Question on filter implementation ...

[Dr.-Ing. Dieter Jurzitza]

Dear listmembers,
I am currently experimenting with the signal - package using i. e. butter() 
and cheby1() functions.

If I do a 
[b a]=butter(2,0.0208333);
[H W]=freqz(b,a,48000);
sig=20*log10(abs(H));
semilogx(sig)

I get a nice and clean plot. Looks as expected. Same behavior when using 
cheby1 ....

However, if I replace
[b a]=butter(2,0.0208333);
by
[b a]=butter(10,0.0208333);

the resulting graph is a mess. Enlarging the degree of the filter beyond 8 
leads to very jumpy, noisy graphs. When I do the same in Matlab no such issues 
show. I found of the coefficients to become very small with that high filter 
degree.

Any hints how to improve this besides not designing so steep filters? Is there 
an option to increase the internal resolution?
Thank you for any feedback,
best regards

Dieter Jurzizta
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Dr.-Ing. Dieter Jurzitza                    76131 Karlsruhe