EEG bandpass filter in mat lab

Question

I\'m trying to filter theta range (3-8 Hz) from a 10 min long EEG signal with sampling rate of 500Hz. This is my code. Please help me to understand what\'s wrong. Right now

Answer 1

Your code (line 4) gives a filter order, n, equal to 37. I've had issues of numerical precision with Butterworth filters of such large orders; even with orders as low as 8. The problem is that butter gives absurd b and a values for large orders. Check your b and a vectors, and you'll see they contain values of about 1e21 (!)

The solution is to use the zero-pole representation of the filter, instead of the coefficient (b, a) representation. You can read more about this here. In particular,

In general, you should use the [z,p,k] syntax to design IIR filters. To analyze or implement your filter, you can then use the [z,p,k] output with zp2sos. If you design the filter using the [b,a] syntax, you may encounter numerical problems. These problems are due to round-off errors. They may occur for filter orders as low as 4.

In your case, you could proceed along the following lines:

[z, p, k] = butter(n,Wn,'bandpass');
[sos,g] = zp2sos(z,p,k);
filt = dfilt.df2sos(sos,g);
fdata = filter(filt,data)