Converting FIR from IIR

Hi all,
I have designed a FIR filter which has 60 DB SNR.Also I designed IIR filter at 60 DB SNR. FIR taps is around 700 whereas IIR is 12.
So I want to design IIR.But I want linear phase.
How can I achieve linear phase using IIR filters.
Thank you,
Abhijith Start a New Thread

hi Abhijit,

stability, linear phase and causality, u cannot achieve all the three
simultaneously in an IIR filter, (pole -zero plot)
now u have stability and causality.

u get only 2, and u have to give up the 3rd one, if u want linear phase, u
need to give up either stability or causality.

if u give up stability, ur filter is of no use and if u give up causality,
its probably not easy to implement it.
So FIR filters are the only approach to linear phase filters, u may use the
optimal recursive method to get minimum taps, but thats the best u can do.
Pradeep

On 9/19/07, i...@yahoo.co.in wrote:
>
> Hi all,
> I have designed a FIR filter which has 60 DB SNR.Also I designed IIR
> filter at 60 DB SNR. FIR taps is around 700 whereas IIR is 12.
> So I want to design IIR.But I want linear phase.
> How can I achieve linear phase using IIR filters.
> Thank you,
> Abhijith
>
>

--
winners dont do different things, they do things differently.... Start a New Thread

It's possible. You can refer to FILTFILT command in MATLAB. You have process
the signal by buffer.

Wei Start a New Thread

Abhijith-

> I have designed a FIR filter which has 60 DB SNR.Also I designed IIR filter at 60 DB SNR. FIR taps is around 700 whereas IIR is 12.
> So I want to design IIR.But I want linear phase.
> How can I achieve linear phase using IIR filters.

Generally it's not possible to achieve exactly linear phase response with IIR
filters, but there are things you might try:

1) You could design an all-pass, phase-compensation IIR filter and apply in cascade
with your current IIR filter. Design the phase response to "equalize" the overall
phase response as needed. This requires an "arbitrary design method", not one based
on mathematical formula (i.e. not Butterworth, Chebyshev, elliptic, etc). You can
check out this page:

http://www.icsi.berkeley.edu/~storn/fiwiz.html

and search for "Allpass design".

2) You could use a modified FIR technique that gets you "mostly linear" phase
response, but reduces the number of taps. Again, check out the above page and search
for "Arbitrary group delay constraints".

3) You could use a different IIR filter design method, like Bessel, that has somewhat
more linear response in transition regions. It sounds to me like you're currently
using an elliptic filter method.

Note that these techniques all increase the number of coefficients ('taps'). This is
one reason why people talk of "tradeoffs" when they compare IIR and FIR filters.

-Jeff Start a New Thread

For a filter with linear phase response you need a symmetric impulse response. Sorry, that's FIR territory.

You may be able to shrink your FIR for example using multirate techniques. The exact way to proceed depends on the problem.
Another potential implementation is via FFT, but I'd start looking at the requirements that demand a 700 tap FIR first.

-mn Start a New Thread

You need to include a complementary all-pass filter to counter the phase
change produced by the IIR. Put the two in series.
David P. Reaves, III
TransLanTech Sound, LLC
Home of the Award-winning "Ariane Sequel" Digital Audio Leveler

In a message dated Thu Sep 20, 2007 5:08 am (PDT), "i...@yahoo.co.in"
writes:
>
> Hi all,
> I have designed a FIR filter which has 60 DB SNR.Also I designed IIR filter
> at 60 DB SNR. FIR taps is around 700 whereas IIR is 12.
> So I want to design IIR.But I want linear phase.
> How can I achieve linear phase using IIR filters.
> Thank you,
> Abhijith
>

David P. Reaves, III
TransLanTech Sound, LLC
Home of the Award-winning "Ariane Sequel" Digital Audio Leveler Start a New Thread

Abhijith,
Send me your FIR coefficients and I will convert them into an IIR with linear phase in the passband using my software (Sketch-A-Filt).
Regards,
Bobby

----- Original Message -----
From: i...@yahoo.co.in
To: a...
Sent: Wednesday, September 19, 2007 12:31 PM
Subject: [audiodsp] Converting FIR from IIR
Hi all,
I have designed a FIR filter which has 60 DB SNR.Also I designed IIR filter at 60 DB SNR. FIR taps is around 700 whereas IIR is 12.
So I want to design IIR.But I want linear phase.
How can I achieve linear phase using IIR filters.
Thank you,
Abhijith Start a New Thread