[sdiy] Digital Bandpass Filters

[M.A. Koot]

Hello all,

It might not be the right place to ask, but currently I really wouldn't know
where else I could better. If anyone could point me to a better newsgroup on
this matter that'd be great!

I'm currently doing my degreeproject on designing a RTA (Real Time Analyser)
in software code.
Now if there's anybody who doesn't know what an RTA is, in a nuttshell: it's
like a spectrum analyser, displaying the current frequency components in a
sound-signal by showing moving bars for every frequencyband.
I have to design a filterset to simulate this thing, so I'm working my way
with Digital third-Octave IIR Band-Filters.
I try to do this with Matlab, or using National Instruments tools, and
programming the rest in Visual Basic. Though since I have very little
experience with digital filters, I have some problems designing these.

For the lowest bands in the analyser, I will need bandpassfilters with a
very small passband.
For example the utter lowest one needs a low Cutoff of 0,712 Hz and High
cutoff of 0,898 Hz.
That means a Passband of only 0,186 Hz! And I still have to maintain only a
low attenuation in the passband following the IEC Norm.

While checking out some filters from National Instruments, I found that they
are quite nice, but when I try to design these lowest bandfilters, I have to
make them small and I have to crank up the Order to keep the curve steep.
The problem is only, that even if I increase the order, it's still not
enough, and the curve doesn't make it to the top of the band before it's
going down again (the high cutoff).
Not only that, if I increase the filter order to over 50, the filter seems
to go crazy, and that's no option either ;)
I'm using elliptical filters at the moment, as they seem to have the highest
possible steepnes of all, but obviously not enough.

It's probably not very suprising with this kind of a small passband, but it
just can't be wider. And since a hardware RTA can do it by using it's own
digital filters, it must be able for me to do it too in software.

Does anybody maby have experiency with this? Experiences with digital
bandfilter design, or could anyone maby point me out a newsgroup which is
related to this subject? I would be very very thankfull.

With Kind Regards,
Michiel Koot

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