unexpected shift after downsampling using decimate
16 vues (au cours des 30 derniers jours)
Afficher commentaires plus anciens
n = 1000;
t = 1:n;
x = rand(1,n) - 0.5;
x = sin(2*pi/100*t);
y = decimate(x,4);
figure('color','w');
ha = axes('nextplot','add','box','on');
plot(ha,t,x,'k','marker','.');
plot(ha,t(1:4:end),y,'r','marker','o'); addkeycb;
There is a shift of 3 points.
0 commentaires
Réponse acceptée
Star Strider
le 21 Nov 2014
Add the default filter to produce the correct decimation:
y = decimate(x,4,'fir');
3 commentaires
Star Strider
le 23 Nov 2014
My pleasure!
It may have to do with the difference between the default IIR filter (using filtfilt) and the specified FIR filter (using filter). They have different characteristics, and one may work better in some situations than the other. (I didn’t try resample to see what results it would produce. That might be worth exploring if you’re interested.)
F S
le 6 Août 2019
This answer solves the problem but is technically wrong. The answer from JK below gives you the real explanation and solution, in case you'd rather use the default filter.
Plus de réponses (1)
Jonathan Kohler
le 27 Juin 2017
Modifié(e) : Jonathan Kohler
le 28 Juin 2017
This apparent time shift is due to MATLAB's choice of initial index for the down-sampled data, and only indirectly related to the choice of filter.
As per the documentation ( https://www.mathworks.com/help/signal/ref/decimate.html , under 'Algorithms'), the first point of the original data and downsampled data are chosen to match for FIR filters, and the last point are chosen to match for IIR filters. The reason for this choice eludes me, but maybe the provided reference explains it.
Because of the difference in handling FIR filters, there is in no shift, as pointed out by Star Strider. However, to fix this for IIR filters, you need only change your choice of time values corresponding to the downsampled data. Instead of choosing
t(1:r:end)
You should choose
nBeg = mod(n-1,r)+1;
t(nBeg:r:end)
where r=4 is the decimation factor applied.
1 commentaire
Voir également
Catégories
En savoir plus sur Multirate Signal Processing dans Help Center et File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!