By Steven Swift

I’ve focused on a few areas of recording and mixing in the past, such as studio monitors, automation, pedal chains and audio interfaces. This time I’m focusing on Compression and how it’s used everywhere in modern music.

It’s common knowledge that compression is used on most instruments either going into the desk, interface or pre-recording but compression can be added post-recording and even across the entire track when mastering for a grand total of three different levels of compression.

Different levels in the above text, doesn’t mean different compressors or different settings, it means three separate stages of compression (pre, post and across the entire track). You might find using three different compression models/values or thresholds perfect for your mix.

Let your ear be the judge.

"Compression in its simplest form controls the loud moments of an instrument/vocal ..."

What Is Compression?

It’s all about the law of averages… kind of.

Compression in its simplest form controls the loud moments of an instrument/vocal and increases the quiet moments of an instrument/vocal to give you a consistent average.

We call the loud moments Peaks and the quiet moments Troughs, which all relate to dynamics (volume). Compression is your friend in the modern age of music and every style of music features huge amounts of compression. Jazz and orchestral music in their nature are more organic (not mainstream jazz – Michael Buble for example), so they don’t feature much compression (sometimes there will be no compression).

Most compressors (regardless of model or manufacturer) feature the same similar design and controls. Although we’re looking at software plug-in versions of compressors, outboard (analogue) compressors work exactly the same.

A Short Note

Before we can look at the controls found on compressors, it is worth noting that sometimes Compressors and Limiters are combined on to the same plug-in interface. Limiters are similar to compressors as they both deal with dynamic levels and ranges.

The only difference is that limiters are used to eliminate Peaks without affecting anything else. Once the instrument/vocal hits a certain threshold (volume), a compressor will reduce the peak and average the volume (making it appear louder), but in the same situation a limiter will cut the track at the threshold once it goes above.

Now that we understand the basic principles of limiters and how they are different to compressors, we can get some visualisation on the various interfaces.

Controls

Make-up Gain (gain) – Peak Reduction

Waves CLA 3A

As you can see above (Waves – CLA 3A), this compressor only features two different controls (Gain and Peak Reduction).These two controls function as a traditioanl threshold option, with the Gain control acting as a make-up gain and the Peak Reduction acting as the threshold value.

Make-up gain is the polar opposite of the Peak Reduction. Make-up gain (Gain for the CLA 3A) is there to make-up the difference in volume once the Peak reduction has taken affect. A compressor can only reduce the incoming signal (Peak Reduction), so the make-up gain acts as a counter weight to bring the dynamics back to an acceptable level (after the Peaks and Troughs have been compressed).

Now let’s take the compressor built into Logic Pro, which has more features than our previous entry.

Logic Pro Compressor

Attack – Release

Attack and Release are like push and pull. They are complete opposites but they function together to control your compression. If one is set wrong, the other will suffer and create uneven compression (can be useful in certain situations, pumping for example – more on that later).

Both are measured in milliseconds (ms) and as you might of guessed, Attack controls how long it takes for the compressor to kick in after the threshold has been reached. Release controls how long it takes for the signal to drop below the threshold after the compression has taken affect.

Added Benefits of Attack and Relase

Having a fast attack time will dull the sound as it reduces the initial hit or attack (brightness) of the instrument/vocal. Setting a slower attack time will add punch.

A long release time will increase the sustain of a part. If you combine both fast attack and release settings, you can achieve pumping (Audible compression – creates a bounce by squashing and releasing rapidly)

"Let your ear be the judge"

Ratio

The ratio of your compression will be set as a ratio of something:1. This controls how much compression in dB occurs once the threshold has been crossed. For a ratio of 6:1, you will have 1dB out for every 6dB across your threshold.

For example, if your threshold was -6dB and you had a peak of 3 dB, that’s 9 dB over the threshold. With a ratio of 6:1, you would have 1.5dB added to -6 giving you -4.5dB.

Knee

The knee control affects how abrupt or smooth your compression kicks in at the threshold value. A soft knee (higher value in logic pro) will curve the entry of the compressor, so it slowly appears instead of a hard knee (in the logic pro compressor example) which has an abrupt entry kicking straight in to your maximum ratio as soon as it hits the threshold.

Threshold

I’ve left this one till last as it’s the same as the Peak reduction in our first compressor. When using a Threshold fader, you have to set your value in dB instead of using the plus and minus (in the CLA 3A example).

Other Forms of Compression

You can find compression for individual tracks or apply a single compressor to a bus to affect multiple tracks at the same time with the same compression settings (Bus compression - it saves adding a compressor to every channel just to duplicate the settings).

Slate Digital VBC Bus Compressor

Side-chain Compression is where you use another input to control the compression level of the original signal. Sounds confusing but just think of it as compressing in response to another source – known as Ducking.

Ducking can be used to compress the bass when the kick drum hits, or when a DJ finds it necessary to speak over a track. The vocal will be side-chained so the track is compressed in response to the added vocal.

Multi-band Compression is similar to side-chain compression but you can alter different compression settings for each frequency band. This is great as a mastering tool to compress an entire track.

UAD Precision Multiband

I hope you enjoyed reading my article and if you have any feedback please don’t hesitate to get in touch!! 

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Steven Swift