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Basic Bi-quad Filters in Core

February 26, 2012
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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.

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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.

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  1. Formant Filters « salamanderangram

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