Compression: The Big Squeeze

It's rare to walk into any studio, large or small, and not see at least one or two compressors in use. Large studios use multiple compressors, and some mixing boards have compressors built into each input channel. A compressor basically works like an automatic level control, restricting the overall dynamic range of an input signal. In other words, it limits the amount of variation between the loudest and the softest sounds.

Proper use of a compressor can prevent distortion from overload during loud sections of a song, smooth out unintentional peaks and drops in a performance, and provide additional sustain for electric guitars. Improper use can add unwanted noise or accentuate noise already present in a signal. Excessive use can destroy the dynamics of a piece. With that in mind, let's take a look at how compressors work, and how they can be used to enhance your recordings.

How a Compressor Works

If you take a look at a compressor's front panel (Figure 1 shows the front panel Rocktron's 300G compressor), you can see there aren't a huge number of knobs that need adjustment in order to get the job done.

The most important control is the threshold control. It tells the compressor the level that the signal must be at in order to start working, and it's expressed in decibels, abbreviated dBs. So, for example, if you are processing your guitar signal, and you set your compressor's threshold to -30dB, the guitar must be louder than -30dB in order for the compressor to care. If your guitar signal comes in at -31dB, nothing happens. If the signal is -29dB or higher the compressor starts to work by reducing the gain of the signal. But how does the compressor know how much to reduce the gain?

The ratio knob sets the amount, or severity, of compression of the input signal once the threshold has been exceeded. It determines the change in the output level that results from a particular input level, and is also expressed in dBs. For example, if the ratio is set to 2 (that really means a ratio of two to one [2:1]), then for every 2dB of incoming signal in excess of the threshold, there will only be a 1dB increase in the output level. Higher ratios, such as 4:1 and 8:1 mean more reduction in the output level, and also mean the output signal is even closer to the level of the threshold.

Extreme amounts of compression, ratios of 10:1, 15:1, or higher, are known as limiting, because the output will increase very little over the threshold, effectively creating an upper limit for the loudness of the sound. Many compressors can be set to an infinite ratio (look for the infinity symbol), meaning that the compressor will not exceed the threshold for any input signal. This can be very valuable when doing digital recording, when exceeding the 0VU mark produces nasty digital distortion, since unlike analog tape, there is no headroom above 0VU.

The attack control lets the compressor know how long to wait before starting to do its job, once it figures out that the input threshold has been exceeded. Attack is measured in milliseconds (ms), with attack times of 1-100 milliseconds considered 'fast', and greater than 100 milliseconds considered 'slow'. The faster attack times will tend to compress high-level peak transients, such as drum hits, or bass guitar slaps, whereas the slower times will let the quick peaks pass uncompressed. Why use slower attack times? There are occasions when the original sizzle or bite of a sound will be lost when using attacks of less than 100 milliseconds. As long as allowing the peaks to be heard does not cause distortion further on down the signal path, you may prefer the sound that using longer attack times will give you.

Compressors without an attack control tend to automatically use faster attack times when the volumes are much higher than the threshold, and slower attack times when the input levels are just over the threshold.

A fourth control found on many compressors is the release control. When the signal drops back below the threshold level, the release control tells the compressor how long to take to get back to normal, when it is no longer modifying the input signal. You'll usually see release times either measured in milliseconds or seconds, with release times of 1000 milliseconds (1 second) considered 'fast', and greater than 1000 milliseconds considered 'slow'.

Slower release times tend to smooth out the signal, and for guitars, can help increase sustain. However, if the release is set too long, the gain will remain turned down for a while after the loud sound has ended. This may result in causing quiet sounds that occur between loud sounds to be turned down as well. There's also a danger with setting release times too fast. Quick release times will make the compression effect too noticeable, to the point where you can hear a very unnatural 'pumping' or 'breathing' sound as the signal level goes up and down. In general, the release should be set fast enough to recover in time to process the next bass slap, power chord or drum hit.

Again, like the attack control, some budget compressors might not include a release time control. Some models simply have switches for 'slower' and 'faster' release. If there is no release control at all, the manufacture has designed the compressor to use a release time to fit the most common musical situations.

Some compressors have an additional control called a peak level control and like the threshold control, it's expressed in dBs. Using this control, you can set a level which will not be exceeded at the output, even when you have a very quick, loud sound, such as a snare drum hit. This control gives you additional flexibility, because now the ratio control can be used for smaller compression ratios (4:1 or lower), while you still have an upper limit in place to avoid possible overload or distortion due to unexpectedly high signal levels.

Additionally, you'll probably have an output gain control to set the final output level. This is needed because the compressor is reducing the level of signals that cross the threshold, so an additional stage of gain is used to bring the signal back up to a nominal level.

What's "Soft Knee" Compression?

Another approach to compression is what's known as soft knee or over easy compression. Instead of a user-defined threshold control, a soft knee compressor is always working, automatically applying more compression (a higher ratio) as the signal gets louder. The compression is very low to minimal when the signal is very soft. Some manufacturers apply a bit of soft knee compression to the signal even when they provide a threshold control. It simply means that a bit of compression is beginning as the signal approaches the threshold, and the user-defined ratio is applied once threshold is reached. Some musicians and engineers like the soft knee approach, claiming a warmer sound due to the gradual transition from uncompressed to compressed sound. It's also claimed to make the compression a bit less noticeable.

