Re: FIR Filter limitation (or not?)


"rover8898" <rover8898@xxxxxxxxxxx> wrote in message
news:1133216698.967625.187630@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Hello everyone,
>
> OK. So I got the point that the response of a digital filter just
> repeats itself at a period of Fs. Also it seems that a digital signal
> has only frequency components from -Fs/2 to +Fs/2. If I undestood
> correctly, this is because all signals when digitized, are, in the
> frequency domain a series of the frequency spectrum of the undigitized
> signal interspaced at intervals of Fs. Depending on the bandwidth of
> the undigitized signal and the Fs, absence or presence of aliasing will
> be determined.
>
> So basically, if aliasing occurs, the *digital signal* (-Fs/2 to +Fs/2)
> will have frequency components of the original signal that are above
> Fs/2. That is where those unwanted "high frequencies" end up; they
> double back in the relevant [-Fs/2 to +Fs/2] frequency range if there
> is aliasing .If there isn't any aliasing, it implies that the input
> signal was already properly cleaned up. And if the input signal
> contains frequencies components up to Fbw, and we decide to sample the
> input signal at Fbw/10 because 95% of the relevant signal lies under
> Fbw/30, we would be commiting a huge no-no for the frequency components
> from [Fbw/20 to Fbw] will fold back and corrupt the retrieved data,
> despite these components being quite weak in strength.Rigth ?
>
> A low pass digital FIR filter whose [Fpassband=0.45*Fs and
> Fstopband=0.47*Fs] serves no practical purpose if its purpose for being
> is to attenuate high frequencies. Right?
> It will supress only 6-10% of the input frequencies (depending on the
> transitions bands). In other words:
> the [0-0.45 0.55-1.45 1.55-2.45 2.55-3.45....]*Fs frequencies of the
> undigitized input signal will pass unhindered through the digital
> lowpass FIR filter. It certaintly not a viable antialiasing filter.
> Rigth?
>
> -Roger

Right. Good study. The antialiasing filtering needs to be done
pre-sampling. If you're doing sample rate conversion, then you could well
*also* need to do antialiasing filtering in the discrete time world - and
*there* you use digital filters.

Fred


.

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