On Thursday, February 28, 2013 9:30:12 AM UTC+13, Riheen wrote: > Hi , > > I am doing EEG signal processing. Suppose, my data is sampled with 500Hz. So, according to Nyquist theorem there is 250 hz spectrum. I filtered the signal 0-64 Hz. I need 0-8Hz band. Now how much decomposition level is required?? 3 or 5??
I find it easier to work in timescales, not frequencies: dt=1/500; seconds Level 1: 2*dt Level 2: 2^2*dt Level 3: 2^3*dt etc
Then, 8 Hz => 1/8 s = 2^n*dt 2^n=500/8 n=log2(500/8)=5.9 so you need to decompose to level 6, and the approximation will be 0 to 8Hz (approximately).
Note, however, that these timescales are not exact. In fact for ocean waves and using mother wavelet 'db5' (which fits ocean waves very nicely), I find (from zero crossing analysis) that the times scales go like: Level 1: 1.5*2*dt Level 2: 1.5*2^2*dt Level 3: 1.5*2^3*dt etc whence: n=log2(500/8/1.5)=5.4