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Wavelet Multiresolution Representations

The Gabor representation uses the same temporal resolution in each frequency band . However, a that is a good resolution at a low frequency may not be a good resolution at a high frequency. Therefore, in a multiresolution representation higher frequency bands may have a smaller (finer) than lower frequency bands. Of course, the Gabor relationship still holds, so the frequency resolution must increase (i.e. become coarser) at higher frequencies. This is often acceptable, however, since the ratio of to the frequency remains constant (so this is also called a ``constant Q'' representation, since ).

In the most common arrangement, the central frequencies of the frequency bands increase by powers of 2, . Therefore, the widths of the frequency bands also increase by powers of 2, , but the time resolutions decrease (become finer) by powers of 2, . In this case the elementary functions are generated by contracting and translating a single mother wavelet: [ W_jk(t) = W_00[ 2^k ( t - j;t_0) ] , ] for and . The Gabor elementary function, or a slight variant of it called the Morlet wavelet, can be used as a mother wavelet. The signal then is represented by a linear superposition of wavelets: [ (t) = _k=0^N _j=0^2^k T;/;t_0 c_jk W_jk(t) . ] The generation of the signal is controlled by the triangular array of coefficients . Like the continuous Gabor transform, there is also a continuous wavelet transform that represents the coefficients in a continuous field. Also like the Gabor transform, the wavelet transform allows independent control of frequency content and time-evolution. However, because of the essentially exponential measurement of frequency ( in the wavelet vs. k in the Gabor), translation along the frequency axis causes dilation or compression of the signal's spectrum. A shift of changes the instantaneous spectrum from to . Much more could be said about the information processing affordances of these representations, but it is beyond the scope of this paper.


next up previous
Next: Constraint satisfaction Up: Representation of Motion Previous: Gabor Representation

Bruce MacLennan
Wed Oct 2 16:55:07 EDT 1996