Find the difference between signal values

Dear Matlab community,

is there a way to calculate the difference between signal values at given times?

I want to find the differences between the value of blue signal at the beginning of the red pulse and at the end of the red pulse, and also do it for every red pulse.

[Vinay Gurram]

Hi Bogdan,

Scope has feature called Triggers which will help you in this case. Kindly refer this it might help: https://www.mathworks.com/help/simulink/ug/scope-trigger-panel.html

Answer 1

[Mathieu NOE]

hello

if you export the data to matlab array you can use this code; it has the benefit to use linear interpolation between samples , so maximal time accuracy

I created somme dummy data, I assume you can easily adapt it to your own needs

hope it helps

Code :

clc
clearvars

%% dummy data
% repetition frequency of 3 Hz and a sawtooth width of 0.1 sec.
% The signal is to be 1 second long with a sample rate of 1kHz.

t = 0 : 1/1E3 : 1;  % 1 kHz sample freq for 1 sec
D = 0 : 1/5 : 1;    % 5 Hz repetition freq
y1 = pulstran(t,D,'rectpuls',0.03); 
y2 = sin(2*pi*5*t -0.7)+ 1*t;

threshold = 0.5; % your value here

[t0_pos1,s0_pos1,t0_neg1,s0_neg1]= crossing_V7(y1,t,threshold,'linear'); % positive (pos) and negative (neg) slope crossing points 
% ind => time index (samples)
% t0 => corresponding time (x) values 
% s0 => corresponding function (y) values , obviously they must be equal to "threshold"

% y values for signal y2 at trigger time stamps (interpolated)
s0_pos2 = interp1(t,y2,t0_pos1,'linear');
s0_neg2 = interp1(t,y2,t0_neg1,'linear');

figure(1)
plot(t,y1,'b',t0_pos1,s0_pos1,'db',t0_neg1,s0_neg1,'+b','linewidth',2,'markersize',12);grid on
hold on
plot(t,y2,'r',t0_pos1,s0_pos2,'dr',t0_neg1,s0_neg2,'+r','linewidth',2,'markersize',12);grid on
hold off
legend('signal 1','signal 1 positive slope crossing points','signal 1 negative slope crossing points',...
  'signal 2','signal 2 positive slope crossing points','signal 2 negative slope crossing points'  );


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function [t0_pos,s0_pos,t0_neg,s0_neg] = crossing_V7(S,t,level,imeth)

% [ind,t0,s0,t0close,s0close] = crossing_V6(S,t,level,imeth,slope_sign) % older format

% CROSSING find the crossings of a given level of a signal
%   ind = CROSSING(S) returns an index vector ind, the signal
%   S crosses zero at ind or at between ind and ind+1
%   [ind,t0] = CROSSING(S,t) additionally returns a time
%   vector t0 of the zero crossings of the signal S. The crossing
%   times are linearly interpolated between the given times t
%   [ind,t0] = CROSSING(S,t,level) returns the crossings of the
%   given level instead of the zero crossings
%   ind = CROSSING(S,[],level) as above but without time interpolation
%   [ind,t0] = CROSSING(S,t,level,par) allows additional parameters
%   par = {'none'|'linear'}.
%	With interpolation turned off (par = 'none') this function always
%	returns the value left of the zero (the data point thats nearest
%   to the zero AND smaller than the zero crossing).
%
%	[ind,t0,s0] = ... also returns the data vector corresponding to 
%	the t0 values.
%
%	[ind,t0,s0,t0close,s0close] additionally returns the data points
%	closest to a zero crossing in the arrays t0close and s0close.
%
%	This version has been revised incorporating the good and valuable
%	bugfixes given by users on Matlabcentral. Special thanks to
%	Howard Fishman, Christian Rothleitner, Jonathan Kellogg, and
%	Zach Lewis for their input. 

% Steffen Brueckner, 2002-09-25
% Steffen Brueckner, 2007-08-27		revised version

% Copyright (c) Steffen Brueckner, 2002-2007
% [e-mail]

% M Noe
% added positive or negative slope condition

% check the number of input arguments
error(nargchk(1,4,nargin));

% check the time vector input for consistency
if nargin < 2 | isempty(t)
	% if no time vector is given, use the index vector as time
    t = 1:length(S);
elseif length(t) ~= length(S)
	% if S and t are not of the same length, throw an error
    error('t and S must be of identical length!');    
end

% check the level input
if nargin < 3
	% set standard value 0, if level is not given
    level = 0;
end

% check interpolation method input
if nargin < 4
    imeth = 'linear';
end


% make row vectors
t = t(:)';
S = S(:)';

% always search for zeros. So if we want the crossing of 
% any other threshold value "level", we subtract it from
% the values and search for zeros.
S   = S - level;

% first look for exact zeros
ind0 = find( S == 0 ); 

% then look for zero crossings between data points
S1 = S(1:end-1) .* S(2:end);
ind1 = find( S1 < 0 );

% bring exact zeros and "in-between" zeros together 
ind = sort([ind0 ind1]);

% and pick the associated time values
t0 = t(ind); 
s0 = S(ind);

if strcmp(imeth,'linear')
    % linear interpolation of crossing
    for ii=1:length(t0)
        %if abs(S(ind(ii))) > eps(S(ind(ii)))    % MATLAB V7 et +
        if abs(S(ind(ii))) > eps*abs(S(ind(ii)))    % MATLAB V6 et -    EPS * ABS(X)
            
            % interpolate only when data point is not already zero
            NUM = (t(ind(ii)+1) - t(ind(ii)));
            DEN = (S(ind(ii)+1) - S(ind(ii)));
            slope =  NUM / DEN;
            slope_sign(ii) = sign(slope);
            t0(ii) = t0(ii) - S(ind(ii)) * slope;
            s0(ii) = level;
        end
    end
end

% extract the positive slope crossing points 
ind_pos = find(sign(slope_sign)>0);
t0_pos = t0(ind_pos);
s0_pos = s0(ind_pos);

% extract the negative slope crossing points 
ind_neg = find(sign(slope_sign)<0);
t0_neg = t0(ind_neg);
s0_neg = s0(ind_neg);

end

[Bogdan Miroschnitschenko]

Thank you a lot, it is really not easy.

[Mathieu NOE]

My pleasure