function out = halfinterp(n,fp,l,x)
b = firhalfband(n,fp,'dev');%firhalfband(n,fp);   %半帶濾波器系數
hf = mfilt.firinterp(l,b);     %使用半帶濾波器設計一個多相插值濾波器
x_real = real(x);
x_imag = imag(x);
out_real = filter(hf,x_real); %實部和虛部分開濾波
out_imag = filter(hf,x_imag);
outt = (double(out_real) + j*double(out_imag));
out=outt*sqrt(var(x)/var(outt));  %使得輸入和輸出的功率相同以便比較