sigmoid_art.m

matlab中噪声模型

function [xx, x] =sigmoid_art(N,a,c)
% [xx x] = sigmoid_art(N, a, c);
% N  = length of pulse (in samples)
% a and c are parameters of the sigmoid fn - see sigmf.m
% 
% uses SIGMF function to generate artificial baseline shifts. The
% rate of which the ramp from 0 to 1 happens depends upon a and c.
% Defaults are a=-2; c=5; The more negative a becomes, the steeper 
% the ramp. If a becomes positive then the signal is inverted.
% alues of c other than 5 lead to asymetric waves .. e.g.
% [xx x] = sigmoid_art(100,-1,0);
% gives a 100 point vector which rapidly ramps to unity from 0 then
% drops rapidly back to zero with a discontinuity at the 51st element.
% xx produces a continuous pulse where the second half is a mirro reflection
% of the first half of the wave.
% 
% see PULSTRAN for generating a continuous time series of this output 
% (but remember that artefacts are not periodic!!!!)
%
%   See also: SINC_ART, SINC SQUARE, SIN, COS, CHIRP, DIRIC, GAUSPULS, 
%             PULSTRAN, RECTPULS and TRIPULS  

%   Author: G. Clifford, 8-10-03
%   Copyright 2003 MIT

if nargin < 3
  c = 5;    
end
if nargin < 2
  a = -2;
end
if nargin <1
    N = 100;    
end

% make a time vector
spacing = (50/N) * 0.2;
t=(spacing:spacing:10);

% make a sigmoid
y = sigmf(t,[a c]);

% normalise
y=y/max(y);

% reflect at N/2 samples
ord = [length(y):-1:1];
z=y(ord); 

%invert it and add it back on.
x = [-1*y+1 y];
xx = [-1*y+1 -1*z+1];

% normalise amplitude
x = x/max(x);
xx = xx/max(xx);

if nargin <1
    figure;
    plot(xx);
end