Glenn Kasten | 715183b | 2016-01-27 17:21:35 -0800 | [diff] [blame^] | 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 | } |