A limiter is a signal processor that allows signal to pass un-attenuated under a certain threshold and to attenuate the peaks of the signal over a set threshold.
A limiter is a more extreme version of compression. Compression works in exactly the same way as a limiter but it allows more dynamics as
The parameters available for control on a limiter are;
Threshold – the set point at which a limiter will start to attenuate signal
Compression Ratio – The amount that signal will be attenuated. For example 1:1 means for every DB of signal, one DB of signal will be passed. 2:1 would mean for every 2 DB of signal, 1 DB of signal will pass.
Attack time – The time taken for the limiter to react to signal that exceeds the given threshold.
Release time – The time taken for the limiter to stop affecting the signal after limiting a peak.
Hold Time – The time that the limiter will maintain gain reduction.
Ceiling – This Specifies the maximum output level.
Input Gain – This adjusts the gain before limiting is applied.
Output Gain/Make up Gain – This adjusts the gain after limiting is applied.
Lookahead – This applies to digital limiting only and is a buffer for preempting quick transients that are coming up.
True Peak Limiting – A limiter that can pre-empt and attenuate inter sample peaks
Dither – This prevents artefacts being created when converting an audio file to a lower bitrate.
Knee – This refers to how quickly the limiter clamps down on the signal after the threshold is crossed. Hard knee refers to a fast reaction and a soft knee refers to a more gentle approach to the onset of the limiter.
It's good to be aware that not all of the above parameters will be available on every limiter. Here are some examples;
Fab Filter Pro L2
This plug-in is what we refer to as Brickwall limiter. This means that it has a fixed ratio of infinity:1, therefore any sound fed in over the set ceiling will be attenuated.
You can manipulate the level of limiting by changing the input gain and the output level (Ceiling)
Avid Dyn3 Cl Limiter
This plug in can work as both a compressor or a limiter by changing the ratio accordingly. To work as a limiter the ratio must be set high enough. Typically this would be from 20:1 up to infinity:1.
As you can see from the two above examples; limiting can be approached in quite different ways. The Fab Filter limiter allows you to feed the signal into the limiter by increasing the input gain – this method would be more useful in the mastering stage, whilst the Avid Limiter makes it easier to destructively limit the signal via changing the threshold and can produce a more dynamic result.