2-sided Zmod

Rotating shafts experience a an elliptical motion called whirl. It is important to decompose this motion into a forward and backward whil orbits. The current function makes use of two sensors to generate a bi-directional spectrogram. The method can be extended to any time-frequency distribution
%
% compute the forward/backward Campbell/specgtrogram
%
% INPUT:
% y (n x 2) each column is measured from a different sensor
% ///////
% __
% |s1| y(:,1)
% |__|
% __
% / \ ________|/
% | | | s2 |/ y(:,2)
% \____/ --------|/
%
% Fs Sampling frequnecy
%
% OUTPUT:
% B spectrogram/Campbel diagram
% x x-axis coordinate vector (time or Speed)
% y y-axis coordinate vector (frequency [Hz])