Synchronising two different signals
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Hello,
I am quite new to signal processing and I am trying my best to synchronize and align two signals that come from different devices.
One has a sample rate of 50 Hz and is recording acceleration with 3 axis (x, y and z). The other is recording ECG at 250Hz.
I have time marks of when both devices are turned on and off (which is different by max 10-20 seconds). Both seems to have 'time-drift', meaning that when looking at the data size, it never perfectly equals the size of Time*SampleRate. And even after resampling the 250Hz signal at 50Hz both data sizes have a small difference.
I have been trying different methods to align those signals but there is always a little delay between the two of them and I don't know what to do. I tried removing the time drift, synchronizing them with time-series, even the cross-correlation (even though I am not sure this can work since one of my signal has 3 axis)… Nothing seems to work.
So is there a way to know if there is major problem in one of the signals ?
Or is there another way of aligning/synchronizing those signals ?
I linked the two unfiltered signals.
Thanks,
Marine
2 Comments
What are we to do with these?
Uz = unzip('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1157478/Data.zip')
EKG = readtable(Uz{5})
Acc = readtable(Uz{7})
Just looking at them, I have no idea.
.
Marpe
on 15 Oct 2022
Accepted Answer
I got this far, however I have no idea what you want to do with them at this point —
% clear
% close all
% datapathECG = %Data path;
% dECG = dir(datapathECG);
% nd = length(dir);
Uz = unzip('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1157478/Data.zip');
% EKG = readtable(Uz{5})
% Acc = readtable(Uz{7})
%ECG data
for n=10
% file=[datapathECG,dECG(n).name];
% tbl = readtable(file);
tbl = readtable(Uz{5});
ecg=table2array(tbl);
[~,~]=size(ecg);
Fs1 = 250;
%Resampling of ECG from 250Hz to 50Hz
ecg=resample(ecg,50,Fs1);
[Necg,~]=size(ecg);
end
%Acceleration data
% close all
% datapathAcc= % Data path to data files
% dAcc=dir(datapathAcc);
% nd=length(dir);
for n=5
% file=[datapathAcc,dAcc(n).name];
% tbl = readtable(file);
tbl = readtable(Uz{7});
acctab=tbl(:,1:3);
Tacc=tbl(:,10);
acctab = table2array(acctab);
[Nacc,dum]=size(acctab);
Fs2=50;
end
%Remove drift in both
%ECG
StartECG = datetime(2022,10,11,07,56,40,'TimeZone','UTC');
StartECG.Format = 'hh:mm:ss';
Stop = datetime(2022,10,11,08,57,10,'TimeZone','UTC');
Stop.Format = 'hh:mm:ss';
Fs = 50;
Time = seconds(Stop - StartECG);
Nth = (Time.*Fs);
drift = Nth - Necg;
drift = drift/Fs;
drift = Nth/drift;
drift1 = round(drift/Fs);
ecg(drift1:drift1:end,:) = [];
[Necg,d]=size(ecg);
%Acceleration
DT = datetime(Tacc.Var10, 'InputFormat','yy-MM-dd''T''HHH:mm:ss''Z''','TimeZone','UTC');
DT.Format = 'hh:mm:ss';
DTnew = fillmissing(DT,'linear');
StartR = DTnew(1,1);
disp('Real = ');
disp(StartR);
StartR = datetime(StartR,"TimeZone","UTC");
StartR.Format = 'hh:mm:ss';
%Have to change everytime
StartTH = datetime(2022,10,11,07,55,20,'TimeZone','UTC');
StartTH.Format = 'hh:mm:ss';
disp('Theory = ');
disp(StartTH);
delay = abs(StartR - StartTH);
StartACC = StartR - delay;
Stop = datetime(2022,10,11,08,56,10,'TimeZone','UTC');
Stop.Format = 'hh:mm:ss';
Fs = 50;
Time = seconds(Stop - StartACC);
Nth = (Time.*Fs);
drift = Nth - Nacc;
drift = drift/Fs;
drift = Nth/drift;
drift2 = round(drift/Fs);
acctab(drift2:drift2:end,:) = [];
[Nacc,d]=size(acctab);
%Time for ECG
time=StartECG;
time.Format= 'hh:mm:ss';
s1=seconds(1/Fs2:1/Fs2:Necg/Fs2);
T=time+s1;
Tecg = timetable(T',ecg);
%Time for Acceleration data
time=StartACC;
time.Format= 'hh:mm:ss';
s1=seconds(1/Fs2:1/Fs2:Nacc/Fs2);
T=time+s1;
Tacc = timetable(T',acctab);
TT = synchronize(Tecg,Tacc,"union","nearest");
stackedplot(TT.Time,TT.Variables);
Or, to put it differently, I do not see a problem. You have several different signals all sharing the same time vector. (I have no idea what they refer to, or what you want to wo with them.)
.
6 Comments
Marpe
on 15 Oct 2022
Star Strider
on 15 Oct 2022
The ECG signal (that I assume is ‘Column 1’) does not look like an ECG signal to me. The amplitudes are all wrong and there is significant noise.
Are you attempting to demonstrate postural effect (or something else) on heart rate? The ECG is going to have some sort of delay in response to any external disturbance simply because that is the way physiology works.
What effect do you intend to demonstrate?
Marpe
on 15 Oct 2022
Star Strider
on 15 Oct 2022
I have never done anything with ballistocardiography. An interesting study is Estimation of the respiratory rate from ballistocardiograms using the Hilbert transform and there are several other open-source articles in a Pub Med search i did just now, many of which appear to be interesting.
The data need to be pre-processed if you are to make any sense of them. First calculate Fourier transforms (fft or pspectrum) on all of them to determine the appropriate filter characteristics, then filter them. Geting rid of as much of the noise as possible is important.
There will be a delay between the ECG, pulse (most reliably detected and characterised by LED plethysmocardiography) and ballistocardiography signals because there is inherent inertia in all the physical responses. (There are delays between the carotid and pedal pulses simply because of the transit time of the pulse wave, the physical characteristics of the arterial system, and the distance between them, for example.) The ventricles require time after the ECG excitation signal to contract and then to eject the blood that filled them during diastole. They all have mass, viscosity, and elasticity. (I modeled all this decades ago simply out of curiosity and because I had easy access to all the signals. I am certain that much better models now exist that you could identify and estimate.) So when you have filtered the signals to get rid of as much of the noise as possible, it could be interesting to use the System Identification Toolbox functions to see if a model characterising them could be identified and estimated.
I leave that to you!
Marpe
on 15 Oct 2022
Star Strider
on 15 Oct 2022
My pleasure!
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