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FIR Filter limitations (or not?)

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rover8898

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limitation of fir filter

Hello,

I am quite new to the field of digital FIR filters (I always worked with the old-fashionned analog ones, but I digress).
There is an important aspect to FIR filters that eludes me:
A low pass digital FIR filter filters out all frequencies in the stopband (to the set attenuation). This is quite obvious. Now what remains unclear to me is that if the digital filter can only recover sucessfully frequencies up to Fs/2 (Fs=sampling frequency), in order to either filter them if they lie in the stopband or let them pass if they lie in the passband, does it (the digital FIR filter) also filter out frequencies above Fs/2 (since they lie in the stopband)? Or, are the frequency components of above Fs/2 of the input signal not filtered, but rather emulate something akin to "noise". Basically, if a digital FIR lowpass filter can only filter out frequencies up to Fs/2, it would imply that an a simple [20dB/dec lowpass analog filter] would be able to filter frequencies that a [-100dB FIR lowpass filter with Fs/Fc=1.1] would not be able to. This seems quite odd as all litterature indicates that digital FIR filters far out-perform analog filters. Also, if digital filters (lp, hp, bp, bs) have a severe limitation to their upper-frequency-of-proper-operation, it would mean that all digital filters require an analog lowpass filter at their input that can be tunned on the fly w.r.t Fs, as to remove the frequency components above Fs/2 prior to the digital filtering. This seems highly unlikely.
What am I not undestanding properly?

Thanks in advance,
-Roger
 

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