Mixers – Knob city or simple switches? Howard Turner from Studio Wizard gets on the Busses…
Once upon a time everything was recorded in mono. With one mic. How simple was that? A mic, a mic amp, a recorder. Then someone in the 30’s decided that it sounded better if he also laid some close mics on the soloists to bring the solos up in the recording. That meant some signals had to be mixed together, and the mixer as we know it was born.
The earliest mixers had just two purposes; 1) They allowed you to introduce more than one input, and send them out to more than one destination. So they had to ‘route’ signals. 2) They also allowed the relative levels of these multiple signals to be adjusted with ‘faders’ so that the output wasn’t too loud or quiet.
Oh, then someone decided that adjusting the frequency response of the signal might be a good idea and equalisation or ‘EQ’ was invented – although even in the days of the Beatles the EQ was a switch marked ‘pop’ or ‘classical’ – in classical mode the signal went straight through, in pop mode all frequencies above 5k were cut…
3 track Multitrack tape came along in the early 60’s and brought with it the concept of recording some signals whilst listening back to others, and the mixer started to look slightly like the ones we still see today, with sections dedicated to the functions it has to perform; inputs, groups and monitoring, each with identical controls for all the different in’s and out’s
The path is clear.
So there are two sound sources in a studio: the current live performance, and the stuff playing back from the recorder that has already been done. There are also two destinations: the recorder to record the current performance as a ‘overdub’ on another track, and the main mix output, where engineer and performer get to listen to a rough approximation of the final result (the ‘monitor’ mix).
So the mixer is divided up thus:
The Input Section, where the inputs come in.
The Monitor section, where the pre-recorded parts are listened to.
The (sub)Group section, where signals are adjusted for level before they go off to the recorder.
The Master section, where the final mix output controls are lumped together with all the other bits of the mixer that haven’t got a home, like Control Room volume, Aux outputs etc…
Lets look at a simple desk layout –this desk has 3 inputs, 2 sends to tape, 2 tape monitors and a single mono output.
Diag 1. A basic mixer. A format that carries on to this day and is still extensively modelled in software and digital products.
The long wires that the inputs and outputs feed their signal onto, or take their signal off of, are the ‘Busses’. A name whose origins date back to the heavy metal ‘buss bars’ that went up and down the length of old industrial revolution factories.
The earliest desks had these sections split up in their layout, generally thus:
Diag 2. The ‘Split’ format mixer. Easy to understand, but quite wasteful of space.
And here’s one we made earlier. A rare example of a compact split format console the Allen & Heath Mix wizard 20-8-2 – clearly showing the separate sections for inputs, outputs, tape monitors etc, although the group faders have been omitted to save space. (photo courtesy http://www.allen-heath.com)
Big enough to sleep on! Split consoles are so space-hungry that this type of design was generally used in large studio consoles, like the MTA980 here, although even here the in-line design is now king. (Photo courtesy www.toftaudiodesigns.com)
Not Fade Away.
We admitted earlier that performers may need to listen to a mix so different from that which the engineer has that a separate set of mix faders needs to be provided. These produce a mix totally separate from that made by the engineer, the source of these signals is before the engineers’ faders and unaffected by their movement – this ‘Auxilliary’ mix is therefore before or ‘Pre’ Fader.
Diag 3. Pre-Fade. The same mixer input channel with a pre-fader Auxiliary (usually on a knob, not a fader) for creating headphone monitor mixes.
Another use for an auxiliary send would be to feed signals to a reverb or other time delay effect. Now when we vary the level of the unaffected ‘dry’ signal with a fader, we expect the reverb for that signal to vary in proportion, so we need to pick up a feed for this sort of auxiliary send after (‘post’) the fader.
Diag 4. Post-Fade. It’s that mixer again, with a post-fader Auxiliary option for feeding a fader-related signal to reverb and other associated effects. On most desks today, some Aux’s are pre, some are post, and some can be switched either way…
Now auxiliaries are fine for ‘effects’ (i.e. things that take the original signal, mess with it and then are added to the mix via another mixer channel). But what about signal processing that affects the whole signal? Like a gate, or a compressor, or an external EQ? Now here we need a mechanism by which we can effectively ‘insert’ an external processing box into the signal chain, by way of, unsurprisingly, an ‘insert point’.
Diag 5. Insert point. Now we have a way to drop an external processor into the signal path, replacing the original signal with the processed version. The insert is provided by either a pair of, or single, stereo, jack – specially wired to break the path when something is plugged in. With nothing plugged, the signal passes uninterrupted. Some big mixers offer insert points both pre and post EQ. We have shown the most common position in an analogue desk – post. It must be noted that in all digital desks, the analogue insert is before the A-D converters, and thus is pre-EQ.
Lets pay a bit more attention to the input stage now. As most of you probably know, the first choice facing you when plugging a signal into a mixer is: ‘mic’ or ‘line’? The mixer will only work it’s best with a signal lying between quite distinct level (voltage) limits (see ‘Tech Terms’), so it’s the job of the input stage to boost or cut the input signal to match what the desk likes. Most ‘line’ level signals (synths, outboard, recorders etc) have outputs pretty close to line, and only need a modicum of adjustment to meet with the mixers approval – these use the ‘line’ input setting to provide a small amount of trim. Microphones on the other hand, have signal outputs a couple of powers of ten below line, and so need these tiny signals boosting heavily, and a separate specialist amp section is employed to achieve this high gain result. At this stage you will probably also encounter a ‘Pad’ which will shift the gain window of either of these two input stages down by 20 dB (to avoid distortion with louder signals), and a phase reverse switch (more of which another time I’m sure!) which (briefly) makes sure that in multi-mic set-ups the mic signal waveforms are rising and falling in unison and not cancelling each other out. Last but not least is the switch for microphone 48volt phantom power for condenser mics.
