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Quick Guide To EQ And Filters

Confused about shelves, notches and band passes filters? We roll down the slopes and sweep through frequency-charted waters...

If you've never tweaked an EQ control, you've never mixed! So goes some old studio engineer's saying. Without a doubt, EQ is the most used - and abused! - of all studio effects and it's an essential part of virtually all types of synthesis. That's because it works on the most essential element of sound - the tone. In fact, at its simplest, all EQ does is cut and boost certain frequencies of sound. Think of a hi fi's treble and bass controls - they're basic EQ controls. But, of course, technology has made EQ rather more sophisticated than that...

The good old days

EQ is short for 'equalisation'. In the good old days of recording - we're probably going back as far as 78s here - the recording process was significantly less sophisticated than it is today and it would typically lose the higher frequencies.

EQ was devised as a corrective process to boost the higher frequencies to compensate for the loss to make the recorded sound 'equal' to the original. That's no longer the case with modern recording equipment and EQ is now used more as a creative effect.

No limit

The human range of hearing is typically quoted as running from 15Hz to 20kHz although this varies considerably with the individual. In practice it's generally not that wide and we lose sensitivity to the upper frequencies with age and by listening to loud rock and dance music so many people's upper limit may be closer to 15kHz.

You might expect this to be the range of frequencies you'd be interested in EQing. However, experiments have shown that frequencies above our normal range can affect our perception of the sound in subtle ways so don't ignore them.

The combination of frequencies is what gives a sound its distinctive tone (see Oscillators for more about this) and when you EQ a sound you boost and cut specific frequencies which alters the tone.

In control

There are several controls typically used in filters and EQ effects:

	Cutoff frequency/Cutoff point. The frequency at which a filter comes into play and starts to affect the sound. This determines what part of the frequency spectrum the filter works on.
	Attenuation. This is the opposite of amplification - in other words it reduces the target frequencies.
	Level/Gain. A level or gain control allows you to cut (attenuate) or boost (amplify) a set of frequencies.
	Bandwidth/Q/Resonance/Emphasis/Peak. The range of frequencies that will be affected by the filter either side of the cutoff point. It is usually measured in Hertz so it relates to frequency although it would be more useful if it was measured in octaves.

The terms 'bandwidth' and 'Q' are most commonly used in recording circles. 'Bandwidth' is the most useful because it tells you exactly what the control does. Q is an abbreviation for 'Quality' or, more exactly, 'figure of Quality' although these fuller descriptions are never used. 'Emphasis' and 'Peak' are rarely used in modern parlance.

Resonance is particularly common in synthesis. It boosts the frequencies around the cutoff point and can be used to put the filter into self-oscillation - not something you'd want to do with a recording.

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