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Mixer KnobsArticle from Making Music, July 1987 |
What a confusing bank of knobs, switches and dials on that mixer, eh? We sort them all out. S'easy
Always an inquisitive sort of chap, Martin Sheehan found out what all the mixer's knobs do.
THE RECORDING MIXER is a clever thing, though not as mysterious as it may at first appear. The apparent complexity of all those knobs can cause the casual observer's jaw to gape. The awe-struck onlooker can be calmed to some extent by the explanation that most of the rows of knobs perform the same tasks but on a different sound or instrument.
The rows themselves can be divided into three basic types: Input; Group & Monitor; and Master. These types of row relate directly to terms such as "8 into 4 into 2" or "24-16-2" which are used to describe the number of each type of channel sported by a particular mixer. 16-8-2 implies sixteen input channels, eight groups, and a stereo pair of master outputs.
Having broken the desk down into these three basic sections, we can sort out the individual knobs and switches that go to make up our hypothetical mixer.
At the top of each input channel is the Input Gain control. The idea of this knob is to level out the strength of each type of input signal. To assist in this a Pad switch is used which has the ability to attenuate, or cut, the strength of an incoming signal by 20dB. This is necessary as there is a wide difference in original signal level between, say, a dynamic mike (low) and a keyboard (high).
A Phase switch may also be present near the top of the input channel and this is used to invert the polarity of any signal which may be coming in 'upside down'. This can happen with incorrectly wired microphones or some bohemian makes of equipment. It is most noticeable as a loss of bass when a pair of mikes is being used on the same source or where an instrument is being both miked-up and DI'd (directly injected into the mixing desk). A switch labelled Phantom may also appear near the top of each input channel and is used to supply power to condenser microphones (not to activate a ghost in the machine).
Now comes the Equalisation (EQ) section, which may be as simple as a three-band high, mid and low system, or might boast sweep EQ on the mid section. Sweep EQ involves a pair of knobs for each band, one to select the amount of boost or cut and the other to select the frequency at which it takes place. An EQ in/out switch can be used to bypass the equalisation circuitry if it is not required.
Next come the Auxiliary Sends. There are two types of these, pre-fade and post-fade. Pre-fade auxiliary sends are unaffected by the channel fader and are used to feed a monitoring system which is independent of the signals going on to tape. Post-fade sends have their overall level governed by the channel fader so that when the fader is brought down, so too is the level of the auxiliary send. These post-fade sends are used to feed effects (FX) units such as echo and reverb: the reason for them being under the overall control of the channel fader is so that the level of effect will change with the level of original signal. At least one of each type of auxiliary send will be available, and on some mixers pre/post switches enable you to choose the function of some of the auxiliaries.
The Pan control and Routing switches follow. These are used to dictate where the signal will go after it leaves the input channel. Pressing the first routing switch, which may be labelled Mix or L-R, will send the signal directly to the stereo master outputs. Subsequent switches may be labelled 1-2, 3-4, 5-6 and so on up to the total number of groups available. Pressing any of these switches will route the signal to the relevant group. The pan control will proportion the signal to the left or right in the case of the master outputs and to the odd or even numbered groups as selected on the routing switches. Each group output feeds one track of the multitrack recorder. Routing a number of input channels to the same group is an easy way to adjust the level of a bunch of signals under the control of a single fader — very useful for percussion, a brass section, and backing vocals.
A pre-fade listen (PFL) button will be situated somewhere near the foot of each channel, and is used to route the signal from that channel to the monitors while excluding all other signals — very handy for tracking down naughty noises. Near the PFL button there is also likely to be a Mute switch. This is used to silence the channel without having to pull the fader down, invaluable for dropping out certain tracks from a mix which need to be brought in again later. Not having to move the fader ensures that the level of that channel will not have altered when it is reintroduced into the mix. The Fader itself completes each channel — somewhere close to the fader there may be an Overload-indicating light emitting diode (LED). This LED lights up just before the signal level gets too much for the circuitry to handle.
These facilities will be duplicated anything from four to 48 times, depending on the input size of the mixer.
These sections on the desk won't have as many knobs as the input sections but their functions can seem less straightforward.
A group/tape button, which may be labelled Tape Return or Tape Monitor, switches the monitoring between either the group output or the multitrack tape input. Below this switch is a knob which controls the level into the stereo monitor mix of the selected signal.
More Auxiliaries will follow, both pre and post-fade, for use as monitor and FX sends respectively. As with the input channels, some of these group auxiliary sends may be switchable pre or post. A PFL switch will be available exclusively to monitor each group, which is completed by a fader controlling the overall signal level from that group, normally to its respective input on the multitrack.
The third section of the mixer will be situated either at the right hand end of the desk or between the input channels and the groups.
A Master Send for each auxiliary controls the overall output level of that auxiliary to the foldback amps, effects units and so on. Each master auxiliary send is likely to have an associated AFL (after fade listen) button which allows its signal to be monitored. Auxiliary returns may be present to mix effects back into the stereo master outputs, and each return will have an associated pan control to position its signal between the left and right channel. The two faders at the bottom of the master section control the level of the main stereo outputs.
There is a variety of other controls likely to be dotted around the master section of the mixer. A talk-back switch, with associated microphone socket and level control, allows the engineer to talk to the artists in the recording area. A slate button sends a controlled level of tone to the different sections of the mixer for line-up purposes; a 2-track Replay or Monitor switch will route the output of the mastering machine into the monitors; and a Mono switch may also be provided so you can check the mono compatibility of your stereo mix.
Considering that all mixers do basically the same thing it is amazing how dissimilar the models from different manufacturers can appear. This is a lot to do with different approaches to layout, and also the fact that the same knob can have as many different names as there are manufacturers. Next time you come across an unfamiliar mixer, the puzzle of the knobs will soon seem less of a riddle.
Feature by Martin Sheehan
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