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Relate lowpass filter cutoff frequency with real frequency (Hz) to filter from a sampled signal?

Started by Unknown August 21, 2009
Hello to all,

My problem is the following:

I have a signal x that was sampled with Fs = 8000 Hz and sampled 1000 samples.

I want to filter the signal using a truncated version of an ideal
discrete lowpass filter getting the filter coefficients b with the
sinc function:

k = -100:1:100 ;
b = (1/Wc) sinc (Wc * k) ;

I want to filter frequencies higher than 2500 Hz, I must somehow
relate the Wc (filter cutoff frequency) with the 2500 Hz frequency
that I want to filter from my sampled signal at 8000 Hz.

I thought that Wc = 2*pi*(2500 / 8000) but, when I when I plot the fft
the frequency response is not what I would expect, as such nor the
filtered signal is filtered correctly.

I filter using

x1 = filter(b,[1],x) ;

My question is how do I relate Wc with the 2500 frequency cutiff and
the 8000 sampling frequency ?

Thanks a lot!
Mio