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Earlier today I asked a question about EQ and got back some helpful information which actually clarified equalization completely for me. However I'm confused on what's the difference between equalization and filters. Is there a difference or are the terms used interchangeably? If there's a difference what is it ? Below I posted the link to the question I asked about equalization earlier.

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A filter is usually a particular kind of EQ where volumes at certain frequencies are only cut and not boosted. Most of the time a filter cuts everything above a certain frequency (low-pass filter) or everything below it (high-pass filter). There are others like notch and bandpass filters that either filter or "let pass" adjustable bands of frequencies somewhere in the middle.

But the point is that a filter uses EQ to "filter out" frequencies rather than boost them whereas EQ usually refers to the ability to cut or boost.

  • Isn't it more correct to say that an EQ is a particular type (or use of) a filter (filters, to be specific)? – endorph Mar 7 '17 at 4:31
  • No, not in my opinion. You could stretch the common meaning of filter and apply it to a different context such as calling a compressor a filter that filters out dynamic ranges. Or by saying an EQ filters frequencies whereas as an air filter filters particulates. But in the common meaning of "filter" in relation to EQ, it's a subset of what EQ is. It's cut-only EQ. – user37496 Mar 7 '17 at 5:00
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    I guess. Maybe we're using different definitions. Filters (in the electronics sense) are used to implement EQs, so it's pretty strange to say that a filter is a kind of EQ. – endorph Mar 7 '17 at 8:39
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EQ is a type of filter. The difference in naming is mainly because of the different ways different types of filters can be used.

Typically, EQ is adjusted for individual instruments to improve the mix (to avoid two instruments "competing" for the same range of audio frequencies, etc) and remains constant for the duration of the piece. The EQ controls usually show a display of amplitude against frequency, calibrated in well defined units like Hertz and decibels.

On the other hand many musical instruments have "built-in filters" which affect the sound - for example a wah-wah mute on a trumpet, the una corda pedal on a piano, the bowing position on a string instrument (sul ponticello / sul tasto), the many types of effects pedals on electric guitars, etc. These can be varied from note to note, or sometimes within a single note, as the performer wishes.

Similar filter effects can be created after recording the audio, or on audio that is generated entirely by electronics. Filters used this way usually have controls that don't show an "objective" measurement of what they do to the sound like an EQ filter display, but are designed to be easy to use in real time.

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A "filter" is not a well-defined term: basically it is used for any kind of sound processing intended to leave information in place. A low-cut filter is used for removing rumble noises and attenuates the signal increasingly with lower frequencies. It is a special form of a high-pass filter: there are also "shelving" high pass filters which have a frequency response ending on a "shelf", meaning that they don't attenuate more than a given amount even as frequencies get lower. A graphical EQ typically has shelving filters as its first and last control.

A notch filter cuts out a certain frequency.

All of those filters are "linear" and "time-invariant", meaning that you cannot distinguish running through signals through separate (identical) filters and then summing the results from summing the original signals and running them through a single filter, and if you send a delayed signal in, the result will be the response for the undelayed signal, just appropriately delayed.

A graphical EQ is a whole filter bank typically arranged with logarithmic frequency steps designed in a manner where the output will be identical to the input (apart from scaling) as long as all controls are at the same height. So basically all individual filters in a graphical EQ tend to have a "shelf", for all but the first and the last in the middle of their range. It is either that, or the overlap of the individual filter ranges is so much that the total frequency response comes out flat nevertheless.

Now all of that are linear filters, but there is also non-linear filtering like the one used for noise suppression or echo compensation (both of which tend to be done by piece-wise linear filtering but with the filter being constantly adapted to the signal, making the end result non-linear with regard to the overall input).

I don't really know a rigid definition of "filter" that would apply to everything commonly called a "filter" but excluding intentional distortion (various kinds of vacuum tube sound simulation including "clean", tape drive simulation, plain old overdrive and so on).

So basically it's more or less convention when some input-derived signal processing is labelled "filter" and when not. There is no hard and sticky rule people would agree on.

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