function output = mnzpk( Z, P, K )
% mnzpk : modified of MATLAB zpk function. 
%   Input arguments: 
%         Z: list of zeros[Hz]
%         P: list of poles[Hz]
%         K: the DC gain. If there are pole or zero at 0, the gain at 1Hz
if mnismember([Z P],0)
    output = zpk(Z*2*pi, P*2*pi, abs(prod(P*2*pi+1i*2*pi)/prod(Z*2*pi+1i*2*pi))*K);
else
    output = zpk(Z*2*pi, P*2*pi, prod(P*2*pi)/prod(Z*2*pi)*K);
end

end