Spectrogram of Two Beating Chirps

N=512; M=64;
c=chirp(0:1/(2*N-1):1,0,1,512);
s = c(:) + circshift(c(:),N/32);
specgram(s,N,1,M,M-1);
% specgram(sig,Nfft,fs,winlength,novlp);