Compression

A compressor is a robot that turns the gain down#

Imagine a robot whose job is to turn the fader down when something gets too loud. That is a compressor. Rectify and smooth a copy of the signal, invert it, and use it to drive the gain of the original. If you flip it round, you get a gate.

What is compression? Gain across time. What is EQ? Gain across frequency. The time part is the defining feature. Every compressor has attack and release, whether or not it gives you knobs for them. Set both to zero with an infinite ratio and you have a hard clipper. At any lower ratio, an instantaneous compressor bends the top of the waveform over at a slope instead of flattening it, so it is a waveshaper rather than a clipper. Without the time component, a compressor, a limiter, a gate and an expander are all just waveshapers, which is to say distortion.

The transfer curve does not show the compression#

Everyone knows the chart with input level on one axis and output level on the other, and a bend at the threshold. People think that bend is the curve of the compression. It is not. It says that a signal coming in at x level will come out at y level at some unspecified point in time in the future. How does it go from x to y? Linearly, convexly, concavely, in a zigzag? The chart does not tell you.

That journey over time is the most important thing about any dynamics processor, and it can be measured and seen instead of mystified. The typical shape is a convex attack and a concave release.

Most compressors exhibit the same curves#

I never claimed all compressors sound the same. Unlike EQ, compression has genuine flavours: old analogue circuits really did behave differently, and some compressors are frequency-dependent or add EQ and distortion. But most compressor plugins, at any price and whatever the vintage branding, apply essentially the same vanilla gain-reduction curves behind a different interface. Many expensive ones sold as program-dependent do not react in a frequency-dependent way, which a swept test exposes.

Topology labels such as FET, VCA, opto and vari-mu name a component, not a sound. The differences come mostly from the time-constant ranges each technology was coerced into using because of associated technical limitations. The famous console bus compressor is a standard VCA compressor: low ratio, about 30 ms attack, about 100 ms release, a bit of soft knee, and almost any compressor can do it. So learn one full-featured compressor with wide ranges, and know the settings instead of auditioning ten plugins for half an hour.

A fast attack takes punch away#

People think a fast attack makes things punchy. It is the opposite. A fast attack erodes the transient; a slower one lets it through and fattens the body. An instantaneous attack on a snare gives you a flat, inflated cat sneeze. A fast attack with a slow release on the mix bus sounds like someone sat on your mix. Classic bus compression is the reverse: around 30 ms attack, 50 to 100 ms release. First decide whether the track needs bus compression at all. Many do not, and "glue" will not fix a mix that does not work.

Bass is the exception that wants a short attack, otherwise it sounds percussive and weird.

Ratio has sharply diminishing returns: each doubling halves the remaining change, so 8:1 is enough for almost anything. And remove any compressor you cannot show helps at a matched level.

The sidechain is where the tricks are#

Sidechain ducking can be used in every mix, rock and metal included as well as electronic. In guitar genres the classic is ducking the bass from the kick. If bass is making a bus compressor pump, you can high-pass the detector instead of reaching for a different compressor.

The nuance is the curve over time#

Truly novel compression is hard to create, which is why so little exists, and the nuances come in the shape of the gain reduction over time. To the best of my knowledge, Convexity is the first plugin that lets you draw that time curve yourself, although it is admittedly experimental. I also borrow models from other disciplines: Black-Scholes compression.