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Recording Techniques (Part 6)

The Mixing Console (2)

Part 6: The mixing console. David Mellor continues his close examination of the recording studio's most complex and expensive piece of equipment.

The mixing console has probably the steepest learning curve of any item of studio equipment. It has so many functions and so many possibilities, and so many knobs and switches that it is bound to take some time to come to terms with. The good news is that once you know one mixing console thoroughly, it's a lot easier to learn your second, easier still the third, etc... Mixing consoles are all slightly different, but in any one sound engineering field - studio, PA, theatre, etc - the functions have to be similar because the various makes of console all have a similar job to do.

In last month's installment I got as far as describing some of the various types of EQ (equalisation) that may be found on a typical mixer. EQ is a very subjective area and two mixers which appear to be identically specified EQ-wise may sound very different. In fact, assuming a satisfactory performance in the areas of noise, distortion and crosstalk etc, it is the equaliser section that is of predominant importance in determining the sound of the console.

But the functions that the console provides are just as important as the overall sound - more so for some applications. There are so many consoles available that it's impossible to go through them all, but the description that follows is of the features that are common to most. For now, let's see just what the console is supposed to do; next month we'll start doing it for real.


Figure 1. Auxiliary sends. The switches follow the convention where the legend on the button shows what happens when the switch is pressed in. The legend above the button shows the situation when the switch is left out.

Following the signal path further down the channel strip we find the auxiliary send controls. These are extra signal paths out of the console, separate from the main audio outputs. Figure 1 shows what a typical aux send section might look like. Figure 2 shows the internal view of what is happening. But why do we need auxiliary sends? What can they do that can't be achieved with the normal group outputs?

Each auxiliary on a console is like a separate mixer, independent of the main mix set up on the channel faders. Whatever the position of the faders, a completely different balance of sounds can emerge from each of the auxiliary outputs. So a mixer with four auxiliaries can provide five completely different mixes of the same sound sources. Four auxiliaries, by the way, is a good number to have for a small console, eight is better and more versatile for a larger model. PA engineers probably wouldn't be satisfied with 20, but that's a different story.

The first and foremost use for auxiliary sends in a music recording studio is for foldback. A group of musicians in the studio will want to be able to hear themselves and each other very clearly as they play. They will also need to hear any tracks already recorded on tape, and perhaps a click track to help them keep time. For this they will each need a set of headphones. The foldback is simply the signal supplied to these headphones. Auxiliary 1 can be used as the foldback send on each channel. (Sometimes an auxiliary is actually labelled 'Foldback'. However, it is still just an ordinary auxiliary and can be used for other purposes). The guitar, bass, drums, keyboards and whatever are mixed using the Aux 1 controls to provide a musically balanced signal completely independent of the levels of the signals going to the multitrack tape recorder. The balance between the instruments in the foldback is purely for the musicians' benefit, since it will not affect what is recorded on tape. It's up to the musicians to decide amongst themselves how loud to have each instrument in their headphones. The overall level in the phones is determined by the auxiliary master control.

Musicians, as any engineer will know, are never satisfied with the foldback balance. So far, I have assumed that one aux send will provide a foldback signal for everyone concerned. But it is more than likely that each musician will have his or her own preferences and two or more auxes will have to be used to provide different foldback mixes (requiring a different headphone amplifier for each mix, of course).

The one thing that must not happen with foldback mixes, once they are set to the musicians' requirements, is for the levels of the instruments to change. You may, for the purposes of the recording, want to adjust fader levels as the players become more enthusiastic and confident in their parts, to set the correct level going to tape. This means that the aux sends used for foldback must be prefade. In other words, the signal is taken from the channel from a point in the circuitry before the fader. On pre-fade, the setting of the fader has no effect on the level of the signal going to the aux output. The signal routing for pre-fade can be traced in Figure 2.

Figure 2. Auxiliary circuitry.

The other major use of aux sends is for effects, principally reverb. If Aux Sends 1 and 2 are being used for two foldback mixes, Aux Send 3 will very probably be patched into the reverb unit, although you can of course use any auxiliary for any purpose. Particularly during mixdown, a separate balance of signals is sent to the reverb unit using the aux send. This time the level coming from each channel to the reverb should be controlled by the fader, as well as by the aux send control. Usually, if you fade a channel down you want its reverb to die away too. This is done using the post-fade setting on the auxiliary. The signal is now taken from a point after the fader, so there are now two positions in the channel module - the aux level control and the fader - where the level of the aux send is determined. This is also shown in Figure 2. The best mixing consoles have a Pre/Post switch for each aux, so that you can combine pre-fade and post-fade sends any way you like. To cut costs however, many consoles have one switch per pair of auxes, which is a bit of a nuisance.

The aux master, one of which is provided for each aux send, is the master level control for each aux bus. If you want to change the overall level of the signal going to the foldback or reverb, then adjust the aux master. If one channel needs tweaking, use the individual aux control on the channel.


