function x = fmcubic(N, a)% fmcubic -- create a signal with an interesting cubic frequency modulation%%  Usage%    x = fmcubic(N)%%  Inputs%    N    length of the signal%    a    degree of warping, -0.5 =< a < 1 (optional, default is 0.5)%%  Outputs%    x    signal% Copyright (C) -- see DiscreteTFDs/Copyrighterror(nargchk(1,2,nargin));if (nargin <2)  a = 0.5;endif (a<-0.5 | a>= 1)  error('a out of range')endn = (1:N)';P = polyfit([0 N/4 N/2 N],[-pi a*pi 0 pi],3);x = exp(j*(P(1)*n.^4/4 + P(2)*n.^3/3 + P(3)*n.^2/2 + P(4)*n));