biquad~ coefficient help?


    Jul 01 2011 | 8:52 am
    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!

    • Jul 01 2011 | 2:04 pm
      well, you can do them all with onepole~.
      but for biquad just use [filtercoeff~].
    • Jul 01 2011 | 3:35 pm
      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.
    • Jul 02 2011 | 12:26 am
      Check out these two equations which turn bandwidth to quality factor and the reverse, hopefully it will help:
    • Jul 02 2011 | 1:30 am
      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.
    • Jul 02 2011 | 4:05 am
      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!