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Bessel Filter cutoff frequ.
Posted:
Aug 29, 1996 2:19 AM


Hi folks,
Th. Krauss wrote two bessel filter functions: besselap and besself, where besself just invokes besselap and lp2lp (lp2bp ...) in order to transform cutoff frequencies, etc.
besselap is not more than a lookup tabel of filter coefficients. But, as stated in the help text, 3 dB cutoff frequency decreases as the order of the filter increases. This actually is pretty annoying ... (yes, you won't get correct cutoff frequencies with besself)
Where do the coefficients come from? Why are they so odd? Any possibility to solve the problem easily?
Remark: I know of some books listing bessel filter coefficients, which result in correct prototypes, i.e. omega_cutoff=1 independent of N. Therefore I'd like to get my questions answered for academical reasons only!
Sincerely
 Joerg Beilharz beili@ifaesp3.ee.tuberlin.de
TU Berlin, Sekr. E2 Phone: +49 30 31424893 Inst. Mess u. Automatisierungstechnik Fax: +49 30 31425717 Einsteinufer 19, D10587 Berlin, Germany 