"Hang on a minute, he's missed a bit!" Well, yes I have. I've postponed the explanation of the pan and routing section, which is positioned above the fader, until a bit later. For the moment, I'm sticking within the channel module before describing how it slots in with everything else. And when you come to think about it, the fader is the next step in the signal chain anyway. The signal doubles back, physically if not electronically, before hopping on a bus (see last month's issue for info on mixing console buses).

The fader simply controls the level of the signal in the channel, and that's it - almost. Among all the controls on the console, the fader is the only one that you operate in a straight line. The simple reason for this is because it allows you to see at a glance the relative levels of all the channels. Mixing with rotary controls would be a pain. Before linear faders were invented there were things called 'quadrant faders', where the knob moved in a circular path - a vertically orientated circle - over electrical stud contacts. Apparently these are still popular with engineers who work to picture, because you can feel where they are without having to look at them.

The fader's job is to control the level of the signal between minus infinity decibels and +10dB. No fader yet made can reduce the signal level completely to zero, even at its lowest setting. Expensive consoles have microswitches built into the fader so that when it is brought all the way down, the signal is physically switched off.

Figure 3. Fader internals.

Figure 3 shows how a fader works electrically. The signal comes in at the top of the resistive track and makes its way towards earth - zero volts. All the way down the track the available level of the signal reduces. (It is actually amplified by 10dB before the fader. That's how the fader allows you to boost the level, even though the mechanical part of it can only reduce it.) The sliding contact on the fader picks off the signal at a point along the track, and the position will determine the signal level.

It's probably a good idea here to point out that faders are very sensitive to dirt. It's important that fades are smooth and click-free. If the sliding contact has to climb over a lump of cigarette ash then it probably won't be. Keep the ashtray, and your can of Coke, well away from the faders. Also, when you are cleaning the console, brush the dust away from the fader slots, not into them.


Just above the fader there will be a solo or PFL button. PFL stands for 'Pre Fade Listen' and is sometimes verbalised as 'piffle' by jargon mongers in the trade. There are several ways in which this button can operate. Sometimes a console offers just one (possibly wrongly labelled) way, whilst top class consoles may have several options.

'PFL' (Pre Fade Listen) is the simplest system. If the button above the fader is labelled 'PFL', then pressing it will allow you to listen to that channel alone on the monitor speakers, at a fixed level unaffected by the fader. You may press the PFLs on several channels simultaneously if you wish, but they will all be mixed at their top-of-the-fader levels.

'Solo' is where you hear the channel with its fader setting intact. This is sometimes known as AFL (After Fade Listen).

'True Solo' is an interesting concept. PFL and AFL work by switching the channel's signal from the mix bus to a separate solo bus, the monitor output being switched to this bus also. True Solo doesn't use an extra bus. If you press the button on Channel 1, all the other channels will be muted, thus allowing you to hear Channel 1 only. If you are mixing when you press this type of solo button, then you wreck the mix. (It would be worse still if you were broadcasting!) The other types of solo are non-destructive. The advantage of True Solo is that there should also be a solo safe button on each channel. If you press this on all your effects returns, then soloing any channel will give you that channel in its correct place in the stereo image and with its reverb and other effects intact. True Solo may not be usable in all circumstances, but it is very powerful when used with care. I wish more consoles offered it.

Figure 4. Signal flow between channels, groups (subgroups), and masters.


It's time now for the signal to emerge from the confines of the channel module. But where should it go?

Figure 5. Signal routing.

These days, most medium size consoles have a number of channels, eight or 16 groups, and a stereo master output. The signal flow is shown in Figure 4. As you can see, you can route the signal from the channel directly to a group and/or to the master, or to a group and then from the group to the master. When going from channel-to-group-to-master, the group is usually called a subgroup. It hasn't actually changed physically but it uses the technique known as 'subgrouping', so when I refer to groups and subgroups I am talking about the same thing, but operating differently.

The routing section of the typical console will look rather like that in Figure 5. Notice the arrangement of buttons: Mix, 1-2, 3-4, 5-6, 7-8. 'Mix' will be used, obviously, when mixing down to stereo. It is also used for monitoring, because in many consoles the mix bus and the monitor bus - both stereo buses - are one and the same. '1-2' means that the signal passes through the pan pot (short for 'Panoramic Potentiometer') before passing to Bus 1 and Bus 2. The pan pot is rotated to the left to send the signal to Group 1, to the right to send it to Group 2. If the control is central, then the signal goes to both buses in equal amounts. All the routing buttons work in this way.

A step up from this arrangement is where each bus has a separate button. This gives greater flexibility and better crosstalk and noise performance. Of course, it costs more.


Figure 6. Group faders and subgroup controls.

In Figure 6, you can see the group faders and associated subgroup paraphernalia. As I mentioned earlier, it's a matter of usage. One would talk about a group when the signal is leaving the console directly after the fader, a subgroup when it is going on somewhere else before emerging from a different output.

