1*05767d91SRobert Wu /* 2*05767d91SRobert Wu * Copyright 2019 The Android Open Source Project 3*05767d91SRobert Wu * 4*05767d91SRobert Wu * Licensed under the Apache License, Version 2.0 (the "License"); 5*05767d91SRobert Wu * you may not use this file except in compliance with the License. 6*05767d91SRobert Wu * You may obtain a copy of the License at 7*05767d91SRobert Wu * 8*05767d91SRobert Wu * http://www.apache.org/licenses/LICENSE-2.0 9*05767d91SRobert Wu * 10*05767d91SRobert Wu * Unless required by applicable law or agreed to in writing, software 11*05767d91SRobert Wu * distributed under the License is distributed on an "AS IS" BASIS, 12*05767d91SRobert Wu * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13*05767d91SRobert Wu * See the License for the specific language governing permissions and 14*05767d91SRobert Wu * limitations under the License. 15*05767d91SRobert Wu */ 16*05767d91SRobert Wu 17*05767d91SRobert Wu #ifndef RESAMPLER_HYPERBOLIC_COSINE_WINDOW_H 18*05767d91SRobert Wu #define RESAMPLER_HYPERBOLIC_COSINE_WINDOW_H 19*05767d91SRobert Wu 20*05767d91SRobert Wu #include <math.h> 21*05767d91SRobert Wu 22*05767d91SRobert Wu #include "ResamplerDefinitions.h" 23*05767d91SRobert Wu 24*05767d91SRobert Wu namespace RESAMPLER_OUTER_NAMESPACE::resampler { 25*05767d91SRobert Wu 26*05767d91SRobert Wu /** 27*05767d91SRobert Wu * Calculate a HyperbolicCosineWindow window centered at 0. 28*05767d91SRobert Wu * This can be used in place of a Kaiser window. 29*05767d91SRobert Wu * 30*05767d91SRobert Wu * The code is based on an anonymous contribution by "a concerned citizen": 31*05767d91SRobert Wu * https://dsp.stackexchange.com/questions/37714/kaiser-window-approximation 32*05767d91SRobert Wu */ 33*05767d91SRobert Wu class HyperbolicCosineWindow { 34*05767d91SRobert Wu public: HyperbolicCosineWindow()35*05767d91SRobert Wu HyperbolicCosineWindow() { 36*05767d91SRobert Wu setStopBandAttenuation(60); 37*05767d91SRobert Wu } 38*05767d91SRobert Wu 39*05767d91SRobert Wu /** 40*05767d91SRobert Wu * @param attenuation typical values range from 30 to 90 dB 41*05767d91SRobert Wu * @return beta 42*05767d91SRobert Wu */ setStopBandAttenuation(double attenuation)43*05767d91SRobert Wu double setStopBandAttenuation(double attenuation) { 44*05767d91SRobert Wu double alpha = ((-325.1e-6 * attenuation + 0.1677) * attenuation) - 3.149; 45*05767d91SRobert Wu setAlpha(alpha); 46*05767d91SRobert Wu return alpha; 47*05767d91SRobert Wu } 48*05767d91SRobert Wu setAlpha(double alpha)49*05767d91SRobert Wu void setAlpha(double alpha) { 50*05767d91SRobert Wu mAlpha = alpha; 51*05767d91SRobert Wu mInverseCoshAlpha = 1.0 / cosh(alpha); 52*05767d91SRobert Wu } 53*05767d91SRobert Wu 54*05767d91SRobert Wu /** 55*05767d91SRobert Wu * @param x ranges from -1.0 to +1.0 56*05767d91SRobert Wu */ operator()57*05767d91SRobert Wu double operator()(double x) { 58*05767d91SRobert Wu double x2 = x * x; 59*05767d91SRobert Wu if (x2 >= 1.0) return 0.0; 60*05767d91SRobert Wu double w = mAlpha * sqrt(1.0 - x2); 61*05767d91SRobert Wu return cosh(w) * mInverseCoshAlpha; 62*05767d91SRobert Wu } 63*05767d91SRobert Wu 64*05767d91SRobert Wu private: 65*05767d91SRobert Wu double mAlpha = 0.0; 66*05767d91SRobert Wu double mInverseCoshAlpha = 1.0; 67*05767d91SRobert Wu }; 68*05767d91SRobert Wu 69*05767d91SRobert Wu } /* namespace RESAMPLER_OUTER_NAMESPACE::resampler */ 70*05767d91SRobert Wu 71*05767d91SRobert Wu #endif //RESAMPLER_HYPERBOLIC_COSINE_WINDOW_H 72