Hi Nick,
Since I wrote that for the Faust project some time ago, I should be able to
figure out the answers. :-)
Email me offline if you still have any questions. If so, I will have to
track down ncauer and install it again, as it appears not to be in the
Octave distribution, and it's not in my current set up (or at least not in
my search path).
- Julius
On Fri, Mar 3, 2017 at 2:49 PM, Nick Thompson <nctho...@gmail.com> wrote:
> Hello all,
>
> I've been studying the implementation of the 3rd order elliptic (Cauer)
> lowpass filter in the Filter library (such as defined here
> https://github.com/oknytt/faust/blob/551d63a27e5853ec911b5f4
> d9fabfcd09885a117/libraries/filter.lib#L1394), and I'm confused about the
> derivation of the second- and first-order filter coefficients.
>
> I understand the analog prototype designed with `[z,p,g] =
> ncauer(Rp,Rs,3);` in Octave, and I think I have a good understanding of how
> to factor the transfer function into cascaded first- and second-order
> sections. What I don't understand is the use of `poly` in Octave to find
> the coefficients, and why the frequency of this elliptic filter is only
> governed by the last coefficient in the final first order filter?
> Intuitively it seems like so many more of those coefficients should depend
> on the frequency of the filter, especially if we were to consider multiple
> sample rates. My knowledge in this domain is fairly rudimentary, but are
> there some assumptions here that I'm missing?
>
> I've asked the same question on StackExchange (
> http://dsp.stackexchange.com/questions/38095/analog-filter-
> prototype-to-direct-form-second-order-cascade) as I understand that this
> list might not be the best place for this discussion, but any explanation
> you can offer me here would be greatly appreciated!
>
> Cheers,
> Nick
>
>
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--
Julius O. Smith III <j...@ccrma.stanford.edu>
Professor of Music and, by courtesy, Electrical Engineering
CCRMA, Stanford University
http://ccrma.stanford.edu/~jos/ <http://ccrma.stanford.edu/>
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