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Figure 2’s bank of filters represents well the idea of the octave-band structure of Haar wavelets, which gives pyramid structure frequency localization as shown in Figure 3.

Here, I must point out that an under sampling operation is done after every filtering. It must be understood that the choice of the Haar wavelet in our system was one made for simplicity. However, we had in mind to investigate the influence of the choice of the wavelet function in our results but, in order to test the robustness truthfully, we had to give up the idea in favor of the addition of extra robustness testing procedures. The next operation is to add a pseudo random sequence N , in fact a Gaussian distribution of mean zero and variance one, to the coefficients of the medium and high frequency bands (i.e. all the bands except the lowest one which is represented by the top left corner in
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