Good morning,
the other day I was lashing up some tests for butter.m in the signal
package. To my surprise, the low-pass filter showed much higher
stopband attenuation than I'd expected.
Now I wonder whether that's a bug or a feature due to compatibility
with Matlab.
Could someone who has access to Matlab please run the attached script
and post the results?
Thanks in advance,
Alex
--
Dr. Alexander Klein, Diplom-Mathematiker
Physiologisches Institut | TransMIT Zentrum
Raum 543 | für Numerische Methoden
Aulweg 129 | Heinrich-Buff-Ring 44
35392 Giessen | 35392 Giessen
function retval = sinewave (f, r, s, a)
n = length (f);
ns = round (r * s);
retval = zeros (ns, n);
for k = 1:n
retval (:, k) = a(k) * sin (2 * pi * (1:ns) / r * f(k))';
endfor
endfunction
sf = 1000; sf2 = sf/2;
data=[sinewave(5,sf,10,1),sinewave(10,sf,10,1),sinewave(50,sf,10,1),sinewave(200,sf,10,1),sinewave(400,sf,10,1)];
[b, a] = butter ( 1, 50 / sf2 );
filtered = filter ( b, a, data );
damp_db = 20 * log10 ( max ( filtered ( end - sf : end, : ) ) )
[b, a] = butter ( 4, 50 / sf2 );
filtered = filter ( b, a, data );
damp_db = 20 * log10 ( max ( filtered ( end - sf : end, : ) ) )
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