The Form & Function Of Dynamics Processors

Some dynamics processors make the side-chain control voltage available for connecting to a neighboring unit, or to tie internal channels together.

Slaving or linking dynamics processors causes the units to operate simultaneously when only one unit or channel exceeds the threshold setting.

This feature preserves stable stereo imaging and spectral balance.

All dynamics processors carry out gain control as a function of side-chain level.

Some use the internal signal as shown in Figure 1 and some use an external or Key Input as shown in Figure 2.

The only difference between a compressor, limiter, AGC, de-esser, ducker, or gate, is the type of side-chain detector, the gain computer attributes and the type of gain control element used.

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The introduction of DSP (digital signal processing) dramatically changed the implementation of dynamics processors. In traditional analog designs, there is no practical means of “looking ahead” or statistically analyzing the content of a signal, instead requiring a function to respond to events that have already occurred.

The supporting circuitry for filtering and dynamic control of attack and release is complex and expensive with limited accuracy.

At the heart of analog designs are gain control elements, usually voltage-controlled amplifiers, or voltage-controlled attenuators (both abbreviated VCA), with these typical specifications:

* Maximum gain linearity: 2% over 80 dB
* Maximum gain range: 80 dB
* Temperature dependence: 0.33% / °C (change in gain transfer accuracy)
* Distortion: 0.05% THD+N
* Signal-to-noise ratio: 94 dB re 0 dBV
* Cost: $2 per gain element
* Sources: Very limited

Certainly respectable numbers, but digital designs can do better. The most significant advantages are the ability to analyze a signal before it is processed and statistically analyze recent history. These abilities allow a wide range of new topologies offering superior performance.

The incremental cost of a single function implemented in DSP is very small, resulting in significant cost reduction when requiring multiple functions. Digital signal processing offers both greater accuracy and reduced cost.