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70 lines
1.9 KiB
70 lines
1.9 KiB
/*
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* Copyright (C) 2016 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#include <math.h>
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#include <audio_utils/limiter.h>
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#undef USE_ATAN_APPROXIMATION
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#ifdef USE_ATAN_APPROXIMATION
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static inline float atan_approximation(float in)
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{
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static const float kPi_2 = 1.5625; // pi/2 ~= 2.0 * atan_approximation(1.0)
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if (in <= 1.0) {
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return in / (1.0 + 0.28 * in * in);
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} else {
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const float in_inverse = 1.0 / in;
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return kPi_2 - in_inverse / (1.0 + 0.28 * in_inverse * in_inverse);
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}
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}
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#endif
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float limiter(float in)
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{
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static const float crossover = M_SQRT1_2;
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float in_abs = fabsf(in);
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if (in_abs <= crossover) {
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return in;
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}
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float out;
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#ifdef USE_ATAN_APPROXIMATION
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// atan approximation
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static const float factor = 4.3125;
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static const float ratio = 1.0 / factor;
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out = crossover + atan_approximation((in_abs - crossover)*factor)*ratio;
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if (out > 1.0) {
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out = 1.0;
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}
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#else
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// polynomial spline
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static const float A = 0.3431457505;
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static const float B = -1.798989873;
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static const float C = 3.029437252;
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static const float D = -0.6568542495;
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// Cubic solution Ax^3 + Bx^2 + Cx + D
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if (in_abs < M_SQRT2) {
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// TODO Use Estrin's method P3
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out = ((A*in_abs + B)*in_abs + C)*in_abs + D;
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} else {
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out = 1.0;
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}
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#endif
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if (in < 0) {
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out = -out;
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}
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return out;
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}
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