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Topic: DWT decomposition level of EEG
Replies: 1   Last Post: Feb 27, 2013 10:31 PM

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Derek Goring

Posts: 3,892
Registered: 12/7/04
Re: DWT decomposition level of EEG
Posted: Feb 27, 2013 10:31 PM
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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



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