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Commit a5c85850 authored by Glenn Kasten's avatar Glenn Kasten Committed by Android Git Automerger
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am a19c8102: Merge "A sinc resampler for Audioflinger. It\'s not enabled yet,... 

am a19c8102: Merge "A sinc resampler for Audioflinger. It\'s not enabled yet, but fully functional and apparently working. It need more "quality" tests. In the 48->44 KHz, it takes about 25% of the CPU time."

* commit 'a19c8102':
  A sinc resampler for Audioflinger. It's not enabled yet, but fully functional and apparently working. It need more "quality" tests. In the 48->44 KHz, it takes about 25% of the CPU time.
parents 712205bd a19c8102

tools/resampler_tools/fir.cpp

+29 −13
Original line number Diff line number Diff line
@@ -54,24 +54,33 @@ static double kaiser(int k, int N, double alpha) { 
int main(int argc, char** argv)

{

    // nc is the number of bits to store the coefficients

    int nc = 16;

    int nc = 32;



    // ni is the minimum number of bits needed for interpolation

    // (not used for generating the coefficients)

    const int ni = nc / 2;



    // cut off frequency ratio Fc/Fs

    // The bigger the stop-band, the less coefficients we'll need.

    double Fcr = 22050.0 / 48000.0;



    // nzc is the number of zero-crossing on one half of the filter

    int nzc = 12;

    int nzc = 16;

    

    // alpha parameter of the kaiser window

    // Larger numbers reduce ripples in the rejection band but increase

    // the width of the transition band. In reality there doesn't seem to be

    // a good reason to choose a big number because of the limited range

    // of our coefficients (16 bits).

    double alpha = 3.0;

    

    // cut off frequency ratio Fc/Fs

    double Fcr = 20000.0 / 44100.0;

    // the width of the transition band. 

    // the table below gives some value of alpha for a given

    // stop-band attenuation.

    //    30 dB    2.210

    //    40 dB    3.384

    //    50 dB    4.538

    //    60 dB    5.658

    //    70 dB    6.764

    //    80 dB    7.865

    //    90 dB    8.960

    //   100 dB   10.056

    double alpha = 10.056;	// -100dB stop-band attenuation

    

    // 2^nz is the number coefficients per zero-crossing

    // (int theory this should be 1<<(nc/2))
@@ -93,13 +102,20 @@ int main(int argc, char** argv) 
        double x = (2.0 * M_PI * i * Fcr) / (1 << nz);

        double y = kaiser(i+N, 2*N, alpha) * sinc(x);



        int yi = floor(y * (1<<(nc-1)) + 0.5);

        if (yi >= (1<<(nc-1))) yi = (1<<(nc-1))-1;        

        long yi = floor(y * ((1ULL<<(nc-1))) + 0.5);

        if (yi >= (1LL<<(nc-1))) yi = (1LL<<(nc-1))-1;        



        if ((i % (1 << 4)) == 0) printf("\n    ");

        printf("0x%04x, ", yi & 0xFFFF);

        if (nc > 16)

        	printf("0x%08x, ", int(yi));

        else 

        	printf("0x%04x, ", int(yi)&0xFFFF);

    }

    printf("\n};\n");

    return 0;

 }



// http://www.dsptutor.freeuk.com/KaiserFilterDesign/KaiserFilterDesign.html

// http://www.csee.umbc.edu/help/sound/AFsp-V2R1/html/audio/ResampAudio.html



 
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