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Thread: Fast Convolution Filtering with Teensy 4.0 and audio board

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  1. #11
    Senior Member DD4WH's Avatar
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    Oct 2015
    Central Europe
    Just to confirm: minimum phase lowpass (real valued) FIR filters built with MATLAB also work well with the code and seem to exhibit low latency (as far as I can measure) and good filter effects. (latest code is on github)

    I built 512 tap, 1024, 2048, and 4096 tap filters and they all have the same 128 sample delay. So this problem is solved, the partitioned convolution does what it should.

    MATLABsī Filterbuilder has some severe restrictions: minimum phase is only possible with real valued lowpass filters and equiripple design, not with complex valued bandpass filters and choosable window [all of which would be needed for SDR IQ filtering]. Additionally calculating a 4096 tap minimum phase filter needs 31 minutes on a standard 2.4GHz laptop!!!

    Now, the last and most complex problem to solve is the following:

    * we need an algorithm that runs on the Teensy 4.0 to calculate COMPLEX bandpass FIR filter coefficients with minimum phase OR
    * an algorithm that is able to transform linear phase coefficients to minimum phase (that runs on the T4) OR
    * MATLAB code to calculate complex bandpass minimum phase filter coefficients for sizes 512, 1024, 2048, 4096 . . .

    Any volunteer or ideas on that would be very welcome!
    Last edited by DD4WH; 11-09-2019 at 10:55 AM.

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