1 /*
2  * Copyright 2022 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 #ifndef FLOWGRAPH_LIMITER_H
18 #define FLOWGRAPH_LIMITER_H
19 
20 #include <atomic>
21 #include <unistd.h>
22 #include <sys/types.h>
23 
24 #include "FlowGraphNode.h"
25 
26 namespace FLOWGRAPH_OUTER_NAMESPACE::flowgraph {
27 
28 class Limiter : public FlowGraphFilter {
29 public:
30     explicit Limiter(int32_t channelCount);
31 
32     int32_t onProcess(int32_t numFrames) override;
33 
getName()34     const char *getName() override {
35         return "Limiter";
36     }
37 
38 private:
39     // These numbers are based on a polynomial spline for a quadratic solution Ax^2 + Bx + C
40     // The range is up to 3 dB, (10^(3/20)), to match AudioTrack for float data.
41     static constexpr float kPolynomialSplineA = -0.6035533905; // -(1+sqrt(2))/4
42     static constexpr float kPolynomialSplineB = 2.2071067811; // (3+sqrt(2))/2
43     static constexpr float kPolynomialSplineC = -0.6035533905; // -(1+sqrt(2))/4
44     static constexpr float kXWhenYis3Decibels = 1.8284271247; // -1+2sqrt(2)
45 
46     /**
47      * Process an input based on the following:
48      * If between -1 and 1, return the input value.
49      * If above kXWhenYis3Decibels, return sqrt(2).
50      * If below -kXWhenYis3Decibels, return -sqrt(2).
51      * If between 1 and kXWhenYis3Decibels, use a quadratic spline (Ax^2 + Bx + C).
52      * If between -kXWhenYis3Decibels and -1, use the absolute value for the spline and flip it.
53      * The derivative of the spline is 1 at 1 and 0 at kXWhenYis3Decibels.
54      * This way, the graph is both continuous and differentiable.
55      */
56     float processFloat(float in);
57 
58     // Use the previous valid output for NaN inputs
59     float mLastValidOutput = 0.0f;
60 };
61 
62 } /* namespace FLOWGRAPH_OUTER_NAMESPACE::flowgraph */
63 
64 #endif //FLOWGRAPH_LIMITER_H
65