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Commit bafa561d authored by Andy Hung's avatar Andy Hung
Browse files

Improve resampler filter computation speed 



Approximate speed improvement is 10-15% for filter computation,
which is floating point intensive. This will be important
for devices without hw floating point support.

Change-Id: I10b4e778c8d632b52218a777504b092c189e437f
Signed-off-by: default avatarAndy Hung <hunga@google.com>
parent 2ade3000

services/audioflinger/AudioResamplerFirGen.h

+25 −1
Original line number Diff line number Diff line
@@ -365,6 +365,28 @@ static inline double I0(double x) { 
    }

}



/* A speed optimized version of the Modified Bessel I0() which incorporates

 * the sqrt and numerator multiply and denominator divide into the computation.

 * This speeds up filter computation by about 10-15%.

 */

static inline double I0SqrRat(double x2, double num, double den) {

    if (x2 < (3.75 * 3.75)) {

        return Poly7(I0Term<0>::value, I0Term<1>::value,

                I0Term<2>::value, I0Term<3>::value,

                I0Term<4>::value, I0Term<5>::value,

                I0Term<6>::value, x2) * num / den; // e < 1.6e-7

    }

    num *= Poly9(-0.13544938430e9, -0.33153754512e8,

            -0.19406631946e7, -0.48058318783e5,

            -0.63269783360e3, -0.49520779070e1,

            -0.24970910370e-1, -0.74741159550e-4,

            -0.18257612460e-6, x2); // e < 10^(-7.13).

    double y = x2 - 225.; // reflection around 15 (squared)

    den *= Poly4(-0.34598737196e8, 0.23852643181e6,

            -0.70699387620e3, 0.10000000000e1, y);

    return num / den;

}



/*

 * calculates the transition bandwidth for a Kaiser filter

 *
@@ -645,6 +667,7 @@ static inline void firKaiserGen(T* coef, int L, int halfNumCoef, 
    const double xstep = (2. * M_PI) * fcr / L;

    const double xfrac = 1. / N;

    const double yscale = atten * L / (I0(beta) * M_PI);

    const double sqrbeta = sqr(beta);



    // We use sine generators, which computes sines on regular step intervals.

    // This speeds up overall computation about 40% from computing the sine directly.
@@ -663,7 +686,8 @@ static inline void firKaiserGen(T* coef, int L, int halfNumCoef, 
                double x = static_cast<double>(ix);



                // sine generator: sg.valueAdvance() returns sin(ix*xstep);

                y = I0(beta * sqrt(1.0 - sqr(x * xfrac))) * yscale * sg.valueAdvance() / x;

                // y = I0(beta * sqrt(1.0 - sqr(x * xfrac))) * yscale * sg.valueAdvance() / x;

                y = I0SqrRat(sqrbeta * (1.0 - sqr(x * xfrac)), yscale * sg.valueAdvance(), x);

            } else {

                y = 2. * atten * fcr; // center of filter, sinc(0) = 1.

                sg.advance();