One aspect of mixing that is often misunderstood is that of controlling dynamics with on-board compressors, limiters, and gates that are prevalent on almost all digital consoles. Outboard dynamic processors can seem even more daunting, with their multitude of controls.
Many newer consoles have made it easy to get started by providing templates that can be selected for different instruments. But rather than rely on someone else's idea of proper settings, you're much better off if you understand what the dynamic processing is doing, instead of just dialing-in a template that may or may not be applicable to your situation.
Let's explore the various functions and controls, so you know what they actually do - and how to use them to improve your mix.
REDUCING GAIN
The Compressor
The compressor is the heart of most dynamic control offerings and, in fact, is virtually the same as the limiter, but just not as aggressive.
Typical controls include Threshold, Ratio, Attack Time, Knee, Release Time, Make-up Gain, and in a larger desk, or an elaborate piece of outboard equipment, there may be more selections.
Threshold (in dB). We'll start with this. Threshold determines the signal level that causes the compressor to ‘kick-in.' Unlike an equalizer, a compressor does nothing until the threshold has been exceeded. At that point, it begins to reduce the gain of the channel, subgroup, or output – wherever it may be patched.
Ratio (in proportion). The amount or proportion of level reduction is controlled by the Ratio setting. A 2:1 ratio means that for every 1 dB of signal level over the threshold, a 0.5 dB reduction in gain will occur. A 10:1 ratio means that a 10 dB reduction in gain will take place for every 1 dB over the threshold.
Attack Time (in ms). This control determines how quickly the compressor reacts after the threshold has been crossed. A short setting will compress fast transients such as percussion instruments, or the plucking of a banjo, and breath ‘pops.' A longer setting will let the initial transients pass through unaffected, then begin to clamp down to keep the overall level from becoming too loud.
Knee. Not all compressors have a knee adjustment, but most modern digital compressors will offer several choices, or even user-defined choices. Knee refers to how rapidly the gain reduction established by the Threshold and Attack controls will reduce the level. A ‘hard knee' will act almost instantly, while a ‘soft knee' will take longer to reach full gain reduction. The effect of the Knee occurs only after the Attack time has been exceeded.
Release (in ms). The release control determines how fast the compressor ‘recovers,' that is restores to full gain, after the threshold has been exceeded. It may, or may not, also be affected by the
Knee setting. There are things to watch out for. If you set too fast of an attack and release, lower frequency notes may cause the compressor to “pump,” that is to respond to every note played on a bass or other low register instrument, causing a very undesirable effect.
Make-up Gain (in dB). Make-up gain allows you to restore the overall output of the compressor if it's been set to heavily compress, which will reduce the output gain (sometimes considerably), depending on the settings you've chosen. This control let's you restore gain to the signal path.
As you can see, all of these controls interact together. It can take some time and experimentation to learn how to derive the audible results that you're seeking.
What are compressors used for?
Compressors can assist greatly in leveling, or smoothing out, an uneven performance by vocalists or instrumentalists, usually at 2:1 or 4:1 ratio settings. They can be set so that only very loud transients are reduced in level, such as a dynamic mic that's plugged into a channel with phantom power turned on, or an extremely loud snare hit. They will also tame feedback, not by stopping it, but by keeping it from become a run-away train until everyone has their hands over their ears.
Limiters are essentially the same as compressors, typically equipped with the same set of controls, but set to much higher ratios, such as 20:1, 40:1, or even Infinity. A limiter is often used to protect the sound system from overdrive, to keep from saturating a device downstream (such as a recorder or signal processor), and/or to ‘clamp' the overall output of the sound system if there is a need to not exceed a given operating level, such as in areas that require a noise abatement plan. Many compressors do dual duty as limiters, depending on the range of their controls.
Look Ahead Limiting
In this digital day and age there exists ‘look ahead' limiters. Such types sense a high-level transient before it can be heard and then apply the limiter to suppress it. As cool as this is, the downside is that some latency must be introduced to provide the timeline for the limiter to act before the program material has reached the loudspeakers, and thus the listener's ears. In the case of low frequencies such as 42 Hz (the lowest note on a four-string bass), the latency will be around 24 ms (1,000/42), which is usually far too long for most normal applications.
Multi-band Gain Reduction
A multi-band compressor or limiter is a marriage between one (or more) EQ filters and a compressor or limiter. You can usually set frequency points and divide them into segments of the audible spectrum. Let's say you have a decent two-track recording but the bass guitar has places where it dominates, and you can no longer ‘fix it in the mix.' If you to use a single-band compressor/limiter, whenever the bass exceeds the threshold, all of the spectrum will be compressed or limited. But with a multi-band compressor you can set it so that only everything below (or above) a certain frequency will be compressed/limited. These stand-alone units are usually pretty high-end and some can cost a pretty penny. That said, even inexpensive digital consoles often have a provision for this.
GATES: ELIMINATING CROSSTALK AND NOISE
Gates are quite simple. They have a threshold control, as does a limiter or compressor, along with attack and release controls. But they must be set up very carefully so as not to ‘chop off' a quiet note, vocal, or percussion strike that should be heard.
Their purpose is to reduce the normal crosstalk that occurs among the mics on a drum kit – or even all the mics on the stage. They are also useful for gating hum and noise that wasn't dealt with by other means, such as transformer isolation or proper AC grounding.
Some engineers are enamored with gates and wouldn't want to do a show without at least gating the drums. In my experience I've rarely gated any drum kit, or stage, arbitrarily. I will use a gate if a floor tom or kick (or whatever) is ringing out too long after the initial impact. The gate tightens up the offending drum, or perhaps the whole kit, but each gate must be precisely set so as to not eliminate a wanted signal from an unwanted one. It's a good idea to use a compressor before the gate, to even out the signal levels that the gate is responding to.
Multi Band Gates
Of course there are even multi-band gates much like multi-band compressors, but woe be he or she who wants to spend hours adjusting them, only to find that the band is ten times louder in performance than in rehearsal.
De-essers
Many vocalists have a tendency to over pronounce the upper register of their voice. Coupled with an overly bright sound system the exaggerated high frequencies can hurt the ears and ruin an otherwise sweet sounding vocal. Enter the De-esser. It functions largely as a compressor/limiter, but only in the upper frequencies. There seems to be some magic in matching the perfect De-esser to a given vocalist, as least in the outboard analog world. In digital, many parameters are controllable, making it easier to obtain the results that are desired.
Careful control of dynamics, be it live, in a recording, or a broadcast scenario, is one of the most powerful tools you can use to make your mix sound truly professional.
