When designing filters, the transition region can allow some aliasing to reduce the filter load in the last 5% of the frequency spectrum. From the analysis point of view, if the last 5% are just attenuated and/or also aliased plays no role. If the signal is an audio file, those 5% of aliasing could add some real audible distortion to the signal. If the AudioSignal processing is True, no aliasing will occure, but the transition bandwidth will double to 10% of the total bandwidth.

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