biquad~ coefficient help?

Jul 1, 2011 at 8:52am

biquad~ coefficient help?

Hi Folks,

Can someone give me a bit of help dealing with the coefficients for [biquad~]?

I’m trying to do the following:
Lowpass @ 3dB/octave
Highpass @ 3dB/octave
Highpass @ 6dB/octave

I realize I can do the 6dB Highpass with [onepole~], but for a variety of reasons, I’d like to do it all with [biquad~]

Thanks!

#57842
Jul 1, 2011 at 2:04pm

well, you can do them all with onepole~.

but for biquad just use [filtercoeff~].

#207511
Jul 1, 2011 at 3:35pm

How do you do 3dB/octave with onepole~? I though it was fixed at 6dB/octave. As for filtercoef~ and filtergraph~ both, the number I’m struggling with is the Q number. On a lowpass, is Q=1 a -6dB/octave? If so, does that me Q=0.5 is a -3dB/octave? Thanks.

#207512
Jul 2, 2011 at 12:26am

Check out these two equations which turn bandwidth to quality factor and the reverse, hopefully it will help:

– Pasted Max Patch, click to expand. –
#207513
Jul 2, 2011 at 1:30am

Hey Luke, thanks for that! Am I correct in thinking that a bandwidth of 1 means 1 octave and a bandwidth of 2 means 2 octaves? What’s puzzling me is that even when I widen the bandwidth, it seems like the slope is still at 6dB/octave. Perhaps it’s the way [filtergraph~] is drawing it, but it seems that if I set the bandwidth to 2, then it drops a total of 12dB inside that band, and if I set it to 3, it looks like it’s dropping 18dB. Either way–assuming I’m seeing it correctly, which is always a risky assumption–it looks like it is still maintaining a slope of 6dB/octave.

– Pasted Max Patch, click to expand. –
#207514
Jul 2, 2011 at 4:05am

Bandwidth of a filter and the slope are independent of each other. For a brief explanation of slope, have a look at the following link I found: http://www.soundfirst.com/Filter_Slopes.html

Q is a factor of bandwidth and center frequency: http://www.sengpielaudio.com/calculator-bandwidth.htm

Hope this helps!

#207515

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