function  output=demod(input)
len=length(input);
fc=455000;   %載波頻率
Rs=128000;
fs=5*Rs*4;
w=2*pi*fc/fs;
j=sqrt(-1);
N=150;       % 濾波器的階數為(N+1)  
A=[1,1,0,0];
F=[0,1/(5*2),1/(5*2),1]; % 128000/640000
lpf=firls(N,F,A);
t=0:len-1;
%t=3:len+2;
Idesignal=input.*cos(w*t);
Idesignal=conv(Idesignal,lpf);
Idesignal=Idesignal(N/2+1:N/2+len);
Qdesignal=input.*sin(w*t);
Qdesignal=conv(Qdesignal,lpf);
Qdesignal=Qdesignal(N/2+1:N/2+len);
output=2*Idesignal-j*2*Qdesignal;