diff options
Diffstat (limited to 'Alc')
| -rw-r--r-- | Alc/mixer.c | 104 |
1 files changed, 87 insertions, 17 deletions
diff --git a/Alc/mixer.c b/Alc/mixer.c index 4917477a..e7a924bf 100644 --- a/Alc/mixer.c +++ b/Alc/mixer.c @@ -133,17 +133,87 @@ static inline ResamplerFunc SelectResampler(enum Resampler resampler) return Resample_point32_C; } + +/* The sinc resampler makes use of a Kaiser window to limit the needed sample + * points to 4 and 8, respectively. + */ + #ifndef M_PI #define M_PI (3.14159265358979323846) #endif -static float lanc(double r, double x) +static inline double Sinc(double x) { - if(x == 0.0) return 1.0f; - if(fabs(x) >= r) return 0.0f; - return (float)(r*sin(x*M_PI)*sin(x*M_PI/r) / - (M_PI*M_PI * x*x)); + if(x == 0.0) return 1.0; + return sin(x*M_PI) / (x*M_PI); } +/* The zero-order modified Bessel function of the first kind, used for the + * Kaiser window. + * + * I_0(x) = sum_{k=0}^inf (1 / k!)^2 (x / 2)^(2 k) + * = sum_{k=0}^inf ((x / 2)^k / k!)^2 + */ +static double BesselI_0(double x) +{ + double term, sum, x2, y, last_sum; + int k; + + /* Start at k=1 since k=0 is trivial. */ + term = 1.0; + sum = 1.0; + x2 = x / 2.0; + k = 1; + + /* Let the integration converge until the term of the sum is no longer + * significant. + */ + do { + y = x2 / k; + k ++; + last_sum = sum; + term *= y * y; + sum += term; + } while(sum != last_sum); + return sum; +} + +/* Calculate a Kaiser window from the given beta value and a normalized k + * [-1, 1]. + * + * w(k) = { I_0(B sqrt(1 - k^2)) / I_0(B), -1 <= k <= 1 + * { 0, elsewhere. + * + * Where k can be calculated as: + * + * k = i / l, where -l <= i <= l. + * + * or: + * + * k = 2 i / M - 1, where 0 <= i <= M. + */ +static inline double Kaiser(double b, double k) +{ + if(k <= -1.0 || k >= 1.0) return 0.0; + return BesselI_0(b * sqrt(1.0 - (k*k))) / BesselI_0(b); +} + +static inline double CalcKaiserBeta(double rejection) +{ + if(rejection > 50.0) + return 0.1102 * (rejection - 8.7); + if(rejection >= 21.0) + return (0.5842 * pow(rejection - 21.0, 0.4)) + + (0.07886 * (rejection - 21.0)); + return 0.0; +} + +static float SincKaiser(double r, double x) +{ + /* Limit rippling to -90dB. */ + return Kaiser(CalcKaiserBeta(90.0), x / r) * Sinc(x); +} + + void aluInitMixer(void) { enum Resampler resampler = ResamplerDefault; @@ -180,23 +250,23 @@ void aluInitMixer(void) for(i = 0;i < FRACTIONONE;i++) { ALdouble mu = (ALdouble)i / FRACTIONONE; - ResampleCoeffs.FIR8[i][0] = lanc(4.0, mu - -3.0); - ResampleCoeffs.FIR8[i][1] = lanc(4.0, mu - -2.0); - ResampleCoeffs.FIR8[i][2] = lanc(4.0, mu - -1.0); - ResampleCoeffs.FIR8[i][3] = lanc(4.0, mu - 0.0); - ResampleCoeffs.FIR8[i][4] = lanc(4.0, mu - 1.0); - ResampleCoeffs.FIR8[i][5] = lanc(4.0, mu - 2.0); - ResampleCoeffs.FIR8[i][6] = lanc(4.0, mu - 3.0); - ResampleCoeffs.FIR8[i][7] = lanc(4.0, mu - 4.0); + ResampleCoeffs.FIR8[i][0] = SincKaiser(4.0, mu - -3.0); + ResampleCoeffs.FIR8[i][1] = SincKaiser(4.0, mu - -2.0); + ResampleCoeffs.FIR8[i][2] = SincKaiser(4.0, mu - -1.0); + ResampleCoeffs.FIR8[i][3] = SincKaiser(4.0, mu - 0.0); + ResampleCoeffs.FIR8[i][4] = SincKaiser(4.0, mu - 1.0); + ResampleCoeffs.FIR8[i][5] = SincKaiser(4.0, mu - 2.0); + ResampleCoeffs.FIR8[i][6] = SincKaiser(4.0, mu - 3.0); + ResampleCoeffs.FIR8[i][7] = SincKaiser(4.0, mu - 4.0); } else if(resampler == FIR4Resampler) for(i = 0;i < FRACTIONONE;i++) { ALdouble mu = (ALdouble)i / FRACTIONONE; - ResampleCoeffs.FIR4[i][0] = lanc(2.0, mu - -1.0); - ResampleCoeffs.FIR4[i][1] = lanc(2.0, mu - 0.0); - ResampleCoeffs.FIR4[i][2] = lanc(2.0, mu - 1.0); - ResampleCoeffs.FIR4[i][3] = lanc(2.0, mu - 2.0); + ResampleCoeffs.FIR4[i][0] = SincKaiser(2.0, mu - -1.0); + ResampleCoeffs.FIR4[i][1] = SincKaiser(2.0, mu - 0.0); + ResampleCoeffs.FIR4[i][2] = SincKaiser(2.0, mu - 1.0); + ResampleCoeffs.FIR4[i][3] = SincKaiser(2.0, mu - 2.0); } MixHrtfSamples = SelectHrtfMixer(); |