biquad~ coefficient help?

bkshepard's icon

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!

pid's icon

well, you can do them all with onepole~.

but for biquad just use [filtercoeff~].

bkshepard's icon

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.

Luke Hall's icon
Max Patch
Copy patch and select New From Clipboard in Max.

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

bkshepard's icon
Max Patch
Copy patch and select New From Clipboard in Max.

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.

Steven Miller's icon

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!