先看下fir1中的一段解釋
For filters with a gain other than zero at Fs/2, e.g., highpass
and bandstop filters, N must be even.Otherwise, N will be
incremented by one.In this case the window length should be
specified as N+2.
即高通、帶阻濾波器的階數(shù)應(yīng)該控制為奇數(shù),因?yàn)槿绻A數(shù)為偶數(shù),則在π點(diǎn)必有一零點(diǎn),這對于高通帶阻來說是不允許的,故取階數(shù)為奇數(shù),而你FIR1濾波器階數(shù)為M+1階,所以你的M必須為偶數(shù),所以可以將程序改為
clear all;
clc
wlp=0.2*pi;wls=0.35*pi;wus=0.65*pi;wup=0.8*pi;
B=wls-wlp;
M=ceil(12*pi/B);
M=M+mod(M,2);
wp=[(wls+wlp)/2/pi,(wus+wup)/2/pi];
hn=fir1(M,wp,'stop',blackman(M+1));
freqz(hn)