Filter banks for less ringing artifacts

In image compression using a filter bank, blocking can be reduced by overlapping the basis functions in the lapped orthogonal transform or the lapped biorthogonal transform, whereas ringing can be conspicuous. In this paper, two methods are proposed to suppress ringing while blocking is reduced. One of them uses an unequal length linear phase biorthogonal filter bank. This is an extension of the unequal length orthogonal transform to the biorthogonal type and a generalization of the filter length in the lapped biorthogonal transform. This filter bank can be realized with a lattice structure. By reducing the length of the high-frequency filter on the synthesis side, ringing is compressed to the edge so that it is less visible. Another method introduces the peak-to-peak ratio into the performance criterion of the filter bank design. In this method, ringing can be suppressed by restricting the impulse response of the filter. By means of these methods, ringing can be reduced while coding performance is retained in comparison with the conventional equal length filter banks.