Bessel

The Bessel filter has a maximally flat group delay (in the passband). That is, it's phase response is maximally linear. This is important if one does not want to distort the shape of signals with content at different frequencies. for example, a butterworth filter has an overshoot when filtering a step response, but the bessel filter does not. It's rolloff is even slower than a butterworth, however.

Group delay is defined as - ${\frac{{d}}{{d\omega}}}$$\theta$($\omega$). So flat group delay == linear phase response.

Matlab doesn't include a digital bessel filter design. The help file seems to claim that this is because a digitized bessel filter won't have a flat group delay. However, a directly designed FIR filter should be able to give linear phase anyway, so digitized bessel filter may be unnecessary (http://www.dsprelated.com/showmessage/132491/1.php)

http://www-k.ext.ti.com/SRVS/Data/ti/KnowledgeBases/analog/document/faqs/bes.htm

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