function z = zeroxing(sig,N,step)
% z = zeroxing(sig,N,step)
%
% zeroxing - zero crossing rate of sig with rect. window 1/2N w/ width N
%
% ELEC 301 Project - "Mars Lander the Theologian", aka, "Speaker Verification"
% Sara MacAlpine, Aamir Virani, Nipul Bharani, JP Slavinsky
% WRC (OCEE) Class of 2001 - Fall '99

% Determine bounds of signal
sig_start = 1;
sig_stop = length(sig);

% prepare zero-bordered signal so can start windowing from sig(1) back to
% sig(N-2) - this is a little bigger than needed buy can accomodate different
% windows.
size_sig = size(sig);
if size_sig(1) > size_sig(2)
  new_size_sig(1) = size_sig(1) + 2*N -2;
  new_size_sig(2) = size_sig(2);
else
  new_size_sig(1) = size_sig(1);
  new_size_sig(2) = size_sig(2) + 2*N -2;
end

sig_padded = zeros(new_size_sig);

sig_padded(N:N+length(sig)-1) = sig;

% Set relative bounds on rectangular window - win_start will be the number
% of values below the current index and win_stop will be the number of
% values included above the current index.  Added together, they should
% equal N-1
win_start = floor(N/2);
win_stop = N - win_start - 1;

% Set window value
window = 1/(2*N);

% Calculate zero crossing rate
i=0;
for n = N:step:N+length(sig)-1,
  num_zeroxings = 0;
  sig_inwindow  = sig_padded(n-win_start:n+win_stop);
  for m = 2:N,	
    num_zeroxings = num_zeroxings + abs(sign(sig_inwindow(m)) - sign(sig_inwindow(m-1)));
  end
  i = i + 1;
  zeroxing_stepped(i) = num_zeroxings * window;
end  

z = zeroxing_stepped;