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I have EEG signal with a sampling frequency of 128 Hz, and filtered from 1-30 Hz using a bandpass filter. Now I want to know while decomposing the signal; does DWT decomposes it based on sampling frequency or cutoff frequency? Detailed Coefficients one should be 7.5-30 (if consider cutoff frequency), or 32-64 (if consider sampling frequency).

If we consider the cutoff frequency, we now that upper limit is a bit higher than 30, so how can we correctly justify it?

I am using python pywavelets and want to compute the frequency range of decomposed level.

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THe DWT does not really know about the actual sampling. It do cares about the relative frequency span. And it has not knowledge about pre-processing, like linear filtering.

So in your case, the detail coefficients of the first level of wavelet coefficients correspond, more or less, to the [128/2-128]/2 frequency range.

Because of the initial [0-30] low-pass filter, the above [32-64] first wavelet subband range is likely to contain little useful information.

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