function X = chirpf(x,theta0,dtheta,K);

% Synopsis: X = chirpf(x,theta0,dtheta,K).

% Computes the chirp Fourier transform on a frequency

% interval.

% Input parameters:

% x  : the input vector

% theta0 : initial frequency (in radians)

% dtheta : frequency increment (in radians)

% K  : number of points on the frequency axis.

% Output:

% X: the chirp Fourier transform of x.



% Part of software package for the book:

% A Course in Digital Signal Processing

% by Boaz Porat, John Wiley & Sons, 1997



N = length(x); x = reshape(x,1,N); n = 0:N-1;

g = x.*exp(-j*(0.5*dtheta*n+theta0).*n);

L = 1; while (L < N+K-1), L = 2*L; end

g = [g, zeros(1,L-N)];

h = [exp(j*0.5*dtheta*(0:K-1).^2), ...

     exp(j*0.5*dtheta*(-L+K:-1).^2)];

X = ifft(fft(g).*fft(h));

X = X(1:K).*exp(-j*0.5*dtheta*(0:K-1).^2);