WFILT_DB Daubechies FIR filterbank Usage: [h,g] = wfilt_db(N); Input parameters: N : Order of Daubechies filters. Output parameters: H : cell array of analysing filters impulse reponses G : cell array of synthetizing filters impulse reponses a : array of subsampling (or hop) factors accociated with corresponding filters [H,G] = dbfilt(N) computes a two-channel Daubechies FIR filterbank from prototype maximum-phase analysing lowpass filter obtained by spectral factorization of the Lagrange interpolator filter. N also denotes the number of zeros at z=-1 of the lowpass filters of length 2N. The prototype lowpass filter has the following form (all roots of R(z) are outside of the unit circle): H_l(z)=(1+z^-1)^N*R(z), where R(z) is a spectral factor of the Lagrange interpolator P(z)=2R(z)*R(z^{-1}) All subsequent filters of the two-channel filterbank are derived as follows: H_h(z)=H_l((-z)^-1) G_l(z)=H_l(z^-1) G_h(z)=-H_l(-z) making them an orthogonal perfect-reconstruction QMF. Examples: --------- : wfiltinfo('db8'); References: I. Daubechies. Ten Lectures on Wavelets. Society for Industrial and Applied Mathematics, Philadelphia, PA, USA, 1992.
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