Flexibility really matters when it comes to innovative speaker design. As DIYers and speaker designers ourselves, we came to realize that miniDSP platforms could be used for both basic and advanced digital signal processing applications. While textbook filter implementation like Butterworth, Linkwitz Riley, and Bessel work in 80% of configurations, some speaker designers may want to investigate novel IIR filter implementation like:

- Filters currently not implemented inthe basic mode

- Cascaded filters

- Custom filters developed on 3rd party software

The following section will teach you how to use our advance programming.

DISCLAIMER: The following section assumes that you have some basic understanding of filtering and Digital Signal Processing. Testing the accuracy of a filter before loading it to the miniDSP must be performed ahead. Once loaded inside the miniDSP, you should first confirm the overall transfer function to confirm that your biquad computations are correct. miniDSP cannot be liable of incorrect/unstable plug-in biquad settings being loaded into the platform.

Introducing miniDSP Advanced Biquad Filter Programming

miniDSP filtering is based on 2nd order linear recursive filtering, also called a Biquad digital filter. A biquad contains two poles and two zeroes with a transfer function expressed as follow in the Z domain.

All filters of miniDSP (PEQ/Shelf/Graphic EQ/Low Pass/High Pass) are all implemented using the 5 coefficients of the a 2nd order filter. (a1/a2/b0/b1/b2). Note that a1 and a2 need to have sign inverted (times -1) due to the DSP implementation.

Biquad programming on miniDSP plug-ins is allowed on all biquad objects of the DSP structures. There are currently 2 places were custom biquad can be implemented:

• PEQ filters on input/outputs: With 6 biquad on each input and 6 biquad on each outputs

• Crossovers: With 8 biquads on each output, cascaded filters can be created by stacking 2nd order filters in series.

Adding them up together will give you all the flexibility you need to create your custom processing.

Filter ideas...

Advanced Biquad programming trully opens the door to a new range of filtering applications such as:

• Linkwitz transform
• All pass filters
• Cascaded filters (e.g. two low pass filters in series)
• Filters of up to 16th order (96dB/oct) when cascading 8 biquads.. Still not enough? How about adding the remaining 6 biquad filters from the PEQ outputs and building a ( 8 + 6) * 2nd order = 28th order filter with 168dB/oct attenuation as highlighted in this community tutorial.

Where to start?

• Download the Biquad filter spreadsheet
• Read about biquad filters and how to insure they are stable
• Load a simple biquad and test it with a sound card  first to make sure it is correct
• Start simple, get more creative over time!

Ready to build your own biquad filters?
Check out the miniDSP series (IIR) on miniDSP's webstore.