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94 lines
3.1 KiB
C
94 lines
3.1 KiB
C
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/*
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July 15, 2015
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Iowa Hills Software LLC
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http://www.iowahills.com
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If you find a problem with this code, please leave us a note on:
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http://www.iowahills.com/feedbackcomments.html
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Source: ~Projects\Common\BasicFIRFilterCode.cpp
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This generic FIR filter code is described in most textbooks.
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e.g. Discrete Time Signal Processing, Oppenheim and Shafer
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A nice paper on this topic is:
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http://dea.brunel.ac.uk/cmsp/Home_Saeed_Vaseghi/Chapter05-DigitalFilters.pdf
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This code first generates either a low pass, high pass, band pass, or notch
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impulse response for a rectangular window. It then applies a window to this
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impulse response.
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There are several windows available, including the Kaiser, Sinc, Hanning,
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Blackman, and Hamming. Of these, the Kaiser and Sinc are probably the most useful
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for FIR filters because their sidelobe levels can be controlled with the Beta parameter.
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This is a typical function call:
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BasicFIR(FirCoeff, NumTaps, PassType, OmegaC, BW, wtKAISER, Beta);
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BasicFIR(FirCoeff, 33, LPF, 0.2, 0.0, wtKAISER, 3.2);
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33 tap, low pass, corner frequency at 0.2, BW=0 (ignored in the low pass code),
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Kaiser window, Kaiser Beta = 3.2
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These variables should be defined similar to this:
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double FirCoeff[MAXNUMTAPS];
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int NumTaps; NumTaps can be even or odd, but must be less than the FirCoeff array size.
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TPassTypeName PassType; PassType is an enum defined in the header file. LPF, HPF, BPF, or NOTCH
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double OmegaC 0.0 < OmegaC < 1.0 The filters corner freq, or center freq if BPF or NOTCH
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double BW 0.0 < BW < 1.0 The filters band width if BPF or NOTCH
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TWindowType WindowType; WindowType is an enum defined in the header to be one of these.
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wtNONE, wtKAISER, wtSINC, wtHANNING, .... and others.
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double Beta; 0 <= Beta <= 10.0 Beta is used with the Kaiser, Sinc, and Sine windows only.
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It controls the transition BW and sidelobe level of the filters.
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If you want to use it, Kaiser originally defined Beta as follows.
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He derived its value based on the desired sidelobe level, dBAtten.
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double dBAtten, Beta, Beta1=0.0, Beta2=0.0;
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if(dBAtten < 21.0)dBAtten = 21.0;
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if(dBAtten > 50.0)Beta1 = 0.1102 * (dBAtten - 8.7);
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if(dBAtten >= 21.0 && dBAtten <= 50.0) Beta2 = 0.5842 * pow(dBAtten - 21.0, 0.4) + 0.07886 * (dBAtten - 21.0);
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Beta = Beta1 + Beta2;
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*/
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#ifndef _WFIR_H_
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#define _WFIR_H_
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class WFIR
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{
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public:
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enum TPassTypeName
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{
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LPF, HPF, BPF, NOTCH
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};
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enum TWindowType
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{
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wtNONE,
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wtKAISER,
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wtSINC,
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wtHANNING,
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wtHAMMING,
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wtBLACKMAN,
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wtFLATTOP,
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wtBLACKMAN_HARRIS,
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wtBLACKMAN_NUTTALL,
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wtNUTTALL,
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wtKAISER_BESSEL,
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wtTRAPEZOID,
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wtGAUSS,
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wtSINE,
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wtTEST
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};
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static void BasicFIR(double *FirCoeff, int NumTaps, TPassTypeName PassType,
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double OmegaC, double BW, TWindowType WindowType, double WinBeta);
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private:
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static void WindowData(double *Data, int N, TWindowType WindowType,
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double Alpha, double Beta, bool UnityGain);
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static double Bessel(double x);
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static double Sinc(double x);
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};
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#endif
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