Hi Stefan,
Thanks for the data set. Here's an idea to get you started.
% RRintervals is a list of the intervals between successive R waves
% approximate the time relative to the first interval
t = cumsum(RRintervals);
% make a uniform grid inside the full interval
% I arbitrarily picked a grid at 1 second intervals... you may find something else more useful.
tGrid = 1:55;
% interpolate to get the beats per second on a uniform grid
bps = interp1(t,RRintervals,tGrid,'spline');
% compute the PSD
% units of Pxx are squared seconds/Hz.
% sample rate is 1 Hz.
[Pxx,F] = periodogram(bps,[],numel(bps),1);
% Zero out the DC bin so it fits nicely on screen
Pxx(1) = 0;
% make a pretty plot
plot(F,Pxx);
xlabel('Hertz');
ylabel('s^2 / Hz');
% compute the power in the various bands...
% note that I removed the DC bin earlier, so VLF is somewhat suspect...
vlf = bandpower(Pxx,F,[0 0.04],'psd') % units of sec^2
lf = bandpower(Pxx,F,[0.04 0.15],'psd') % units of sec^2
hf = bandpower(Pxx,F,[0.15 0.4],'psd') % units of sec^2
totPower = bandpower(Pxx,F,'psd') % units of sec^2
% you can then take the ratio of lf, hf, etc. to totPower * 100 to get the percentages etc.
Hope this helps.
-G
