compressors and limiters

To reduce the dynamic range of a recording, dynamics processing is used in the form of compressors and limiters. Typically a compressor or limiter will attenuate the level of a signal once it has reached or gone above a threshold level.

Compressors and expanders belong to a group of sound processing effects that are adaptive, meaning that the amount or type of processing is determined by some component of the signal itself (Verfaille et al., 2006). In the case of compressors and expanders, the amount of gain reduction applied to a signal is dependent on the level of the signal itself or a secondary signal known as a side-chain or key input. With other types of processing such as equalization and reverberation, the type, amount, or quality of processing remains the same, regardless of the input signal characteristics.

Depending on the nature of the signal-dependent processing, it can sometimes be more obvious and sometimes less obvious than processing that is not signal dependent. Any changes in processing occur synchronously with changes in the audio signal itself, and it is possible that the actual signal will mask these changes or our auditory system will assume that they are part of the original sound (as in the case of compression). Alternatively, with signal-dependent quantization error at low bit rates, the distortion (error) will be modulated by the amplitude of the signal and therefore be more noticeable than constant amplitude noise such as dither, as we will discuss in Section 5.2.3.

To determine if a signal level is above or below a specified threshold, a dynamics processor must use some method of determining the signal level, such as RMS or peak level detection.

Other forms of dynamic processing increase the dynamic range by attenuating lower amplitude sections of a record- ing. These types of processors are often referred to as expanders or gates. In contrast to a compressor, an expander attenuates the signal when it is below the threshold level. The use of expanders is common when mixing drums for pop and rock music. Each component of a drum kit is often close miked, but there is still some “leakage” of the sound of adjacent drums into each microphone. To reduce this effect, expanders or gates can be used to attenuate a microphone signal between hits on its respective drum.

There are many different types of compressors and limiters, and each make and model has its own unique “sound.” This sonic signature is based on a number of factors such as the signal detection circuit or algorithm used to determine the level of an input audio signal and therefore whether to apply dynamics processing or not, and how much to apply based on the parameters set by the engineer. In analog processors, the actual electrical components in the audio signal chain and power supply also affect the audio signal.

A number of parameters are typically controllable on a compressor. These include threshold, ratio, attack time, release time, and knee.