There are a number of different types of compressors that every engineer is familiar with, but there’s not a lot of information available about the differences between them and what they’re good at. Most compressor plugins today are based on the four different electronic building blocks that could be used to build a compressor back in the analog hardware days. Here’s a brief look at their characteristics, and what we can do with them.
Optical Style (like the (LA-2A)
A light bulb and a photocell were used as the main components of the compression circuit. The time lag between the bulb and the photocell gave it a distinctive attack and release time. Slow attack and release, use when large transients aren’t present (like vocals). Transparent, tightens up track without being noticed. Adds warmth. Limitations: Won’t control transients, pumps with low end content.
FET Style (like the 1176)
A Field Effect Transistor (FET) was used to vary the gain in the old analog hardware version, which had a much quicker response than the optical circuit. Very fast attack and release with lots of control. Agressive sounding. Best for punch and snap, but adds the most color. Very warm and rich sounding, but won’t work on the mix buss. Limitation: not transparent.
VCA Style (like the dbx 160 or SSL buss compressor)
A Voltage Controlled Amplifier (VCA) circuit was a product of ‘80s technology and had both excellent response time and much more control over the various compression parameters. Aggressive sounding with extreme settings and extended parameter functions. Works on the peaks of the program. Use when the transients are already controlled (mix buss). Excellent for adding punch to drums and buss compression. Doesn’t smooth out volume. Limitation: Doesn’t smooth out volume. Can be thin sounding.
Vari-Gain Style (like the Fairchild 670 or Manley Vari-Mu)
The Vari-Gain compressors are sort of a catch-all category because there are other ways to achieve compression besides the first three. Takes time to react, which adds a “glue” to the mix (like the mix buss or subgroup). Ratio increases with gain reduction. The louder the transient, the harder it’s compressed. Good for a mix buss or to add warmth and fatness. Limitation: Slow attack and release. Won’t solve dynamic issues or increase the punch.
Non-Emulation Digital Compressors
While it’s great to make the compressor plugin based on a previous hardware model, today’s digital compressors can do so much more. Almost every developer now has a compressor that goes way beyond what a hardware emulation can do and that results in parameters and sound that never could have been achieved in the old hardware days. Limitation: While some can provide hardware emulation, not many are particularly good at it.
As you would expect, each of the above has a different sound and different compression characteristics, which is the reason why the settings that worked well on one compressor type won’t necessarily translate to another. The good thing about living in a digital world is that all of the different hardware compressor types have been duplicated by software plugins, so it’s a lot easier (not to mention cheaper) to make an instant comparison on a track and decide which works better in a particular situation.