MATLAB FIR1 function

Hi,

why Does MATLAB FIR1 function use -6 dB cutoff frequency instead of -3 dB ?

I am using MATLAB 6.5 version.

I am trying to design a LPF with the help of FIR1 function.

n = 51; % order of the filter
wn = 0.4 % cutoff frequency
fir1(n,wn); 
freqz(b);
  
Now my cutoff frequency should come on the -3 dB level but it is on -6 dB level.



Regards
Abhishek

Abhishek Agrawal wrote:

> Hi,
> 
> why Does MATLAB FIR1 function use -6 dB cutoff frequency instead
> of -3 dB ?
> 
> I am using MATLAB 6.5 version.
> 
> I am trying to design a LPF with the help of FIR1 function.
> 
> n = 51; % order of the filter
> wn = 0.4 % cutoff frequency
> fir1(n,wn);
> freqz(b);
>   
> Now my cutoff frequency should come on the -3 dB level but it is
> on -6 dB level.

"help fir1"  tells at least that this is what you should expect.

"fdatool" reveals that this is a property of the algorithm they use 
for calculation of windowed fir filter coefficients.
Maximally-flat fir filters instead provide -3dB at the wn-location.

Did you consider posting this question to the comp.soft-sys.matlab 
ng? 

Bernhard

> 
> 
> 
> Regards
> Abhishek