function [up,down] = envelope(x,y,interpMethod)

%ENVELOPE gets the data of upper and down envelope of the known input (x,y).
%
%   Input parameters:
%    x               the abscissa of the given data
%    y               the ordinate of the given data
%    interpMethod    the interpolation method
%
%   Output parameters:
%    up      the upper envelope, which has the same length as x.
%    down    the down envelope, which has the same length as x.
%
%   See also DIFF INTERP1

%   Designed by: Lei Wang, <[email]WangLeiBox@hotmail.com[/email]>, 11-Mar-2003.
%   Last Revision: 21-Mar-2003.
%   Dept. Mechanical & Aerospace Engineering, NC State University.
% $Revision: 1.1 $  $Date: 3/21/2003 10:33 AM $

if length(x) ~= length(y)
    error('Two input data should have the same length.');
end

if (nargin < 2)|(nargin > 3), error('Please see help for INPUT DATA.');
elseif (nargin == 2),  interpMethod = 'linear'; end
  http://www.chinavib.com/forum-viewthread-tid-32967-extra-page%3D1-page-1.html

 
% Find the extreme maxim values and the corresponding indexes
%----------------------------------------------------
extrMaxValue = y(find(diff(sign(diff(y)))==-2)+1);
extrMaxIndex =   find(diff(sign(diff(y)))==-2)+1;

% Find the extreme minim values and the corresponding indexes
%----------------------------------------------------
extrMinValue = y(find(diff(sign(diff(y)))==+2)+1);
extrMinIndex =   find(diff(sign(diff(y)))==+2)+1;

up = extrMaxValue; up_x = x(extrMaxIndex);
down = extrMinValue; down_x = x(extrMinIndex);

% Interpolation of the upper/down envelope data
%----------------------------------------------------
up = interp1(up_x,up,x,interpMethod);
down = interp1(down_x,down,x,interpMethod);