### 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!

well, you can do them all with onepole~.

but for biquad just use [filtercoeff~].

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.

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

-- Pasted Max Patch, click to expand. --Copyallof the following text. Then, in Max, selectNew From Clipboard.----------begin_max5_patcher---------- 266.3ocuPFsaBCBEF9Z3ofP1EfVokxz31c64XZVpUbxRKsoEicy3695APmYI 6psLtfyg+CG9O7cBinaZFz8TxijmIHzILB4k.AT7LhVWLTVUz6uF0pO1r4MZ RnjSO37x5g1NBKWLgUYYLoHExuamjymxZaNxBa2puZURtfOUJfrLwbNrRg1G k4WLn4fqR6bu2pCCIkRVGKsqw55Me3KHyEYQY6gZicrG+3JihsEtx8F6quzo KcgWRsXrEhT8.DTKmCg7LQ1022r0+yF+sytmdim1hZumzm5LEUzurMLqAeAw yXLrk7WP1qLLWLhqK.ziszuWXl7+.eKAfsP4wVL7CzS8anGYM9L9SXxycBE -----------end_max5_patcher-----------

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. --Copyallof the following text. Then, in Max, selectNew From Clipboard.----------begin_max5_patcher---------- 663.3oc2W1sihBCEG+Z7ofP1KvcXv1BEg8t84XmIaPopcCVHPM5tSl8Ye6Gh eLinUYGhYHjZ6oE5+yOOm1xKCrblTrgT6X+M6eXaY8x.KKkIoAqsssbVltYZ dZsZXNKI00oyINd593jMbkcjeioYELNKcIQY96Uzz7ldJS4SWPYy+YEYJWOo vXefmMZrrL.4i8TVred6SPyTukhI+5QXbyqgsZIkkS3JAg1arXEuwJ7.sTS+ iRKPjXRzl0ij+6RhVENN1OK640ACjEdFxBFYsPYuCE4EoYSRYyuIfDhf6IRD PUGzBQhNIQf+WHhxAtIpzVDRdw5R4X5.UFikk3PYYqwI36t3j1HBD..2VZCN ArmGgvyxif6NdLilyIUyqRKW72K+mNDiTYA3HUKTa9I7j9Y7I8yws4P4zZti mPhhj3KUoYrTlPvMJhoctinYMgqspmDoWntApaUTf7WYAvORdMVVCzZwsQ87 BAD5vpzvX07iO6hRIWwZRnqLZaG4u8Em5NDBPWFBPvmbJjjbYHD+ImAXCxFh tuYPKGfgrorx1E4+U2blqKzejr9WlAGN7A2xh0t5hCs+zSdH+gO.8k0.93gx qQxGWXdXWR2zq5GDiOGlC+.OHzG.Y2wPju.WM.TgsQusiGgcAe5CHfhBNG8B 5M5olcw9lr29MGJGPZ+XjVWrpZZyqp4Hu16chLRMmxR4zB1gCJ9nAsflkQT8 2nxkzrxBwt1aEQX.P9gGQpMfC.p.tiZ.S1djbrtdKQDF6DwF3DQWmOzUIYDW C5.WwhsL11nWzJte4WfIZRbBUab+oIroZp+3TroRZr4RZadIRsTm3ny6Z0GZ Ezq3Kz.IkzuA9.CjTXuJISREi6UEYxBVAcPQhFuN3efdzW3I -----------end_max5_patcher-----------

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!

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