Gain Structure for DJs 101
Playing music to an audience, whether it's a stadium rock band or a CD at a kid's birthday party, will involve some fundamental principles of audio production. Basic knowledge of the factors affecting live sound can help DJs avoid common pitfalls and maintain audio quality across venues.
As the front line 'face' of what's being heard, it's in the DJ's best interest to be able to diagnose and minimize problems with the sound system, or at least the part which he or she is directly in control of.
This article is intended as an introduction to the key concepts of gain structure and audio theory, with specific bearing on equipment commonly used by Scratch Live DJs. Good gain structure is essential for a good sounding system, with adequate level. Systems that have large amounts of noise are almost invariably due to poor gain structuring.
What is gain?
Gain is the process of increasing the amplitude of your signal, or 'making it louder'. The reverse process, making your signal quieter, is known as attenuation.
Headroom, as the name implies, is how much room there is between the average level of your signal and the 'ceiling' - the maximum allowable level in a given part of your system.
Clipping describes what happens to a signal when it's amplitude is greater than the system will allow. Clipping can result in unpleasant loud distortion.
Noise is technically any unwanted portion of your signal. Generally in DJing, it will be hiss, hum, or perhaps crosstalk interference from other signals.
An analog audio signal will always accumulate some noise during transmission, and minimising how much is best achieved by keeping the signal as loud (hot) as possible while also avoiding clipping. Signal to noise ratio (SNR) is the measure of how hot or high the desired audio signal is above the noise floor of the system or piece of equipment. Higher ratios meaning less background noise.
Poor gain structuring is almost always the cause of excessive noise in a system.
The noise portion of your signal will be affected by gain in the same way as the rest, therefore it is less desirable for a quiet signal to be transmitted than a loud one, if possible, while avoiding clipping.
For example a turntable phono output is very low level, and if transmitted for more than a couple of meters via standard cables, will accumulate a large amount of noise proportional to the material content. When that signal is later turned up to line level by a phono pre-amp, the result will be very noisy. Note the relevance of this to the SL 1 Thru outputs, if you use normal vinyl in your Scratch Live setup. Try to use the shortest cables you can from the SL 1 to the mixer phono inputs.