The two types theme continues here with digital and analogue. Digital desks have been getting a bad press lately, but largely because people have been comparing the very best analogue with budget priced digital. That is rather like comparing a vintage Ferrari with a modern hatchback.

But the truth be told, there is something wonderful about sitting behind a vast expanse of knobs, buttons and blinking lights, that a computer screen or a touch screen working surface just cannot give you. There is a sense of event about the session in the large studio with the large analogue desk, that the clinical feel of a digital operation cannot achieve. This is often enough to raise the game of the artist to the point where he or she gives that special performance. If that is all the analogue desk ever did, it would still be worth the money.

The Analogue Mixing Desk

The beauty of the analogue desk is its simplicity. Every input channel is the same and every button has just one function. There may be dozens of knobs and buttons on each channel strip, but once you have worked out what each one does and you have worked out what is what on the centre monitor section, you can use the desk and you are on your way.

The sight of an analogue desk would have some people laugh at the idea that they are simple. On the desk in our control room there are 70 knobs and buttons, nine control lights (LEDs) and a large peak meter for each and every one of the 60 channels. That is a grand total of 4,200 buttons and 540 LEDs, not counting the eight stereo inputs and the monitor and mix-down section in the middle. But these are all logically grouped together in such a way that operation is made easy.

The design that has evolved is so elegant that every virtual desk in every computer based system does its best to copy the layout of a classic analogue desk.

Each strip is divided into sections, usually with routing at the top and the main channel fader at the bottom. The purpose of the mixer is to collect all the signals from the various sources and send them on their way to a whole host of storage and processing devices such as multitrack recorders, effects, headphone amps and studio monitors. The largest studio desks designed for use with 48-track recorders and with the ability to tie in many effects and different headphone mixes can have nearly 70 connections for each channel strip.

Some of these connections link to the same (i.e. corresponding) connection on all other channel strips. The mixer does this via a common connection known as a virtual earth bus (also known as a zero-ohm bus). This is a common rail or bus that allows additional signals to be added to the rail without effecting the volume (voltage) of the other signals.

Before the invention of the virtual earth bus, if you brought up the volume of one signal, the others were reduced. If you brought down the signal from the lead vocals, all the other signals coming into the desk were slightly increased. The operator had to quite literally balance the inputs and earlier, audio engineers were actually known as balance engineers.

Internally, the larger studio versions are anything but simple and designing and building a sophisticated desk has rightly been compared to knitting fog. But at the heart of the analogue desk, two simple devices make the whole thing work: the semiconductor and the capacitor.

A semiconductor takes an incoming signal and alters a supply current to become a copy of this signal, but usually at a higher voltage. One can compare this to two streams of water flowing next to one another. One stream is small and thin and the other is large and robust - but level of the small stream is used to operate a sluice gate that regulates the flow of the larger stream. As the small stream rises, the sluice gate rises and the large stream is allowed to rise. If the small stream falls, then the sluice gate falls with it and the flow of water in the large stream is cut back.

The amplifier works the same way. The signal is collected into the op-amp via a connection called the collector. Here the base supply current is allowed through as a direct copy (or analogy) of the original signal and emitted from the op-amp via a connection called (you guessed it!) the emitter.

(Yes, I know that this is a gross over-simplification and for those who are interested in the magic World of electronics should definately do some further reading.) 

It is this ability of an amplifier to create an analogy of an incoming signal that has given us the phrase analogue technology.

The other little gizmo that makes it all work is the capacitor. This is a passive element that lets high frequency signals through, but does not like lows. In technical terms, a capacitor has an impedance that is inversely proportional to frequency. The lower the frequency, the higher the impedance.

So you see, if we just shove a capacitor in the signal path, we create a so-called high-pass filter (the highs can pass, but the lows are blocked). But if we short out the signal path via a capacitor, only the highs will have been shorted out and the lows get through. We have built a very simple low-pass filter. By combining capacitors of various values with op-amps, we can build filters of all different values and characteristics. For example, if we combine a high-pass that filters off all frequencies below 500Hz (500 cycles per second) with a low-pass that filters off all frequencies above 2kHz (2,000 cycles per second) than we have built a band-pass filter for the band 500Hz to 2kHz.

Multiple combined filters placed in a single signal path to alter different frequencies are called an equalizer (eq) and all recording desks have some kind of eq on their input and/or mixdown channels. On the simplest desks this can be just two controls for highs and lows. More advanced forms of eq are parametric and semi-parametric equalisation. A parametric equalizer has three controls, one for cut/boost, one to select the width of the range of the control and a third to select the frequencies. A semi-parametric equalizer does not have a width control.

On larger desks, the parametric eq is combined with shelf filters in which the operator can choose a frequency, above or below which (depending on whether it is a low-pass or high-pass filter) a second control sets the cut or boost level. Often these shelf filters can be switched to act as semi-parametric filters.

Graphic equalizers are often found as part of domestic hi-fis, but are used in the studio to adjust some monitors and sometimes as effects. Graphic eq's have a collection of faders, usually at least ten and each fader is for a set frequency. This allows the operator to visually create the kind of audio frequency curve he or she is looking for - which is the very obvious reason that they are called graphic.