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Parametric (Peaking) EQ variations

6905 Views 13 Replies 4 Participants Last post by  Chester
Hi; I have been using various programs to design EQ filters over the years including REW WinISD Pro, Crown Audio's IQ for Windows, Aixcoustic's Electri-Q, the UFX plugin (on KX drivers) etc. One thing I have noticed over time is how the Q of Parametric EQ filters varies from program to program; ideally the graph of one parametric EQ in a program (given the same ranges on the frequency and db axes) would look the exact same in another program. This rarely is the case. WinISD Pro (which every released version I know of there is a bug with 'negatively' gained parametric EQ's (ex. 2 parametrics both centered at 1000 hz, both a q of 2, one negative with a negative gain and one with positive (but same) gain will not cancel out). I have found formulas to correct this (and in the 'next release' of Win ISD it should be fixed) however this just goes to show how important it is to double check graphs.

I am currently working on a conversion from REW to Crown Audio's IQ for Windows; if anyone else has any information on this sort of thing it would be greatly appreciated. I am guessing that there hasn't been any discussion about how the Q is calculated on various programs though...
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The problem is that every graphics equalizer may calculate bandwidth differently.

As an example, some EQ's (such as the DSP1124P) define bandwidth as:

Bandwidth (Hz) = centre frequency*(BW/60)*sqrt(2)

So, the Q formula becomes: Q = 60/[(BW/60)*sqrt(2)].

For the R-DES eq, the bandwidth is defined as (1.766*centre frequency/Q).

Many are just the standard Bandwidth = centre frequency/Q.

So for each equalizer you would need to know how bandwidth is defined in relation to the standard Q calculation (√2/BW). One equalizer may define its 2/3 octave as the total width of the filter at its -3dB endpoints, (so a 2/3 octave filter would formulate to Q of 2.12). Another may define only the positive half gain endpoint, so a 2/3 octave filter actually covers 4/3 octave overall. There are many different definitions of bandwidth.

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REW could 'calibrate' itself
REW allows the selection of various EQ's, so that the filters comply with that specific equalizer.

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