[SciPy-user] normalized frequencies to pi radians / sample

[Robert Kern]

On Tue, Mar 18, 2008 at 2:08 PM, Forrest Sheng Bao
<forrest.bao at gmail.com> wrote:
> Hi,
>
> I am trying to use buttord function to compute the order and frequency of a
> Butterworth filter.
>
> But I got confused about the passband and stopband edge frequency. According
> to the doc,
>
>       wp, ws -- Passband and stopband edge frequencies, normalized from 0
>                  to 1 (1 corresponds to pi radians / sample).  For example:
>                    Lowpass:   wp = 0.2,          ws = 0.3
>                    Highpass:  wp = 0.3,          ws = 0.2
>                    Bandpass:  wp = [0.2, 0.5],   ws = [0.1, 0.6]
>                     Bandstop:  wp = [0.1, 0.6],   ws = [0.2, 0.5]
>
> I should normalize the frequency to pi radians / sample. Now, suppose a
> frequency 10Hz and my sampling rate is 100 Hz. Is the normalized frequency
> 0.1?

0.2, I believe. If the sampling rate is 100 Hz, the Nyquist frequency
(pi radians/sample) is half that: 50 Hz.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
 -- Umberto Eco