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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, , 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

% 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);

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