The traditional mixing console has a separate set of controls for each channel. So a 72-channel console that would be typical for a well set up commercial recording studio would have seventy-two identical channel strips, each with routing, EQ, dynamics, auxiliary sends, pan, solo and faders. Did I miss anything?
In the glory days of analog, this was the only way a console could be constructed. Each knob on the control surface is connected to a potentiometer beneath, through which the signal flows. It's a simple mechanical link that cannot physically be extended. The same applies to switches. So the knobs and buttons have to be positioned directly above their associated electronics. The law of 'one knob per function' applies.
The main problem with this is that the console inevitably becomes very wide. So if you have a group of background vocals on low numbered channels, you have to physically move a long way – and when you do it a hundred times during a mix, it is a long way – to adjust those channels. And when you are close enough to adjust them, you are away from the 'sweet spot' between the loudspeakers and are not best placed to make good sonic judgments.
The other problem is that to duplicate the same sets of controls 72 times seems wasteful. And it pushes the price of such a console to astronomical levels.
The answer to this is the 'assignable console'. In an assignable console, there are channel strips as before. But now they only have the very essential functions that you need instant access to, the fader being the most important. You don't need as many channel strips either. For instance, 24 strips can be assigned to cover as many channels as the console has electronics for – perhaps up to 240.
Functions such as EQ and dynamics do not need to be replicated for every channel. One set of controls can be placed in the center of the console, directly between the loudspeakers, so the engineer can remain in the sweet spot no matter what channel he or she is adjusting.
There were attempts in the analog days to design assignable consoles. Only the ones by Euphonix were any real success, and it required the development of digital mixing consoles to make assignability truly practical. In a digital console there is no physical link between a control and its associated electronics. There is infinitely more flexibility for control layout.
Oddly enough, some digital consoles still follow the 'one knob per function' scenario. But these are probably the last of the breed. Assignable consoles such as the Digidesign ICON are rapidly becoming the norm.