Re: AUParametricEq Q parameter?
Re: AUParametricEq Q parameter?
- Subject: Re: AUParametricEq Q parameter?
- From: "James Chandler Jr" <email@hidden>
- Date: Tue, 4 Jan 2005 11:14:58 -0500
What Kevin said. A higher Q has a longer ring-time.
If this Q parameter is the typical EE definition, 'musically useful' parametric
equalization is in the range of 0.5 to 10. Once Q gets above about 5 or 10, the
ring time can get annoyingly long, and it sounds real artificial except for
special FX.
For a parametric EQ, folks are typically interested in the ear's frequency
perception of the width of the cut/boost, rather than the ring-time.
For general 'cookbook' calculations specing a second-order bandpass filter
(again assuming a typical EE definition of Q)--
FL - The lower -3 dB cutoff frequency
FH - The upper -3 dB cutoff frequency
FC - The center frequency
If you don't know the FC, but you have decided on an FL and FH, FC is the
geometric mean of the -3 dB band limits (FC ain't halfway between FL and FH)--
FC = (FL * FH)^0.5;
FractionalBandwidth = (FU - FL) / FC;
Q = 1 / FractionalBandwidth;
=====
Given those relationships, you can use basic algebra to calculate whatever
unknowns you need, given whatever knowns you have.
Some parametric EQ's have knobs labeled Q. Others have Bandwidth knobs which
might be more understandable to customers. Sometimes the Bandwidth knob is
labeled Octaves rather than FractionalBandwidth. And there are some EQ's with
knobs for Upper and Lower Cutoff frequency, but that is rarer.
Anyway, you can always derive the Q from the style of knobs you want to present
to the user.
Take an example of FL = 1000 Hz, FH = 2000 Hz: FC = 1414 Hz, FractionalBandwidth
= 0.707, and Q = 1.414. It would seem that the OctaveBandwidth would be 1 in
this case, but maybe I'm looking at it the wrong way.
=====
Lowpass, Hipass, Notch, and Allpass filters also have a Q. In all the filter
varieties, Q controls the 'steepness' of the frequency response and phase
response in the close vicinity of the center frequency (and of course the ring
time).
=====
A multiband parametric (depending on its design) will not necessarily have the
frequency response you would guess just by looking at the knob settings. The
bands interact, and various equalizer designs have different behavior of
interband interactions.
JCJR
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