A typical mic input stage. Showing all the controls mentioned above.
The Great God Pan
In all these illustrations we’ve been looking at mono signals for simplicity. All you need to do to create a stereo mix buss is to have 2 busses; L & R and a pan pot to fade the signal between them. A pan pot (that’s a potentiometer to you) is basically 2 rotary faders ganged together, except one works backwards so that as one gets turned down, the other gets turned up. In the middle they feed equal amounts to both L & R.
Do the Splits?
Thus far we’ve also just been looking at split format mixers – but they have a big disadvantage – size. What if we incorporated input channel 1, group out 1, & monitor 1 all onto the same strip of metal? This is called an ‘in-line’ mixer, and virtually all recording mixers now follow this format. This makes the mixer around half the width and thus cheaper – and easier to work with too, once you have got your head round the fact that all the controls on each channel strip actually aren’t affecting the same signal path! It also allows, for example, auxes and eq’s to be physically switched between the channel strips input and monitor signal paths, thus making the mixer more flexible and reducing the number of actual knobs on the desk.
An’ In-Line desk’ The MTA 924 inline console featuring 24 fully specified channels and monitors – all in an amazingly compact frame. (Photo courtesy www.toftaudiodesigns.com)
An In-Line channel-strip The Soundtracs Solitaire in-line input channel in detail, showing all the different sections we have talked about combined in an in-line channel – note that a group output trim control is missing – a common omission in in-line designs. (Picture courtesy http://www.soundtracs.com)
The Telephone Exchange
As a mixer is essentially an overgrown set of switches, it is not surprising that in order to give the routing of signals maximum flexibility it incorporates a patch bay. This large array of identical jacks is effectively the sockets on the back of every single item of gear in the studio brought up into one place where experimentation with routing can be achieved without any crawling around the back of things. It also creates a stable grounding environment regardless of how the gear is actually plugged up. Without it, experimentation is awkward and the consequent unstable earthing of the studio is just inviting interference and noise.
A patchbay beside a TL Audio VTC at Bath Spa University.
Whilst we have been talking about analogue mixers so far, pretty much all of what has been said applies to digital consoles too. Why? Well because so far all digital consoles have been designed to model the internal architecture of the analogue desks that preceded them. This made good sense, as engineers didn’t have to learn a new architecture. Indeed once you have got your head round analogue console architecture, the inner workings of your 02r or whatever seem an awful lot less complicated, and the way that you can program changes to the internal architecture of digital desks reveals their true power.
Of course, digital desks ape analogue ones ‘so far’. Because it can only be a matter of time before someone with no previous analogue desk knowledge gets to design a digital desk whose architecture is driven only by the designers’ own creative experience. The first of this breed may be poorly received, if only because our previous experience of traditional desks is no longer applicable and indeed hinders our ability to work on such consoles, but to some, these new desks may well be the key to unlocking their creativity. We await developments…
Every desk will have it’s own quirks in the nature of it’s function, so this article is not intended to be a short-form manual for your desk. What it is, however, is an introduction to the underlying signal structure that exists in any mixer design, be it analogue, digital, or even virtual in your PC! Once you can sit there with your eyes closed and visualise every wire and connection, then the gear will never get in the way of your creativity ever again.
Some light reading:
Sound Recording Practice – Fourth Edition, J Borwick Ed., APRS/Oxford University Press 1994. ISBN 0-19-816381-9
Modern Recording Techniques, D M Huber & R E Runstein, Focal Press ISBN0-240-80308-6
Sound Engineers Pocket Book, M. Talbot-Smith, Focal Press. ISBN 0-240-51612-5
Line Level – When the meters on a desk read 0 (or as we call it 0Vu – that’s ‘volume units’ by the way) then you can be pretty sure that the voltage on the line will be at one of two standard voltages.
+4dBm: The old pro standard that most gear with jacks and xlr’s uses is what we call +4dBm; there’s a long winded historical reason why we ended up with this standard that we won’t go into here – suffice to say that on this scale 0Vu = approx 1.25volts RMS (ie 4dB more than 0dBm which is .775vRMS)
-10dBv: A semi pro standard you find generally in things that have phono (RCA) plugs on them like hi-fi etc. This is based on dBv standard where 0dBv=1vRMS, but in this case when the meters on the gear read 0vu, the actual signal level is tiny; 0Vu= approx 0.3vRMS, (ie 10dB less than 0dBv)
The –10 standard has been designed to allow cheap low voltage power supplies and even 9v battery power to be used in the equipment (if you think about it, it’s obvious that the signal peak-to-peak voltage has to be less than the power supply voltage, so low psu voltages can only sustain low voltage signals without distortion). The payoff in this case is that the signal level is some 12dB lower than in +4 pro gear (so they have difficulty communicating with each other without generating either noise or distortion) the noise floor on –10 stuff is 12dB poorer in theory (and probably even worse in practice) than the pro stuff. Try not to mix the two in your studio – it’ll always cause trouble somewhere ‘down the line’…
Effects – Reverb, Echo, Delay, Harmoniser etc – generally all ‘time delay’ derived effects. These take the signal, mess with it and then you add a little of this into the mix to augment the original ‘dry’ signal. These will be fed from a post fader Auxiliary and returned down a separate channel input or effects return (which is just a crude input with less options than the main ones).
Processors – Compressors, Gates, Eq’s etc. These affect the whole signal i.e. the processed signal replaces the original. These are put into the signal chain via an Insert point.
About the author
Howard Turner has over 30 years experience in the studio business, and for the last two decades his Studio Wizard Organisation has allowed him to stop shouting at musicians and going to sleep on the mixing desk all of the time, instead he gets to design studios and shout at builders for a change… Further information: 07092 123666 web: www.studiowizard.com