function [v_vect, f_vect] = da_f(fc,fstop,fstep)
%
% function [v_vect, t_vect] = da_f(fc,fstop,fstep)
%
% returns the normalized frequency gain for the function used by Dong 
% and Atick to fit the time-kernel of a LGN X-cell. fc is the temporal 
% cutoff frequency in Hz (usually set at 5.5Hz). 

fc1 = 2*pi*fc; %converts to circular frequency (from Hz to cycles/sec)
f_vect = 0:fstep:fstop; %in Hz
f_vectc = 2*pi*f_vect;
v_vect1 = complex(0,f_vectc);
v_vect2 = complex(fc1,f_vectc).^3;
v_vectt = v_vect1./v_vect2;

v_vect = abs(v_vectt);
v_vect = v_vect./max(v_vect);