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Q. What are filters and what do they do?

Published July 2009
By Various

I always hear people talking about low‑pass filters and high‑pass filters and cutting at this and that frequency, but where do you get these filters from? I don't think I have one in my Cakewalk Project 5 software. Are they part of equalisers?

Via SOS web site

This diagram illustrates both low‑pass (high cut) and high‑pass (low‑cut) filtering. The shaded areas in the diagram will be attenuated.This diagram illustrates both low‑pass (high cut) and high‑pass (low‑cut) filtering. The shaded areas in the diagram will be attenuated.

SOS Technical Editor Hugh Robjohns replies: People sometimes use the terms 'EQ' and 'filter' interchangeably, so it's understandable that you might be confused. We've published several introductory guides to EQ, most recently in SOS December 2008 (/sos/dec08/articles/eq.htm), so if you're a bit baffled about the broader subject of EQ, it would be well worth reading this.

Essentially, EQ is used to boost or attenuate (turn down) a range of frequencies in order to shape a sound. High‑pass and low‑pass filters are common in professional equalisers, but less common in budget designs. They are used to define the highest and lowest frequencies of interest in the signal and they pretty much do what their names suggest: let audio above a certain frequency pass (high‑pass filter) or audio below a certain frequency pass (low‑pass filter). Anything outside those limits is attenuated. They are also called low‑cut or high‑cut filters, but the function is the same.

Filters are defined by their slope, which determines the attenuation of signals outside the 'pass' band. Most audio filters on mixing desks (and DAWs) will have a slope of 12dB or 18dB per octave, and in synthesizer filters the slope may be as steep as 24dB per octave. If an 18dB/octave high‑pass filter is set to 80Hz, any audio an octave below that (at 40Hz) will be attenuated by 18dB, and an octave lower still, at 20Hz, it will be attenuated by 36dB... and so on.

High‑ and low‑pass filters generally have much steeper slopes than the more normal equaliser bands (which are typically only 6dB/octave) and are intended for a different purpose. You can't effectively remove rumble with a bass EQ control, but you can with a high‑pass filter. But equally, you can't shape the tone of a bass guitar with a high‑pass filter as easily as you can with a bass EQ control.

Filters are used for 'corrective' equalisation, as opposed to creative equalisation. They are used to clean up a signal, rather than to shape the sound creatively. They only provide attenuation of unwanted frequencies, and there's no scope to boost any part of the frequency range. Of the two, the high‑pass filter is probably the most useful, as it helps to remove unwanted rumbles and other unwanted sub‑sonic rubbish that microphones tend to capture. Most DAW software includes a software EQ that you'll be able to use to perform any of these tasks, and although I'm not personally familiar with Cakewalk Project 5, I notice that it can host third‑party VST plug‑ins, so there are many freeware plug‑ins that you could use if your DAW doesn't have them built in.  

Published July 2009