Using vDSP_deq22 as a bandpass filter
For reasons that may become clearer in future, I needed to use a bandpass filter in an iOS app. The DSP part of Apple’s Accelerate framework makes this lightning fast both for the programmer to implement and for the machine to execute … if the programmer knows how use vDSP_deq22, which is documented, at best, concisely.
The following functions produce the five-coefficient filter definition you need to pass to vDSP_deq22. Since this took me a little while to put together, I thought I’d share.
void makeBandpassFilterWithFcAndQ(float* filter, double Fs, double Fc, double Q) { // Fs = sampling rate, Fc = centre freq // with thanks to http://www.earlevel.com/main/2013/10/13/biquad-calculator-v2/ // and https://github.com/bartolsthoorn/NVDSP double K = tan(M_PI * Fc / Fs); double norm = 1.0 / (1.0 + K / Q + K * K); filter[0] = (float)(K / Q * norm); filter[1] = 0.0f; filter[2] = -filter[0]; filter[3] = (float)(2.0 * (K * K - 1.0) * norm); filter[4] = (float)((1.0 - K / Q + K * K) * norm); } void makeBandpassFilterWithFreqRange(float* filter, double Fs, double Fbtm, double Ftop) { // with thanks to // http://stackoverflow.com/questions/15627013/how-do-i-configure-a-bandpass-filter // -- this sets Q such that there's -3dB gain (= 50% power loss) at Fbtm and Ftop double Fc = sqrt(Fbtm * Ftop); double Q = Fc / (Ftop - Fbtm); makeBandpassFilterWithFcAndQ(filter, Fs, Fc, Q); } |
And you use it like so:
float filter[5]; makeBandpassFilterWithFreqRange(filter, sampleRate, filterLoRate, filterHiRate); vDSP_deq22(rawFloats, rawStride, filter, filteredFloats, filteredStride, numRawFloats - 2); // rawFloats and filteredFloats are pointers, of course |
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