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What is the correct way to extract coefficients (in a form I could use in RecurrenceFilter) from a filter which I have represented as a Mathematica transfer function object, ie. something of the form TransferFunctionModel[...]? I can easily extract the various parts of this object using array indices, but surely there is an "official" method?

I looked at the Mathematica documentation. But, for example, the coefficients used in In[101] here look like they've been copied manually from In[92], rather than obtained through a coefficient extraction function.

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My first question would be why not use RecurrenceFilter[TransferFunctionModel[...],...].

There is more than one way to get the coefficients. The following will work for rational, scalar systems, without delays - which is what is handled by RecurrenceFilter. 

tfd1 = TransferFunctionExpand[tfd][s][[1, 1]]

Reverse@CoefficientList[#, s] & /@ Through@{Denominator, Numerator}@tfd1

{{21, -25, 15, -3}, {1, 3, 3, 1}}

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  • $\begingroup$ In answer to your question: I want to use Mathematica to derive my filter so I can use it somewhere else. Your solution is the same as mine but it didn't seem right to just unpack the internals of Mathematica's structure. $\endgroup$
    – Dan Piponi
    Sep 30, 2014 at 1:11

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