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Hi Rammya,
 
1. Why we choose IIR filter design for practical application, especially for
processing audio signal?
 
We can design an IIR Filter of far less order than an FIR filter giving us
similar desired response. This greatly reduces the hardware resources required.
On the flip side, IIR Filters have a non-linear phase response, which if
acceptable, the filter is best designed as IIR. Also, we have to be very careful
about Stability issues, as IIR Filters have both Poles and Zeros, if not
designed carefully Poles can make them unstable.
 
2. Why not FIR?
 
Answered above.
 
3. Depending on the response there are many IIR filter(butterworth, bessel
etc).Is there any sub division for FIR?

The most commonly used design techniques are : 
1. Window Design Method
2. Frequency Sampling Method
3. Optimum Equiripple Method
 
4. What are the disadvantage of FIR filter design over IIR?

Answered above.
 
There are several other differences between FIR and IIR filters, I have listed
the most basic, but crucial ones.
 
Hope this helps.
 
Regards,
Rajmathi S.
      

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