FIR filter coefficients

hi,

I am trying to implement digital FIR filter in my DSP kit.I know the order, sampling frequency and my signal bandwidth. I want to design Multi band pass filter.

Currently I am working on "fdatool" in Matlab for FIR filter designing. I am trying to make two filter convolve and then get the desired results. One LPF and other bandpass. I successfully get the coefficients in the Matlab workspace and the result of convolution is to some extent satisfactory but the problem is I don't know anything about the filter structure that is used by the original source codes. Cause when I copied the coefficients from the source file and then implemented it Matlab the result was as expected, but when I change the filter structure like Direct or Transpose or anything, nothing happens as oppose to my designed filters whose respone changes as I change the structure.

   What could be the reason for that...

[Honglei Chen]

Hi Talha,

I don't quite follow your question. Could you clarify what you do, what changes and what does not change?

Thanks.

[Talha]

Here are the specs my sampling frequency is 500 Hz, I want to design a multi band pass filter having following characteristics like:

Its total bandwidth is 150 Hz and there is notch from 49 to 51 Hz.

I am trying to make one like this but so far haven't achieve good results.

[Rick Rosson]

I am not understanding it either. Please tell us:

1. What you want to do

2. What you have tried so far that has worked

3. What you have tried so far that has not worked

4. What you are seeing in MATLAB that tells you it is not

working (e.g. error message or incorrect behavior).

Thanks!

Rick

[Talha]

Hi Rick, here's the answers:

1. Make a multi band filter

2. Made a multi band filter by convolution of Low pass and a bandstop at 150 and 49-51 Hz respectively.

3. Burned the code, but the noise didn't go

4. Matlab is showing me the results are fine as in dB diagram but the practical implementation says its otherwise.

I already have a same configuration multi band filter with a notch at 60 Hzsupplied by the company. Its coefficients and when I make my own are different.

[Honglei Chen]

Hi Talha,

Did you try to look at the response of filter before you burnt the code? Do you see a notch at 50Hz? In addition, do you really have to do it in an FIR filter? From your spec, I'd try a notch filter rather than cascading a lowpass and a bandpass together. Type the following command for details if you have Filter Design Toolbox or DSP System Toolbox.

help fdesign.notch

[Talha]

yes the response was pretty close to good actually. The notch was at 50 Hz. and Yes I require a FIR cause its the only one that is supported by the DSP. And a notch is only available in IIR.

Answer 1

[Veera Kanmani]

Use FDA tool