Dr. Alexander Klein wrote: > Am 28.09.2011 um 21:18 schrieb Jeffrey Cunningham: > > >> Here you go: >> >> damp_db = >> >> -0.0426 -0.1679 -3.1179 -14.1980 -27.3152 >> >> >> damp_db = >> >> -0.0003 -0.0015 -3.0103 -53.2194 -103.2218 >> > > Jeff, > > thank you! So, butter in Octave is equivalent to butter in Matlab. > > I reviewed my test cases, and it turned out that the excessive > attenuation was due to the test signals being too close to the nyquist > frequency. On the other hand, I wouldn't have thought that a signal at > 80% of the nyquist limit would provoke such problems. I really wonder > if the effect is a consequence of the IIR-nature of the filter ... > > All the best, > > Alex > > Alex,
This is simply an artifact of the conformal mapping from the continuous-time domain to the discrete-time domain. Remember that a continuous-time Butterworth filter has infinite attenuation at infinite frequency, and the frequencies at infinity are mapped to the Nyquist edges. Just as an aside, there is no real reason to test for attenuation or other filter response characteristics. If the filter routine works and the coefficients are correct, then the final filter will work just fine. I am not saying, of course, that you shouldn't build tests like this, but it is somewhat overkill. Bob Bob ------------------------------------------------------------------------------ All the data continuously generated in your IT infrastructure contains a definitive record of customers, application performance, security threats, fraudulent activity and more. Splunk takes this data and makes sense of it. Business sense. IT sense. Common sense. http://p.sf.net/sfu/splunk-d2dcopy1 _______________________________________________ Octave-dev mailing list Octave-dev@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/octave-dev
