Linear Phase Filtering

The frequency shelving Butterworth filters are the basic 'building blocks' used to design various types of filters provided by DART XP Pro. Belonging to the class of recursive infinite impulse response (IIR) filters, the Butterworth filters guarantee fast processing (which is an obvious advantage) but introduce nonlinear phase distortions, i.e. they do not preserve the shape of the input signal. This sounds like a major drawback but whether or not should be regarded as such depends strongly on the purpose and context of filtering. The point is that our auditory system is practically insensitive to phase distortions introduced by filters, i.e. despite the apparent differences in output signals yielded by linear phase (shape-preserving) filters and their nonlinear phase counterparts the sound we hear is exactly the same.

On the other hand, whenever filters are used for local processing only, preservation of the shape of an input signal is a desirable property - otherwise some discontinuities are likely to appear at the boundaries of the selected block.

Whenever you would like the filter to preserve the waveform shape check the Linear phase box - the signal will be filtered twice : first in the forward, than in the backward direction. Such bidirectional processing allows for the linear phase condition to be fulfilled at the price of slightly slowing down the computations.