Re: Why does fir2 do such strange thing for odd filter order (even number of taps)?

[Lev Serebryakov]

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On 13.11.2014 05:32, Mike Miller wrote:

>> And I don't understand "Add zeros to interpolate by 2, then pick
>> the odd values below." part. Why does it do this 
>> interpolate-and-take-odd-values? Why it is required only for
>> even number of taps? Why does it takes odd values? Is it
>> critical, or even ones will work too?
>> 
>> I've read several textbooks, articles and tutorials about
>> FFT-based FIR designing, and no one mention special processing
>> for even number of taps (odd order). Every source I read is in
>> agreement with method, which is used for even order — mirror
>> specter, take ifft(), rotate. It is clear. And second branch is
>> complete mistery for me.
> 
> Have you read about the differences between type I and type II FIR 
> filters?
 Of course. It is hard to miss this topic :)

> A type II filter is a symmetric filter with an odd order. This is
> what this second case in fir2.m that you are asking about is
> dealing with.
 I don't understand, why it is done in such strange way :)

> The interpolate-ifft-decimate is done to get the half-sample group
> delay necessary for a type II filter.
 I've been thought other way around: Type II is symmetrical/even taps
(by definition) and half-sample delay is only property. But now I
start to understand this oversampling thing, yes.

 Thank you.

- -- 
// Black Lion AKA Lev Serebryakov
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