[sdiy] filters with independent resonance 'location'

[Theo]

I included something like that as one of the filtermodes for my q-o-matic
filterbank, the whole q-o-matic filterstructure is still in the test phase
though.

Instead of the obvious option to put a LP and BP filter in parallel I use 2
filters in series, well kind off.
The first filter is a LP or HP filter, the output of this filter is the
audio output.
The second filter is a BP filter and the output feeds back to the first
filter.
In other words the second filter is only in a feedback loop.
The two filters also have their normal resonance path.
So this filter combination has 2 cutoff, 2 resonance and a feedback
parameter.


The drawing below makes sense in fixed font mode.

                   res1
            +-------<-------+
            |               |
            |               |
in >-----[LP/HP]------------+----> out
            |               |
            |    feedback   |
            +--<--[BP ]--<--+
            |       |
            |       |
            +--->---+
              res2

The idea needs some more testing so I don't know yet if the results are
interesting enough to keep this filtermode.
Because of  LP or HP filters create phase shifts it may also be interesting
to use those in the feedback loop .


Theo


From: Jeffrey Pontius <jpont at stat.ksu.edu>


> In general, a typical lowpass or highpass filter has the resonance set
> around the cutoff frequency.  Are there filters available, and if not
> available could one be reasonable constructed, that would allow the
> resonance 'location' to be set independently of the cutoff frequency?
>
> This is certainly not a new idea, but I have not seen or read about such a
> filter.
> Thanks, Jeff
>