'How To' Articles

A Beginner’s Guide to Compression

COMPRESS TO IMPRESS

Compression is one of the most frequently used effects for audio production. Considered to be an essential studio tool, applying compression to every track in a mix, individually or overall, is an industry standard procedure.

A compressor has the ability to radically alter sounds, or even a whole mix if need be. Despite having a crucial role in production though, compression might not always be audible in the same way as say, a phaser, reverb or other noticeable effects can be.

As such, this has given compression a reputation for having a kind of “invisible” quality. But for most mixing purposes, compression usually winds up becoming the “glue” which keeps a mix together.

So what exactly is compression? What does a compressor do? How, when and where do you use it?

A compressor controls the dynamics of a sound signal. It helps to regulate volume and maintain a level of consistency when applied to that signal. It acts on “peaks and valleys” of sound, so that loud signals are reduced and soft signals are increased.

By reducing loud signals, a compressor ensures that excessively loud volume doesn’t overwhelm a mix and by increasing soft signals, it ensures that quieter sounds are more audible.

Contributed by Byron Struck for the Roland Australia Blog

*Top: An uncompressed waveform. Bottom: A compressed waveform*

It’s important to note that compression usually does all of this without affecting the tone of an instrument or the overall sound of a mix. This is why compression is at times referred to as an “invisible” effect, as we mentioned earlier.

Although, some compressors (like tube or valve compressors) can impart their tone onto whatever they’re acting on. Compression tones like these are usually desirable and its part of why they’re used. But we’ll get to that later.

Like most effects, using compression depends on what we want to use it for. In the case of an instrument, we might use a slight amount of compression on say, a string instrument to increase the decaying notes, to even out the sound, or to increase sustain.

We might use it aggressively on a kick drum, in order to increase the impact or “punchiness” of the beater hitting the drum.

Here’s an example of a drum beat with no compression.

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And here’s an example of the same drum beat with heavy compression. Notice the difference?

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If you noticed the snare drum ghost notes more and the pumping and breathing of the dynamics, you were hearing the side-effects of heavy compression. You might also notice the final cymbal hit sustain is much more present.

This isn’t normally a compression setting you’d use on drums because it’s too severe, however in this example, it outlines the difference between the two beats and what compression can add.

In an overall mix, we’d use compression to make sure that everything has a consistent volume level. We can also use compression to boost the apparent, (RMS or average) volume of our mix and make it louder if need be.

Compression is available in different forms. We can use hardware compressors, software compressors, digital, analog and in some cases, even naturally occurring compression. One of the reasons why audio tape has such a desirable sound quality to it is because of naturally occurring compression.

This kind of compression is in fact distortion of the signal, referred to as tape saturation. It brings a certain sonic texture to the music recorded onto the tape that becomes pleasing to the ear.

In fact, you’ve probably heard compression of some form on nearly every recorded song you’ve listened to, even if you couldn’t spot it immediately!

WHAT’S IN THE BOX?

A compressor usually has a combination of, or all of, the following controls – Threshold, Ratio, Attack, Release and Gain.

*A standard compressor interface with fundamental compression controls.*

Here’s how they work.

Threshold: Determines the volume level at which the compressor starts to lower the incoming sound signal. When the input volume is above the set Threshold level, the compressor begins to work. This is how we decide how much we want the compressor to act on our sound source.

A high Threshold will reduce the volume of louder signals that the compressor is working on, lower Thresholds might compress everything, even lower volume signals.

Ratio: Once the input signal exceeds the Threshold, the Ratio will determine by what factor the out level reduces to. We can “squash” our signal quite dramatically by increasing the Ratio with this function.

So if we have an input signal that is 4 decibels over the Threshold and a Ratio of 4:1, the output will be 1 decibel above the Threshold.

This is where we see formulas such as 2:1 or 4:1 – as the numbers to the left of the colon increase, the more they are compressed post-Threshold. Slope is another term for describing Ratio.

Attack Time: Attack controls how quickly the compressor kicks in and starts to work on the input signal that has exceeded the Threshold.

So, once the input exceeds the level we set on the Threshold, we can choose to have the compressor act immediately or later. Long Attack times make things more aggressive or punchy, shorter Attack times soften the blow.

Release Time: The Release Time determines the length of time the compressor takes to return to its pre-compression state, once the level drops below the Threshold.

The longer the Release time, the more the compressor is acting on the sound. A shorter Release time means that the compressor allows for a more natural decay from the sound source.

Output Gain: Also known as makeup gain, this allows us to increase or decrease the volume of the now-compressed signal. Essentially, it’s a volume control for the compressed output signal.

This is important because contrary to some common beliefs, compression reduces the dynamics of a sound. As such, it makes a sound smaller – not larger.

SAME, BUT DIFFERENT…

Generally speaking, there are a few different kinds of compressors. All compressors control volume and dynamics, but there are certain specific things that some compressors have which separate them from others. Let’s take a look.

- VCA (Voltage Controlled Amplifier)
  Very clean quality, lots of versatility and with specific control over gain reduction, a VCA compressor won’t overly colour sounds. This makes them popular for using in studios and on instruments.Their integrated circuit chips (IC) which contains transistors that track your incoming signal (i.e. voltage) is what makes for their excellent gain control. The BOSS CS-3 is an example of a VCA compressor.

- OTA (Operational Transconductance Amplifier)
  Similar to a VCA compressor except that the output isn’t in voltage, but variable current. These types of compressors are a very specific circuit, based on the CA3080 chip, or it’s more contemporary counterpart in the LM13700. Used for the original guitar compression pedals because of their natural, organic quality.

- FET (Field Effect Transistor)
  Similar to a VCA compressor but with a few unique features. Using transistors to emulate tubes, they can add a colour to the sound via the Input gain control, which is desirable in some situations.

- Optical
  Using a light source such as a bulb or LED, along with a resistor, these compressors are very natural sounding and quite smooth. The light source will increase or dim in relation to the input level, while the resistor will respond to the brightness of the light and will adjust the gain in accordance to it. The resistor operates in a voltage divider or a feedback loop and the overall quality to this ingenious design is that optical compressors can range from nearly invisible to very apparent.

Valve or Tube
Usually similar to optical compressors, except with the addition of tubes for gain, these compressors add excellent tone, smoothness and warmth.

A WORD ON SIDECHAINING

More of a technique than an actual compression device, the Sidechain effect has enjoyed a lot of attention in recent times. This is where an external sound enters into the compressor’s detection circuit to directly influence the signal the compressor reacts to.

In dance music production, often the entire mix goes through the compressor and then a separate dry kick send enters into the Sidechain input. This has the effect of causing the mix to “pump” in time with the music. This has become an important technique in electronic music production.

In the radio, TV and film broadcast worlds, this is also known as ducking, when backing music is run through the compressor and the narrator’s voice enters the Sidechain input. This has the effect of causing the music to “magically” duck down, automatically out of the way for the narrator’s voice.

BANDS IN A BOX

There are other types of compressors, but these are the most common. However, a more modern addition to the compression family is the multiband compressor. Primarily used in DAW software and as a plug-in, multiband compression as its name would suggest, acts on different frequency bands of a signal. As such, it allows for more or less compression to affect high, mid or low frequencies.

By splitting the incoming signal through various filters, each part of the signal has its own compressor added to it before recombination at the output. This has obvious advantages because it allows for elements of your tone to really shine, without the overall character of your sound to degrade.

Because multiband compression frequently appears in software, it’s not always easy to translate to a pedal format. However, the BOSS CP-1X and BC-1X are examples of multiband compression in a stompbox pedal.