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I have a Biquad II filter implementation in my embedded environment and I'm trying to find some equations to generate the coefficients for it.

I have already found the Audio Eq Cookbook, which simply describes how to calculate all of the coefficients for Type I.

LPF: (Taken from Audio Eq Cookbook, Type I Equations)

omega = 2*PI*frequency/sampleRate

sin   = sin(omega)
cos   = cos(omega)

alpha = sin/(2*Q)                                     (if Q is specified)
      = sin*sinh[ ln(2)/2 * bandwidth * omega/sin ]   (if bandwidth is specified)

            b0 =  (1 - cos)/2
            b1 =   1 - cos
            b2 =  (1 - cos)/2
            a0 =   1 + alpha
            a1 =  -2*cos
            a2 =   1 - alpha

What are the equations for type II coefficients?

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  • $\begingroup$ Why would the coefficients depend on biquad type ? $\endgroup$
    – Hilmar
    Mar 22, 2019 at 16:29
  • $\begingroup$ Can I use biquad I coefficients for II type as well? $\endgroup$
    – Can Uysal
    Mar 24, 2019 at 12:25
  • $\begingroup$ hay, i didn't see this until now, 2 weeks later. the coefficients for the Direct Form 1 and Direct Form 2 are the same. but these coefficients have to be massaged a bit if your implementation is a form like Lattice or Ladder or some other State Variable Forms. $\endgroup$ Apr 5, 2019 at 22:01
  • $\begingroup$ @robertbristow-johnson hey robert, please elaborate since I'm desperately wanting to implement a ladder filter. Can you lead me to a reading source? or even better, a math like this simple? $\endgroup$
    – Can Uysal
    Apr 5, 2019 at 23:24
  • $\begingroup$ @robertbristow-johnson lol, robert it is you! you are the author of audio eq cookbook, huge fan! such an honour! :) thank you for this lovely source, It saved me like a years work maybe. Thank you! Your page in musicdsp is down by the way, is there any other sites hosting your original writing? While I find you here, would like to ask another question. When my filter drops down below 200-300 hz, some distortion/noise begin to rumble in low end, especially with high Q. Is there a way to eliminate this? Using direct form II transposed. $\endgroup$
    – Can Uysal
    Apr 5, 2019 at 23:38

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