to open multiple instances to apply to several tracks at once.

Why has compression become even more important today?

Analog tape systems have provided us with a huge number of not only classic but historical recordings, in all styles and forms. One reason for the incredibly warm and rich sound that is associated with magnetic tape is the elasticity of the recording chain. Almost every bit of vintage analog gear, from the microphone and preamps to the console and tape machine, absorbed strong signals by saturating (distorting) in a very pleasant way. As vacuum tube and tape technology is driven slightly over its working limits with high input levels, for example, the resulting harmonic distortion can add a certain sort of "fatness" to the original sound source.

Digital audio recording requires a different set of guidelines to attain optimal results. Unlike analog recording, the digital recording chain is not as tolerant of any audio signals that may exceed the recommended input level of the equipment. This includes digital video cameras, CD and DVD recorders, stand-alone hard disk recorders, computer audio workstations, or any unit that is based on 1's and 0's. The more information you feed into a digital recorder, the more information it has to work with, and the lower the noise level. Unfortunately, any peaking input signals will create a noticeable form digital distortion that is generally neither pleasant nor desired.

Here are some quick links to a variety of hardware and software to help you get familiar with some of the more popular models.

Hardware:
Software:

Should you have any further questions about Compressors or Dynamics Processors, we encourage you to contact us on the phone, online, or in person at our SuperStore in New York City. 1-800-947-9923.

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