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Basic Bi-quad Filters in Core

February 26, 2012

I’ve created some pretty simple biquad filters in Reaktor Core using the information in this text. Biquads are pretty interesting because once you have a structure that acts as Direct Form I,  (a trivial matter in core, see picture) it’s simply a matter of plugging in the right coefficients (which can be easily derived from user inputs). With that in mind, I was able to create a single simple structure that can act as one of eight different types of filter. The extra filter types take up no CPU whatsoever, so it’s a very efficient way to create some pretty versatile filters – one could easily extend the structure to include even more filters at no CPU cost.

The included filter types are:  Low Pass, High Pass, Band Pass, Notch, All Pass, Peak EQ (+18 dB), Low Shelf (-12dB) and High Shelf (-12dB). The decibel values can easily be changed inside the structure or even controlled by a knob with minimum modifications (right now they are simply implemented as core constants).

All filters have 2 poles and 2 zeroes, and creating filters with more poles is a trivial matter – simply cascade another filter directly after the first with the same input values, and it becomes a 4-pole, 4 zero filter.

Notes on use –

  • Type input is an integer 0-7 that controls the type of filter.
  • When the type changes, the P and Q values must be sent back to the filter, too (this is already implemented in the upload)
  • Q input must be greater than 0.

Download here, you can plug it into the Filter Analyzer I posted below to check the frequency response.

3 Comments leave one →
  1. February 26, 2012 10:30 pm

    Thanks anagram; I look forward to trying this.

  2. February 26, 2012 10:31 pm

    Reblogged this on donovick.


  1. Formant Filters « salamanderangram

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