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From: dmcgrath@lennon.austek.oz (David Mcgrath)
Newsgroups: comp.dsp
Subject: Re: Digital Filter Design package (including group delay)
Message-ID: <317@lennon.austek.oz>
Date: 11 Feb 90 22:08:41 GMT
Organization: Austek Microsystems, Adelaide, S.A.
Lines: 36


In article <3288@taux01.UUCP> you write:
>
>The point might have been missed in my short posting, but I obviously
>meant that the designer will be able to specify a variable group delay
>as a function of frequency (and not a fixed delay as in a linear-phase FIR).
>If Anyone knows of such a package, please let me know by posting or e-mail.
>

We have built a fairly general-purpose filtering box at Austek that we have
used to implement such a filter. The box implements a 32768-tap FIR filter on
CD audio data (in real time). The coefficients are downloaded by a PC. One
demo we usually run on this system is a 1/3 octave graphic equaliser, using
the PC and mouse to implement the 1/3 octave display. As a variant of this,
someone here modified the equalizer coefficent calculation code to do a
variable group delay equalizer, so that you can alter the group delay of the
signal by over 1/2 second with 1/3 octave frequency resolution.

We compute the filter response in the frequency domain, so that you can
download into the box a vector of 32768 complex numbers that represent the
complex response of the filter at 32768 frequency bins (from DC to 22.05 KHz).
Since group delay is simply the derivative of the phase response, all you have
to do is integrate the group delay as you count throught the frequency bins,
and generate 32768 complex numbers, all with unity magnitude and phase that is
the integral of group delay.

(You can also specify filter responses as time-domain impulse responses, which
is handy for building reverberation filters etc...)

If anyone out there wants more info on our box, please get in touch with me.

David McGrath,					Phone : +61-8-260-0114
Austek Microsystems,				Email : dmcgrath@austek.oz
Technology Park,
South Australia, 5095,
Australia.