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 WILLY T talks LEVELS on the Front Of House mixing desk.
All vocal and instrument mics; keyboards, MIDI gear and samplers, turntables etc are patched into the Front Of House mixing desk (FOH) via the big thick cable called the Multi-core. Each with differing Signal Strengths or Levels. The mixing desk provides electrical level-matching, so that we can combine or ‘mix & match’ the various signals, sounds and gear, to send out to the power-amps to turn up loud! The desk can cope with all these differences by being able to cut or boost Input and Output levels, so that we can control the signal strength both as it comes into the desk, and as it leaves the desk. But, be careful. All the amplification in the system, at all the various inputs, outputs, EQs, FX and Drive Racks etc, boosts unwanted noise by the same amount as the stuff we want to pump out to the punters!
MIC/LINE LEVELS
As can be imagined, the signal strength coming from the back of electronic gear like a Sampler – technically known as Line Level – is much stronger than that which comes from a mic (known as Mic Level).
All microphones are transducers, they change one form of energy into another. They use sound-waves to move a tiny magnet, to generate an electrical signal. This tiny little generator only puts out a minuscule voltage, so a microphone’s output is extremely low. Mic level is generally considered to be around 2 or 3 millivolts – that’s 1/1000ths of a volt! (But beware, the strength of the signals varies tremendously. Peaks from a meaty kick-drum can hit +4dBv!)
Each mic input on a mixing desk contains a pre-amp or mic-amp. They take the tiny signal from the mic and boost it up to line level so it can be used with the majority of gear in the PA system.
Line level refers to the average or Nominal Operating Level of a piece of audio gear, and generally corresponds to a ‘0VU’ meter reading on the mixing desk. To complicate matters further there are ‘Professional’ and ‘Domestic’ Line Levels, with pro gear running much ‘hotter’. Be aware that mixing and matching ‘Pro’ and ‘Home’ gear can cause level problems like distortion. (For the tech heads: Professional Sound Reinforcement or Broadcast environments operate at a line level of +4dBm. On the VU meters of a +4dBm mixing desk, the 0VU = 1.23 volts RMS. Domestic gear and semi-pro recording gear operates at a line level of -10dBm. On the VU meters of a -10dBm mixing desk, the 0VU = 310 millivolts RMS.)
In a PA system the vast majority of the system is running at line level. Any mic level signals – or DI’d instruments – entering the mixing desk are converted to line level. The signal then stays at line level until it comes out of the back of the power amps; at lot-o-watts! Woo-hoo … make-some-noise!
SYSTEM GAIN STRUCTURE
Each piece of gear in the signal-chain has a level at which the device operates at its optimum. The aim is to have the input level sitting in between Clipping – the level at which the input will distort – and the Noise Floor – the residual, low-level electronic noise of the desk (that characteristic ‘hisss’ sound). Our ears are designed so that strong sounds will mask quieter sounds, so a strong signal masks the gear’s electronic noise.
To get the max out of any system, every time a signal is fed from one piece of gear into the next, whether a mic to mixer, effects unit or power amp, the various gain controls must be adjusted to their optimum levels. This ritual of tweaking the gain controls is known as setting the gain structure, and it doesn’t matter whether you’re working with a Stadium mega-rig, a pair of decks and mixer, or a home studio set-up, if you don’t pay attention to gain structure, you will not get the max out of the gear you’re using, and usually have lots of noise.
Failure to set even one input gain control correctly may lead either to excessive noise – if the gain is too low – or unpleasant distortion – if the gain is too high. We do not want to have mega-hot output signals distorting the inputs of the next bit of gear in the chain. The opposite is also true. If the signal is too quiet it will need to be boosted by the next bit of gear in the chain, which will also boost any noise already in the system. These tweaks must be carried out from the very first input, all the way along the signal path – including the Auxiliary Sends and FX gear – right to the end of the chain, from the mic to the FOH speakers.
FINALLY
Whether the signal path is simply from a pair of decks to your home stereo; or a mega touring rock-rig, leads make a HUGE difference to the sweet sounds – and terrible noise – within your system. Generally, with proper maintenance, gear does not break down – touch wood. It is usually operator error or the leads plugged into it! If anything is going to go down during the gig, you can just about always bet it will be a lead related. Check your leads regularly.
WILLY T runs Performance Recording: providing 16-track live-recording services & fully equipped 24-track location packages. For more info Ph : 0401 431 875. www.myspace.com/willytslaw
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