Re: Drawing an EQ curve
Re: Drawing an EQ curve
- Subject: Re: Drawing an EQ curve
- From: Christopher Penrose <email@hidden>
- Date: Thu, 4 Sep 2003 12:19:27 -0400
On Wednesday, September 3, 2003, at 11:13 PM, Chandrasekhar
Ramakrishnan wrote:
On 9/3/03 11:33 AM, "Robert Grant" <email@hidden> wrote:
If I wanted to show a graphical representation of an EQ curve with
Freq, Gain and Q is there a cheap way to do it?
You can perhaps generate an approximate curve based upon these three
values alone if Apple specifies the effect of the Q parameter. Also,
you can estimate this effect by doing successive Fourier transforms on
test signals (perhaps white noise is a good candidate) where you can.
But this is a haphazard technique which will create an approximate,
piecemeal map of the filter's frequency response. And at times, such
an approximation is probably going to be really distant from the
behavior of the filter.
Better, to get an accurate picture of the frequency curve of a
particular digital filter, having the filter formulae, and the
algorithm used to translate frequency, gain and Q values into its final
filter coefficients, is best. Once you have the filter formulae for a
particular filter, you can sample its Z-transform with arbitrary
resolution. The problem here, obviously, is getting more information
from Apple about the implementation of the filter, and they probably
may consider it to be proprietary. If you have an interest in doing
such anyway or are curious, Ken Steiglitz's "A DSP Primer, with
applications to digital audio and computer music" introduces digital
filter theory and the mathematics of the Z-transform quite well.
http://www.amazon.com/exec/obidos/tg/detail/-/0805316841/
qid=1062692123/sr=8-1/ref=sr_8_1/102-4472252-
1372925?v=glance&s=books&n=507846
Christopher
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