Groups only have faders, and perhaps solo buttons. They are used simply to control the level of a mixed group of channels. (See where the name comes from?) To use the groups as subgroups and mix them into the masters, you need to press the 'Subgroup' button. The level and pan controls will now act on the post-fader output of the group, as in Figure 7. You would use subgroups like this if you had a large number of microphones on a drum kit which you had carefully balanced. If you mixed these into stereo, ie. routed them to a pair of subgroups, then you could adjust the level of the kit as a whole with just two faders, rather than having to painstakingly adjust each individual channel fader. More on this topic later in the series.

Figure 7. Subgroup circuitry.

The output from the groups is the point where you must look at the level of the signal. Too much level going to the multitrack will result in distortion, too little will mean excess noise, so effective metering is called for.

There are three sorts of meters in common use on mixing consoles: VU, PPM, and LED bargraph. The one thing these all have in common is that they should preferably be in alignment with the meters on the multitrack tape recorder. If this is so, you need never look at the meters on the multitrack, just at those close at hand on the mixing console - your workstation if you like.

The letters 'VU' stand for 'Volume Units'. VU meters have been around for decades and show the average level of the signal. This is rather a pity because it's the peak level of the signal which we need to keep a close eye on to avoid distorting the tape. Impulsive sounds like drums will give a very low average reading compared to their true peak levels. You need to interpret the readings VU meters give if you are to produce satisfactory results. Because of the uncertainty involved, VU meters have been dubbed 'Virtually Useless'! Like looking through frosted glass, you can see that something is happening but you are not sure quite what!

PPM stands for 'Peak Programme Meter'. These meters, as you might guess, give true peak readings. They have fast-moving needles which are powered up their scales by special driver circuitry. PPMs are very good, very easy on the eye, and necessarily fairly expensive. VUs and PPMs are distinguished by their markings: the VU meter (typically) has a buff coloured background and a red 'danger zone' at the top of the scale; the PPM (typically) has a black background with a white needle and all-white calibrations.

The LED bargraph, with PPM characteristics, is an alternative to traditional types and is almost standard these days on low and mid-priced equipment. The more the equipment costs, the more LED segments you get in each bargraph, and therefore the more accurate the resolution. Top notch consoles are treated to bargraphs that are probably more expensive than the PPMs they replace. These are for engineers who can't afford to take chances.


Figure 8. Master section controls.

The Master section of the mixing console has two faders - left and right - and all the extra twiddly bits that a good console, in fact any console, needs. The left and right master outputs feed the stereo tape recorder. The monitor outputs go to the power amp and speakers so that you can hear what's going on. Figure 8 shows these extra bits and pieces:

Mix/Tape switch. This lets you hear the stereo output of the console or the output of the stereo tape recorder through the monitor speakers.

Talkback allows you to speak to the musicians in the studio, perhaps through a separate output from the console or maybe through the auxiliary outputs being used for foldback.

Slate lets you make an announcement on the tape, multitrack or stereo, such as "Take 32, introduction - yet again." Often the slate facility records a very low frequency tone on the tape as well as the speech, so that when you are fast winding the tape, this tone can be heard raised in pitch. This helps you find your way between takes.

Oscillator. This is for putting reference tones on your master tape. More on this later in the series.

Monitor Level, Studio Monitor Level, Headphone Level I think need no explanation.

Dim turns down the monitor level by a set amount, usually around 30dB. It is best always to monitor at the same level during the session, for the sake of consistency. The Dim button makes it easy to cut the level by 30dB when you need a quick verbal consultation, then reset to exactly the level you had before.

Figure 9. Monitor section controls.


The final section of the console is the monitor section. In the split monitor style of console, this is a section to itself located to the right of the input channels, usually above the group faders. In an in-line console, the monitor controls are located in the channel modules. This is less easy to explain, but let's say for now that they achieve much the same thing - they enable you to listen to the tracks you have already recorded on the multitrack tape while overdubbing, and also let you make a rough mix as you progress.

A typical monitor section is shown in Figure 9. As you can see, it is a mixer in its own right with level, pan, and auxiliary sends. The aux sends are there so that you can send the signals from the multitrack to the foldback, and also apply reverb if you fancy it. The outputs from the monitor mixer go to the main stereo mix bus, which means that you can use these monitor inputs as extra signal inputs if you wish. Just the job for those additional MIDI-synchronised synths and samplers.

Now that I have covered some of the mixing console basics, it's time to start looking at how the console is used in practice. Next month, I shall be examining the procedures and techniques involved in running a recording session with several musicians, and acoustic and electric instruments - a band in other words. This is where the fun starts!


Read the next part in this series:
Recording Techniques (Part 7)

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As Easy As EZE

Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.


Sound On Sound - May 1990

Donated by: Bert Jansch / Adam Jansch


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Recording Techniques

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7

Feature by David Mellor

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