Signal Processors

All PA Systems are based around audio signals. They are mixed, routed around and amplified to provide various solutions to the user. Often it can be necessary to process these signals, for example to improve the frequency content or dynamics of the sound, add effects and manage audio for amplification over a live speaker system. A signal processor is a general term for any unit that performs this processing, encapsulating a wide variety of devices.

A complex PA System with many audio inputs, outputs and processors requires a patch bay. Often it is impractical to route audio around by manually and directly connecting different devices to each other due to them being in different physical locations or having sockets in hard to reach places. To get around this problem a patch bay is used. When installing a patch bay all the units in a system have their inputs and outputs wired to one piece of hardware mounted in a standard 19" rack with two rows of sockets. Then using "patch" leads (Often with 1/4" jack plugs) the audio from one piece of gear can be patched into another piece of gear by plugging one end of the lead into the top row (Outputs) and the other end into the bottom row (Inputs).

The range of human hearing goes from approximately 20Hz, the lowest audible sound, to 20 kHz, the highest. Sound consists of the amplitude of many different frequencies between this range, different sounds having a different frequency content, for example a bass guitar containing many low frequencies and a crash cymbal containing many high frequencies. One of the most common ways to process a signal is Equalization or, as it is commonly shortened, EQ. Equalization can be used to improve the timbre of a sound, reducing harsh or boomy frequencies and boosting sweeter ones. It can also be used as a "utility", removing frequencies which are causing feedback and reducing unpleasant sounds such as hum and noise. A lot of budget end sound equipment comes with just two or three knobs to control the volume of bass, mid and high frequencies. More fully featured equalizers come in two flavours, graphic and parametric. Graphic EQ, often found in rack units and on the master sections of mixing consoles, splits audio into a number of set frequency bands, anything from around 5 to 31. Each frequency band has a corresponding slider or fader for amplifying or attenuating the level of that frequency. Parametric EQ, often found on the individual channels of mixing desks, allows more specific control. Parametric EQ units have independent controls for the frequency of the sound being "EQed", the amount of amplification or attenuation being applied and often a third for bandwidth, that being the range of frequencies being effected.

Another very common piece of signal processing is compression. Compressors, instead of changing the frequency of the sound, change its dynamics. Put simply dynamics of a sound is the difference between the low and the high amplitudes of the sound, a short, sharp drum hit having a lot of dynamics, a constant rumbling bass note having little. Compressors change these dynamics by reducing the sound when it is at a high amplitude and boost it at a low amplitude, creating a smaller difference between low and high and reducing the dynamics. This is useful for such purposes as making a sound seem audibly louder without needing to increase the peak, or maximum, level of a system.

Feedback is a fact of life when dealing with live sound. Because of the nature of sound waves and electronic circuits sound can leave a speaker system, be picked up by a microphone and in turn leave the speaker system again, once again being amplified. This creates a harsh, deafening ring called feedback. This wants to be avoided at all costs but can be hard to control. Getting ever more frequent in use are dedicated feedback reduction units. These sound processors digitally "kill" the feedback and are placed over the audio before it goes to a speaker system.

In a live PA System the mix created by the user contains the full range of audio frequencies from the lowest bass to the highest treble. Individual speaker drivers cannot accurately reproduce this full range sound so use different designs to deal with different ranges of frequencies, for example a subwoofer reproduces bass frequencies. A crossover is required in this situation. A crossover, usually placed just before the amplifier, splits the full range signal into two or more discreet signals containing only a set range of frequencies matched against the speaker it will be powering.

It is becoming more common in recent years for many of the above signal processors to be combined into a single unit called a speaker management system. Taking the whole mix this can apply EQ, compression, feedback reduction and create multiple crossover signals all in one box.

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