1 /* 2 * Copyright (C) 2016 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 #include <math.h> 18 #include <audio_utils/limiter.h> 19 20 #undef USE_ATAN_APPROXIMATION 21 22 #ifdef USE_ATAN_APPROXIMATION 23 static inline float atan_approximation(float in) 24 { 25 static const float kPi_2 = 1.5625; // pi/2 ~= 2.0 * atan_approximation(1.0) 26 if (in <= 1.0) { 27 return in / (1.0 + 0.28 * in * in); 28 } else { 29 const float in_inverse = 1.0 / in; 30 return kPi_2 - in_inverse / (1.0 + 0.28 * in_inverse * in_inverse); 31 } 32 } 33 #endif 34 35 float limiter(float in) 36 { 37 static const float crossover = M_SQRT1_2; 38 float in_abs = fabsf(in); 39 if (in_abs <= crossover) { 40 return in; 41 } 42 float out; 43 #ifdef USE_ATAN_APPROXIMATION 44 // atan approximation 45 static const float factor = 4.3125; 46 static const float ratio = 1.0 / factor; 47 out = crossover + atan_approximation((in_abs - crossover)*factor)*ratio; 48 if (out > 1.0) { 49 out = 1.0; 50 } 51 #else 52 // polynomial spline 53 static const float A = 0.3431457505; 54 static const float B = -1.798989873; 55 static const float C = 3.029437252; 56 static const float D = -0.6568542495; 57 // Cubic solution Ax^3 + Bx^2 + Cx + D 58 if (in_abs < M_SQRT2) { 59 // TODO Use Estrin's method P3 60 out = ((A*in_abs + B)*in_abs + C)*in_abs + D; 61 } else { 62 out = 1.0; 63 } 64 #endif 65 if (in < 0) { 66 out = -out; 67 } 68 return out; 69 } 70
