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In the above diagram the cables are colour-coded.
To understand the PA you must first understand the route the signal (noise) takes. A microphone is the exact opposite of a speaker. The noise coming into it generates a tiny electical current which goes down the cable into the stagebox. From there it goes to the mixing desk, then to the amplifiers, where it is made a lot louder, before being sent to the speakers where it is sent out to the audience. To get the sounds from your bass guitar, the engineer may put a DI Box (Direct Injection Box) between your bass guitar and the bass amp. It will have a small lead going to the bass amp so you can still hear your bass, and a mic' lead going to the stagebox which sends the sound to the PA. All the microphones on the drums and guitars as well as the vocal mic's will also be plugged into the stagebox along with the bass DI. All the signals from the mic's etc on the stage travel down the multicore to the FOH Desk. (NB: FOH stands for Front Of House, for obvious reasons, and backline is all the bands gear on-stage, like drums and guitar amps etc.) The mulitcore is a large cable with many different leads inside it which feeds each signal from the stage into the respective channel on the desk.
The engineer will work on each seperate channel, adding compression, gates, effects and whatever else he has handy in the FOH Rack beside his desk. He will then create a mix of all the sounds and send that back down the multicore to the stage. At the stage the signal will go straight to the Amp rack where it will go through the crossover, then the amplifiers, then to the speakers. There it is turned back into sound waves and out to the audience. There is also monitors which an engineer has to worry about. Monitors are small PA's which send their mixes to speakers on the stage which the musicians in the band can hear each other in. There can be several different monitor mixes on-stage as every member of the band might need different mixes in order to hear exactly what they need to be able to perform to the best of their ability. In bigger PA's there will be a seperate monitor desk at the side of the stage with it's own engineer so that the engineer on FOH only needs to worry about the audience.
The dreaded feedback loop.
Monitor engineers have to deal with one big problem, feedback. This is caused when the sound that the mic' is sending down the cable to the desk, is coming back out of the monitor beside the mic', then back into the mic' back to the desk, back to the monitor, back to the mic', etc, etc. This is a feedback loop and can be extremely dangerous for both the equipment and everyone's eardrums.
Almost all sound engineers work with the same basic channel listing. It almost always starts with the drum kit and works it's way up through the bass guitar, guitars, any keyboards, acoustic guitars and whatever other instruments the band have, then finishes up with the vocals. There's no rules so everyone can work their own system whatever way they choose but you'll usually find a channel list very similar to this one.
The sound desk that the engineer will be working on is a lot simpler than it looks. Although the number of controls on the desk may seem daunting you only really need to learn how one channel strip works and the rest are identical (More or less!) The masters section is usually at the top right of the desk and simply controls the master level of the sends on the rest of the desk.
There are two inputs into most channels on most desks, a microphone input (3-pin XLR) and a line input which is a 1/4 inch jack the same as a guitar lead uses. The line input is used for signals coming in that are much higher. The mic' input is the one we always hope to use. There will commonly be a switch to change between the two. (So check that first if you're getting no signal!) The first pot (Pot is short for potentiometer, or rotary control knob!) will be the Gain and is used to add power to the signal, therefore increasing it's level. Use gain cautiously as power causes distortion, not the desired effect! The next series of pots is usually the EQ. (EQ is short for equalisation, like the tone controls on your hi-fi) The bigger, and more expensive the desk, the bigger and more sophisticated the EQ section will be. On most desks you'll get Low, Mid and High possibly with sweep pots on each. Then comes the auxillary sends. These send a little of the channel off to wherever you want. In a PA that doesn't have a seperate monitor desk,these would be used to send sounds to the on-stage monitors. They can also be used to send signal to the effects units in the foh rack beside the desk. For example, if the drummer needs to hear a bit more bass guitar than he can hear from the amp, you would send a bit of the channel with the bass guitar to his monitor. If you wanted to add a little reverb to the backing vocals you would use an aux to send a bit of that channel to the effects unit you have set up for reverb. The last section of the channel strip tells the channel where to send the signal. The most obvious choice is the main mix. The mix is the stereo sounds coming out of the speaker stacks at each side of the stage that the audience are listening to. To give the engineer more options you can also send the signal to groups before they are sent to the mix. For example, it's quite common to send all the drums to one group, then to send that group of signals to the mix. If, during the gig you decide the drums are too loud it's easy to turn down the group with all the drums rather than turning down each of the seperate channels on the desk. It's also handy if you want to compress the whole drum kit. You can add compression to the group with just two channels of compression (left & right) rather than several.
So, you get the idea. The desk may look daunting but the skills are not in knowing the desk. That's just the beginning!
At least he's not pointing it at the monitor!
Of course in smaller venues whoes stage is a bit smaller than the one at Wembley Arena there won't really be much point in putting Overhead Microphones above the drum kit. (If someone in the audience complains that he can't hear the cymbals, he probably needs to get his hearing checked!) The mic' on the Rack Toms will probably pick up plenty of cymbals anyway.
