%SP_STZCR Short-time zero crossings.

%       [Y,TSCALE] = SP_STZCR(X,FRAME,OVERLAP,FS) computes the
%       short-time zero-crossing rate of X using a frame size
%       of LENGTH and a percentage OVERLAP between successive
%       frames using a rectangular data window.  The sampling
%       frequency is given by FS. The short-time zero-crossing
%       curve is returned in Y and a time scale corresponding
%       to the end of the data frame is returned in TSCALE.
%       The curve may be displayed with the command
%       'plot(y,tscale)'.
%
%       See also: STMAG, STENG, AVSMOOTH, MDSMOOTH

%       Matlab original by:
%       LT Dennis W. Brown 7-11-93, DWB 11-11-94
%       Naval Postgraduate School, Monterey, CA
%       May be freely distributed.
%       Not for use in commercial products.

%       Ref: Rabiner & Schafer, Digital Processing of Speech
%       Signals, 1978, ss 4.3, pp 127-130.

% window argument is not used but is here to maintain consistency with
% the other sp_ routines.

static (sgn)

stzcr = function (x, frame, overlap, fs, win)
{
  local (x, frame, overlap, fs, win)

  % default values
  y = [];
  tscale = [];

  % must have at least 3 args
  if (nargs < 4)
  {
    error("sp_stzcr: Requires first four arguments.");
  }

  % figure out if we have a vector
  if (min(size(x)) != 1)
  {
    error("sp_stzcr: Input arg \"x\" must be a 1xN or Nx1 vector.");
  }
  
  % work with Nx1 vectors
  x = x[:];

  % variables
  Ns = length(x);                             % number of samples
  N = floor(fs * frame);                      % samples per frame
  Ndiff = floor(N * (1 - overlap/100));       % samples between windows
  L = floor((Ns-N)/Ndiff);                    % number of windows
  y = zeros(L,1);                             % space for answer
  tscale = zeros(L,1);                        % space for time

  % use the absolute value of x
  t = abs( sgn( x[2:Ns;1] ) - sgn( x[1:Ns-1;1] ) );

  % windows with overlap
  for (k in 1:L)
  {
    s1 = (k-1) * Ndiff + 1;                     % start of window
    tscale[k;1] = k * Ndiff/fs;
    y[k;1] = sum(t[s1:s1+N-1;1])/2/N;
  }

  return << tscale=tscale ; y=y >>;
};


% function y = sgn(x)
%
% Returns the signum function.
%
%         { 1    x >= 0
%    y = {
%         {-1    x < 0
%
% by Dennis W. Brown, Last modified: 7/11/93
% Naval Postgraduate School, Monterey, CA

sgn = function ( x )
{
  y = sign(x);
  y = (y >= 0) + (-1) * (y == -1);
  return y;
};