It's Like Having the Fastest Engineer in the West

You can see the advantages that using a compressor might have over the old method of controlling the dynamics of a piece by 'riding' the gain. The engineer who wished to eliminate peaks and high signal levels used to have to listen carefully with hands on faders, trying to anticipate the loud points in the music, and frantically making abrupt adjustments. Once the compressor's controls are set up properly, the compressor is automatically doing all the adjustments in gain, much faster and much more accurately than the best human engineer.

Let's talk about some tips and techniques for setting the compressor's controls for various musical situations. Before that however, let's talk about connecting the compressor to our gear.

Hooking It Up

A compressor is different from a reverb or delay unit in that compression is not an effect to be added to an existing signal. The compressor must process the complete sound and only output the processed version of the sound. The unprocessed signal must not be heard at all; this would defeat the purpose of trying to adjust the level with the compressor.

Keeping that in mind, we can't use the aux send and return loop on the mixing board as we would with a reverb or chorus, since the aux send/return splits the signal. We've got to feed the entire signal into the compressor and direct the output of the compressor back into the mixing board. If your mixer has insert points, you can hook up your compressor to any mixer channel using a TRS (tip-ring-sleeve), stereo-to-mono, "Y" cable. The stereo end of the cable plugs into the mixer channel's insert point, while the two mono plugs are connected to the compressor's input and output jacks. If you make the connections and you lose sound completely, you may have to switch the two mono plugs, since you may be feeding the output from the mixer into the output of the compressor.

Another connection method I use at home is to hook up the output of my guitar multi-effects to the input jack of the compressor, and connect the output of the compressor to an input channel on the mixing board. I have to hook it up this way due to the lack of insert points on the Yamaha ProMix 01 mixer.

If you'd like to process the whole mix, you'll need a stereo compressor. Don't try using two mono compressors because if, for example, you have an unusually loud drum sound panned hard left, only the left channel compressor will start working and the overall sound will drift to the right. A stereo compressor automatically compresses both channels, no matter where the peak sounds occur. Simply connect the left and right outputs from the mixer to the left and right inputs of the stereo compressor. The outputs from the compressor can then be connected to the mix down unit.

What's a Side Chain?

An additional set of connections you'll see on some compressors are known as the side chain input and output jacks. Some compressors refer to the side chain as a 'key input'. Stereo compressors will have two sets of input and output jacks. You can hook up an equalizer to these jacks to provide frequency dependent limiting/compression. One application might be to use the equalizer to remove all the low frequencies from the signal. When the treble-heavy signal is fed back into the side chain input, it tells the compressor to only respond to loud, trebly peaks in the signal. This is often used to remove sibilance from a vocal track.

Another use for the side chain is for 'ducking', the old DJ trick of reducing the volume of the music when doing a voice-over. The vocal input is fed into the side chain input, so that when the vocalist begins speaking, the compressor starts processing the music, lowering the volume. When the voice stops, the music returns to the normal level.

Compression Tips

Here are some starting points for solving various problems in the studio with your compressor (you did go out and buy one, right?) Remember, as always, let your ears be the ultimate guide; if things don't sound right, start tweaking those knobs!

Increasing sustain for guitars and basses

These settings should help increase the sustain of your instrument. If the sound of your guitar loses body, try lowering the ratio a little to around 7:1, but expect less sustain as a trade-off. These settings work great for both rhythm and lead guitars.

Increasing the punchiness of rhythm guitars

A little compression can enhance the texture of your rhythm guitar, as well as making it punchier (but not dominating) in the mix.

Tightening up the entire mix

Maybe you'd like to have your mix sound a bit more 'radio friendly'. Or maybe an acoustic performance has a little too much dynamics for your taste. Use a very small amount of compression on the whole mix, as a final 'seasoning'.

Limiting the signal for digital recording

If you are tracking on or mixing down to a digital machine (DAT or hard-drive), even a small amount of clipping is a major problem. In this application we'll set the compressor to act as a limiter.

Recording vocals

Does anybody still record vocalists? Aw, heck... I guess I can't change the world. One of the most common uses of compressors is to smooth a vocalist's performance dynamics when close miking. Using a compressor can help a singer who lacks good mike technique, by adjusting the gain, especially when you've got a real screamer on your hands (you might set the ratio to 4:1 for them as well).

Evening out the bass guitar

Sometimes, with all their low-frequency energy, an uncompressed bass can use up all the headroom in a mix. In addition, it may sound much better to simply limit the dynamic range of the instrument.

Taming the acoustic drum kit mix

Applying compression to the entire drum mix sometimes gives the overall drum sound more impact. You really need to be careful of over-compression though. With attack settings set too low, loud sounds like a kick drum can cause the compressor to start working quickly and inadvertently lower the sound of other drum sounds on the same beat, such as a high-hat or a cymbal. The cymbal will sound as if it's being 'sucked' down. Usually a drum machine will need little, if any, compression as the internal sounds are normally compressed when recorded.