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December/January 2015

by Reuben Rowntree

Tutors’ Tutorial: Compression

by Reuben Rowntree

Tutors’ Tutorial: Compression

The topic of dynamic range compression (not to be confused with data compression like zip, rar, mp3 etc.) is always worth revisiting, as it is a favourite form of processing for many engineers.
How a compressor works has been covered before so we will take a look at the ‘why’. You may have noticed that older films tend to be more dynamic – they can be very quiet one moment then very loud the next. While this can add excitement it can also be worrying when you are watching Rambo (First Blood) in the early hours of the morning and waking the baby or rousing your grumpy flatmate is an equally terrifying situation.

Compression can reduce the range between the loudest and the quietest parts of the programme and thus make for more consistent and predictable audio.

When recording at home, there can be many reasons for applying compression but it usually boils down to one of two things: to improve consistency or to add attitude/character.

Before we look at these matters, let’’s quickly explain how a compressor works. A compressor reduces the amplitude of loud signals when they exceed a threshold. The ratio control determines how much reduction is applied and the attack and release controls determine the speed of the reaction.

Now, let’’s examine the first problem. Consistency can be problematic if a singer unintentionally moves away from the microphone during a performance. This is obviously not ideal, but a little compression can help to even out the issue. A low ratio with just a few dB of gain reduction will probably do the trick. However the loudest parts will need to be lowered in volume to match the error so fader automation can be a better solution.

If the performance is inconsistent for whatever reason – maybe the singer just got a little too expressive at one point – then compression is especially good at reducing the loud (expressive) part. Again a low to medium ratio with a threshold specifically targeting the loud part (so that the quieter parts do not trigger the compression) will do the trick while still retaining some of the musicality. See the attached image.

The compressor is in a class of its own when it comes to adding attitude to an audio signal. Drums are a great example, where compression can take a moderately roomy drum kit and make it sound like Animal from the Muppets is slamming them in a rocky cavern. The attack, or punch, of the drums and the natural ambiance present in the signal can become exaggerated. To achieve this, a higher ratio and a quicker response time are usually used.

Some manufacturers actually make compression pedals designed for guitar. They are often described as adding sustain. In simplistic terms, compression makes the quieter things sound louder and as such, the decaying tail of a note will increase in volume and therefore appear to last longer. When this is then run through an amplifier’s distortion module, the loudest parts will be squashed further by the distortion. While this can be interesting, it can also quickly reduce the vibrancy of the guitar in some circumstances. Blues and jazz guitarists may prefer a more natural approach, in which case a compressor with mild settings can again target the louder parts of the performance to bring the overall range down.

Judicious use of compression can give your recordings a professional edge, but learning to discern when it is greater consistency or a funkier attitude that you are trying to enhance will change the way these processors are used.

Reuben Rowntree is a lecturer at SAE Institute. In his spare time, he spends way too much money on building hardware compressors. He can be contacted at r.rowntree@sae.edu

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