# REW and FIR



## ttako (Jan 15, 2011)

Hi,

is there any possibility to create a compensating FIR filter with a predefined lenght (nr of taps or max latency, or lowest frequency corner for linear phase) with REW?
I mean a plain wav for convolver and/or a list of coefficients for DSP programing...

If not, is it than planed? or possible for special request?
Thanks,

Tamas


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## JohnM (Apr 11, 2006)

REW doesn't generate FIR filters and there are no current plans to add that capability.


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## EarlK (Jan 1, 2010)

Hi John,

(i) Is it technically possible to write code that exports a subset of the Group Delay display window ( into a wave file or a text file ) ?

> A "Subset" being , any one of the 3 display options , with some smoothing applied .
> With the lowest delay value shifted to Zero Delay ( in my example, 5 ms to 0 ms ) ?

(ii) If so, is it possible to export the reciprocal of the above ? 

> Zero Delay would now correspond to the frequency areas that previously had the largest delays ( see my example ) . 
> The majority of the content would be delayed ( @ 18ms less 5 ms = 13 ms / in my example ) .


> The reciprocal GD export, could then be convolved ( as long as the user doesn't mind the 13ms delay ) to create a kludged-together linear phase response . ( At least, so goes my thinking ) .

:sn:


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## JohnM (Apr 11, 2006)

EarlK said:


> The reciprocal GD export, could then be convolved ( as long as the user doesn't mind the 13ms delay ) to create a kludged-together linear phase response.


Afraid not. What you would need to do is to take the reciprocal of the frequency response and then use an inverse FFT to arrive at the corresponding impulse response. That response, in theory, could be convolved with the original system to create perfectly flat amplitude response and correspondingly flat group delay. Unfortunately the theory does not hold up in the real world, particularly if the response is not minimum phase, so that is merely the starting point for a whole host of modifications to the approach to arrive at something that can be practically applied - that process led to DRC, for example.


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## EarlK (Jan 1, 2010)

Thanks John for the explanation !

:sn:


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