[Dspforum] Matlab IIR filter outputs

[doug wenstrand]

Yea, that makes sense.  In fact, it was what I was trying to describe in
class, but was so concerned about time that I'm sure I didn't do it
adequately.
What you normally see is 5 actual real biquads, and then some more stages :
before the whole thing (+1) after the whole thing (+ 1) and between each IIR
stage (+4).  That gives you a total of 11.  Those stages are written by
MATLAB as biquads (so that you can do the computation easily) but as you
point out, they aren't really much of anything.  When all the denom
coefficients are set to 0, and the numerators are all (except the first one)
what you have is just a gain stage : (output = b*input).  Matlab typically
puts one of these at the front, and/or at the back.  The option is also
there to scatter gain stages in the middle, but usually those are 1.

So yes, feel free to not implement those gain stages as actual biquads,
after all, it is just a single multiply.  You can also get some mileage out
of your suggestion of just folding the gain into the first stage by applying
it to the exported coefficients.   This isn't _exactly_ the same though
right?


On Mon, Nov 3, 2008 at 5:32 PM, Ryan Merton <lucidsmog at gmail.com> wrote:

> Hi,
>
> So if I design an IIR filter with FDATool, set it to single-precision
> float quantization, and output the coefficients at SOS, I end with
> with something curious:
>
> If I have a 10th order filter, I have 5 SOS.  But what I end up with
> are 11 sections.  It appears as though some of the section are just
> constants though.  I think their only purpose is to apply gain.  I
> think I can just multiply the gain into the numerator coefficients and
> be left with 5 sets of numerator/denominator coefficients.
>
> Does that make sense?
>
> Thanks,
> Ryan
>
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