[Discuss-gnuradio] IIR Filter taps

[Tom Rondeau]

Hi all,

So we're working towards integrating the new gr_filter_design tool
that Sreeraj built this summer for our GSoC project. One issue has
come up with it is the specification of the IIR filter taps and how
they are used in GNU Radio. To summarize the conversation between
Sreeraj and myself:

GNU Radio's expected transfer function:
H(z) = \ frac{\sum_{k=0}^{M} b_k z^{-k}}{1 - \sum_{k=1}^{N} a_k z^{-k}}

But gr_filter_design gives the denominator (a_k) in the form:
{1 + \sum_{k=1}^{N} a_k z^{-k}}

So when taking from gr_filter_design, we need to invert the sign of
all taps except the first one.

I feel like we should be able to just copy and paste the a and b
vectors from gr_filter_design straight into a GNU Radio program
(Python or GRC) and have it run. Also, though, I think
gr_filter_design could be used by others running programs other than
GNU Radio. So we should have the program produce filter coefficients
useful to them, too.

>From my background, the way gr_filter_design works currently is the
'right' way and our gr_iif_filter uses the 'wrong' representation. But
I want to hear other people's opinions on this. What's the standard
convention for other IIR filtering tools/programs that people have
used? Should be change the way gr_iif_filter takes in the taps, or
should we change how gr_filter_design produces them?

Thanks for the feedback!

Tom