The distinguishing characteristic of Bessel and Linear Phase Filters is the near constant group delay throughout the passband of the low-pass filter. FilterSolutions normalizes Bessel and Linear Phase filters such that the prototype high frequency attenuation matches the Butterworth filter. The passband attenuation of the Bessel Filter increases with the order of the filter when this normalization is applied. However, FilterSolutions allows the user the option of selecting the desired passband attenuation in dB.
Bessel Filters, modified for equiripple group delay, are frequently referred to as Linear Phase filters. The equiripple group delay adds efficiency: it remains flat further into the stop band. (Refer the description of Delay Filters for more on equiripple group delay). Bessel and Linear Phase filters may be further modified to have a stop-band with transmission zeros.
Below are examples of Bessel and Linear Phase low-pass and step responses.