%傅立葉變換，時域卷積等效於頻域乘積
%濾波操作在時域表現為輸入訊號餘濾波器脈衝響應的卷積
%從頻域上看濾波器操作表現為，輸入訊號的傅立葉變換和脈衝響應的傅立葉變換做乘積
%對於FIR濾波器，濾波器係數即為脈衝響應
%因此，對於FIR濾波器，係數的FFT變換即為濾波器的頻率響應曲線

close all
clear
Fpass = 0.45;  % Passband Frequency
Fstop = 0.55;  % Stopband Frequency
Apass = 1;     % Passband Ripple (dB)
Astop = 60;    % Stopband Attenuation (dB)

h = fdesign.lowpass('fp,fst,ap,ast', Fpass, Fstop, Apass, Astop);

Hd = design(h, 'equiripple', ...
    'MinOrder', 'any', ...
    'StopbandShape', 'flat');
Hlp = Hd.Numerator;
fvtool(Hlp);
HLP=fft(Hlp);
HLP=abs(HLP);
F=linspace(0,2,length(Hlp));
N=ceil(length(F)/2);
plot(F(1:N),20*log10(HLP(1:N)));
grid on;