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Sound Processing Equipment in Live Music
Processors in PA systems are usually used to correct or control elements of the sound. Broadly, they are of two kinds: Equalisers, and Dynamic Processors.
Equalisers control the frequency content of a sound, and in live sound applications include
You won't find a fully-featured professional PA system without a Graphic Equaliser channel on each independent mix.
Not quite as commonplace as the Graphic EQ, but Parametric Equalisers feature in the channel EQ section of most higher-end desks, and their little ‘semi-parametric’ brother is found much further down the ladder (even some budget notepad mixers have a single ‘semi-parametric’ mid).
Dynamic Processors control the dynamic range of a sound (basically, how quiet it can be before it is silent, and how loud it can possibly be), and include
Controlling dynamic range in a PA system is often important and necessary, but some musicians won't have a compressor in the building, let alone on their guitar, voice, keyboard, drums, or - in short - any musical instrument they can think of.
Limiters offer some protection to loudspeakers, and can be useful anywhere that sudden large signal peaks can appear (which can often happen accidentally, if someone switches phantom power on with the mixer channel open, or drops a sensitive microphone).
Expanders can be used as noise gates without such a noticeable ‘chopping’ effect.
Noise Gates can be used as ‘auto-muting’ on channels where no intended signal is present, but can be difficult to use in live performance without unwanted side-effects.
Crossovers are another feature of all but the most basic sound systems, and can range from a couple of knobs to advanced loudspeaker management systems. All of them include frequency filters (so could as well be classed as Equalisers), but most incorporate limiters or compressors too.