Post Wed Oct 31, 2012 10:27 pm

Differences Between Limit, Compress, Expand, Or Gate

Dynamics Processing, Explained: Should I Limit, Compress, Expand, Or Gate?

What's the difference between compression, limiting, gating and expanding?

Whether you're shopping for a dynamic processor for your live sound rack unit or staring at a plugin interface in your DAW, you're going to need to understand the differences these three phrases denote. In a nutshell, they're all describing one type of audio signal effect — dynamics processing — but each of these process types will have a different effect on the audio being processed, and in some cases the difference is radical.

The most common of these dynamics processing types is compression. Originally achieved with units called leveling amplifiers (like the Universal Audio LA-2A), compression looks at an incoming signal and actively reduced its volume. The way it works is this: you set a threshold, which is the volume level at which the compression effect will engage. Anytime the audio signal rises above that threshold, the compressor will reduce the volume at a set ratio; a ratio of 2:1 will half (roughly) the outputted signal, 4:1 will quarter it, etc. In this way a compressor can help tame highly dynamic signals like a snare drum or the human voice and make them more listenable when reproduced (i.e. played back on your headphones). See the below chart:


A very basic compressor will include controls for threshold and ratio, possibly with makeup gain. to boost your newly squashed signal. A device categorized as a simple compressor might also give you controls like Attack and release, which determine how much of a time delay before the effect engages and disengages, respectively. Attack and Release controls are especially helpful when processing drums, where you might want to preserve the initial peak of the drum attack, but compress the ensuing overtones (and then musically craft their decay rate).

Limiting is a type of compression where the ratio is set to ∞:1, i.e. the signal is "capped" at the threshold level. Whereas compressing a signal reduces volume peaks by degrees, limiting does just what is sounds like — it limits. If you set a limiter at a certain db threshold, your output level will never go higher than that threshold. This is helpful when mastering, as limiting will let you maximize the volume of the majority of your track while making sure no errant volume peaks cause unwanted distortion. In a live context, limiting helps protect PA speakers and other sensitive equipment from those same spikes. Limiting can also be used in mixdown, however, to achieve interesting "lo-fi" effects with drums and electric bass. See now what our dynamics chart looks like when we add a limiter with a threshold above that of the compressor:


An expander effects a signal in the opposite way a compressor does, though not in the way you might instinctively think. And expander doesn't increase the put level vs. input level, rather it expands the overall dynamic range of a signal by reducing volume below a certain threshold, making quiet sounds quieter (as opposed to making loud sounds quieter at a certain ratio. While a pure expander isn't a common function on most popular dynamics processor units, adding it to our dynamics chart would look like this:


The relationship between a gate and an expander is analogous to the relationship between a limiter and a compressor. A gate is an expander that reduces signal below a certain threshold by a ratio of ∞:1, effectively muting the signal unless it peaks above the threshold. You'll find gates employed in noise-suppression circuits, but they're also highly useful when mixing drums in a live setting. A well-gated snare drum jumps out in a mix while cutting down on bleed from the other elements of the kit through the snare mic. When we add a gate to our dynamics curve, this is the result:


So when you're trying to decide between, for example, the dbx 266XS Dual Compressor and Gate and the dbx 166XS Dual Compressor Limiter Gate, hopefully this little primer will make the difference between the two units a little clearer, and why exactly you'd want to spend an extra $100 to add limiting to your compressor / gate.