Compressor Circuit Types

Before we get to using the compressor, it's worth briefly covering the subject of circuit types.

With a compressor plugin, you're likely to have the option of choosing the circuit type which will have an effect on the way the compressor sounds. With an analogue compressor, there must be a circuit that detects the level of the incoming signal, which in return gives instruction to attenuate the signal having exceeded a given threshold.

Most digital compressors (plugins) are copies of the way analogue compressors behave. One major factor of the behaviour, or character if you will, of an analogue compressor is the type of circuit it uses to determine the amount of attenuation with respect to the incoming signal. There are many types, however it wouldn't be practical to explain them all now. Following is a brief explanation of three circuit types, Opto, VCA and FET. These are all optional settings in Logic's own compressor plugin which is to be used for demonstration purposes.

Opto (Optical) – named accordingly. This circuit uses a light source and a photocell (phototransistor) or LDR (light dependent resistor). The level of the incoming signal is converted into light using a light bulb or an LED and then detected by the photocell/LDR. The amount of attenuation is determined by how much light the photocell is detecting. The result is a soft and smooth reaction to the incoming signal, which is often desirable in mastering applications. Optos can be a little slow in their response, so when a very fast attack is required, the Opto may not be suitable.

VCA – Voltage Controlled Amplifier. Probably the most common type down to their versatility. Attenuation is determined by detection of the incoming signal's voltage. They can cope with the two extremes very well, very fast, or slow and smooth.

FET – Field Effect Transistor. A transistor based circuit design with an aim to achieve a tube like sound (tube circuitry is often desirable in mastering applications as it can project its own flattering character across the mix). They are typically fast and smooth in their operation. The downside to FET is a high signal to noise ratio.

Compressor circuitry is another subject altogether but for now, don't worry too much. Just be aware that they differ and can each have a different sound. The best thing to do is to familiarise yourself by experimenting with each.

David Eley - TGM Audio

Next page... Setting Up Your Compressor for Mastering.