# Iir\_filter does not work with specified feedback taps?

**URL:** <https://www.ruby-forum.com/t/iir-filter-does-not-work-with-specified-feedback-taps/232678>\
**Category:** GNU Radio\
**Created:** [October 22, 2014, 2:10am UTC](https://www.ruby-forum.com/t/iir-filter-does-not-work-with-specified-feedback-taps/232678 "2014-10-22T02:10:02Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\
**Post date:** [October 22, 2014, 2:10am UTC](https://www.ruby-forum.com/t/iir-filter-does-not-work-with-specified-feedback-taps/232678/1 "2014-10-22T02:10:02Z")

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Hi All,

Any guys who ever used IIR filters? I got problems as below:

I want the IIR filter works as:  
y[n] = 1.8_x[n] + 0.8_y[n-1]  
Then I set the feed forward taps as [1.8], feeback taps as [0.8], just  
like  
self.iir\_filter\_xxx\_0 = filter.iir\_filter\_ffd(([1.8]), ([0.8]), True)

But my testing result show that the feedback taps are not involved in  
computing at all, thus my output turns to  
y[n] = 1.8\*x[n]

I don’t know anything wrong happened to my usage…  
Need help. Thanks

–

Alex,

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**Author:** ![akeem](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/54ee81/32.png) [@akeem](https://www.ruby-forum.com/u/akeem)\
**Post date:** [October 22, 2014, 3:14pm UTC](https://www.ruby-forum.com/t/iir-filter-does-not-work-with-specified-feedback-taps/232678/2 "2014-10-22T15:14:33Z")

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On Tue, Oct 21, 2014 at 8:08 PM, Alex Z. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> But my testing result show that the feedback taps are not involved in  
> computing at all, thus my output turns to  
> y[n] = 1.8\*x[n]
> 
> I don’t know anything wrong happened to my usage…  
> Need help. Thanks
> 
> –
> 
> Alex,

We use a convention where the first feedback tap is assumed to be 1.0.  
So  
you’ll need to make your taps [1.8] and [1.0, 0.8].

Tom