 If you place WINBRF (FIR type Band Reject Filter by Window Function Algorithm),
the impulse response is following. See there is some DC in it.
And see its spectrum. That's the same as WINBRF's frequency response and
0Hz passes through this filter.
That's it for this tutorial. Please understand (1)The spectrum of unit impulse is flat and any frequency has the same amount. (2)The spectrum of impulse response is the same as the filter's frequency response. (3)FIR filter's coefficients is the same as its impulse response. (4)Therefore, if you execute FFT on FIR filter's coefficients, the result should be the same as the filter's frequency response.
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