Limiters (peak limiters, protection circuits)
Most common is a digital plugin. Plugins tend to be much faster, cleaner, and have less overshoot than what you get in the analog-domain.
When you look at an equipment roster of a high-end mastering studio, a compressor will more likely than not be seen in any of the studio’s analog gear. It seems that DSP (digital signal processing) plugins do an excellent job of recreating the dynamic-range control that happens in an analog compressor, yet they don’t quite seem to sound just as good.
What could be the reasons for this? Let’s speculate a little.
When audio comes out into the analog-domain, you get added distortion and noise. These are not necessarily characteristics that will be programmed into the digital circuits (or, algorithms). As a result, you get a subtly different overall presentation of the sound.
The detection circuit (the device used to tell the compressor when to compress the audio passing through it) is what really drives the action of a compressor. Another possibility, which was proposed to me by George Massenberg, is that sample rate for the detector circuit in a digital compressor needs to be much higher than the typical sample rates we are using now, because of the nuances that you typically get at the output of a compression stage. 44.1kHz may be sufficient for the audio passing through the compressor, but it may not present enough detail for the audio that is feeding the detection circuit for the compressor to do as good a job as its analog counterpart. It is speculation, but it is an interesting point to consider.
A lot of the time you will find mastering engineers using an analog circuit not because they’re going to use an EQ to equalize, or a compressor to compress, but because there is something about the filtering that takes place when running audio through that analog gear that changes the sound in a desirable way. So, a great compressor may be used not to compress, but simply due to the tone-shaping sound that is imparted to the program. This seems to be a common factor missing from many current DSP equivalents.