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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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A quick example of this (on a commonly used program):
1. Using WinISD Pro; go to the Transfer Function Magnitude (EQ Filter) menu (after loading a speaker file)
2. Using REW; load look at the Modal EQ with a measurement loaded, check so you can see just the filter response
3. In both programs, load a parametric EQ, with a Frequency: 1000 hz, 24 db gain, and a Q of 1.
4.Pick a frequency (I will use 2000 hz) and note the gain in each program, I get:

WinISD Pro: +18.92 db @2khz
REW: +8.9 db @ 2khz

So clearly both programs are using different methods of calculating the shape of the filter even though they both are using "Frequency" "Gain" and "Q".

EDIT: I realize that this is all in 'software' however depending on the calculation method of the device you are programing you could have a large difference between what you 'should be getting' (according to REW) and what you are actually getting; at least at frequencies other than the center frequency

EDIT/addition 2: I have noticed also that as the frequency is increased on the filter and approaches the Nyquist limit (the maximum frequency representable by the sampling rate) the symmetry is not maintained by the filter (make a filter at 10khz and notice how the half on the 20khz side is 'steeper' than the 5khz side; this is a simulation of how most digital filters work however some manufacturers/products do correct for this (the Crown USM-810 I use being one of them... I called and asked :)) so that is just another thing to be aware of when designing filters.
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Ohh, so perhaps I can find (based upon the various Equalizer presets) one that uses the same calculation method as my software (if I can find one that matches WinISD it would be great because I already have a 'formula' based on the gain and Q of the filter to 'convert' from WinISD to everything else I use :))... Thanks for the tip! If anyone knows what the different methods used to calculate the filters are called it would probably be helpful to others interested to know what they are called.

... or if there is a way to add the USM-810 to the list that would be great too :) (I just noticed that the 'supported devices' have quite a few limitations on various parameters (TMREQ only allows +6 db gain, SMS-1 only goes to 120 hz, etc.)
Thats completely understandable (and makes perfect sense) I am just saying that for me to 'use' those settings to simulate my setup (assuming one of the graphing algorithms matched) doesn't work well :)

Thanks man, I've seen that site before it uses the -3db frequency method which can have some trouble with gains <3db; I typically (in my conversions) measure the gain/2 frequencies and figure out what Q will make that a constant at different gains (usually takes ~10 measurements once a commonality is found) and then create equations to interpolate the values in-between. These are then entered into excel worksheets to make things easier to work with :)
Hmm, it would be cool if we could measure our equipment like we do sound cards; however we could have 5 or 6 eq's we could set on the device so REW could 'calibrate' itself so things would all be on the same calculation :)
True however I have not seen any settings for the USM-810 :)

Anyways; I came up with a conversion to for the REW filters (on a Generic EQ) to a filter in WinISD Pro (WinISD Pro filters use the same equation as Crown's IQ software):
if Gain=X
and the REW "Q" =Y

(WinISD Q)=Y*(1.000009391*exp(0.05756459995*X)-0.000005486433537*exp(-1.355976474*X))

basically the larger the gain, the greater the 'correction' is need to make the Q's equal; then that correction factor is multiplied by the original Q :)
Thanks John :)

I'll try to get you those sweeps today by the way
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