channel_mixer.h (3172B)
1 /* 2 * Copyright (c) 2019 The WebRTC project authors. All Rights Reserved. 3 * 4 * Use of this source code is governed by a BSD-style license 5 * that can be found in the LICENSE file in the root of the source 6 * tree. An additional intellectual property rights grant can be found 7 * in the file PATENTS. All contributing project authors may 8 * be found in the AUTHORS file in the root of the source tree. 9 */ 10 11 #ifndef AUDIO_UTILITY_CHANNEL_MIXER_H_ 12 #define AUDIO_UTILITY_CHANNEL_MIXER_H_ 13 14 #include <cstddef> 15 #include <cstdint> 16 #include <memory> 17 #include <vector> 18 19 #include "api/audio/audio_frame.h" 20 #include "api/audio/channel_layout.h" 21 22 namespace webrtc { 23 24 // ChannelMixer is for converting audio between channel layouts. The conversion 25 // matrix is built upon construction and used during each Transform() call. The 26 // algorithm works by generating a conversion matrix mapping each output channel 27 // to list of input channels. The transform renders all of the output channels, 28 // with each output channel rendered according to a weighted sum of the relevant 29 // input channels as defined in the matrix. 30 // This file is derived from Chromium's media/base/channel_mixer.h. 31 class ChannelMixer { 32 public: 33 // To mix two channels into one and preserve loudness, we must apply 34 // (1 / sqrt(2)) gain to each. 35 static constexpr float kHalfPower = 0.707106781186547524401f; 36 37 ChannelMixer(ChannelLayout input_layout, 38 size_t input_channels, 39 ChannelLayout output_layout, 40 size_t output_channels); 41 ChannelMixer(ChannelLayout input_layout, ChannelLayout output_layout); 42 ~ChannelMixer(); 43 44 // Transforms all input channels corresponding to the selected `input_layout` 45 // to the number of channels in the selected `output_layout`. 46 // Example usage (downmix from stereo to mono): 47 // 48 // ChannelMixer mixer(CHANNEL_LAYOUT_STEREO, CHANNEL_LAYOUT_MONO); 49 // AudioFrame frame; 50 // frame.samples_per_channel_ = 160; 51 // frame.num_channels_ = 2; 52 // EXPECT_EQ(2u, frame.channels()); 53 // mixer.Transform(&frame); 54 // EXPECT_EQ(1u, frame.channels()); 55 // 56 void Transform(AudioFrame* frame); 57 58 private: 59 bool IsUpMixing() const { return output_channels_ > input_channels_; } 60 61 // Selected channel layouts. 62 const ChannelLayout input_layout_; 63 const ChannelLayout output_layout_; 64 65 // Channel counts for input and output. 66 const size_t input_channels_; 67 const size_t output_channels_; 68 69 // 2D matrix of output channels to input channels. 70 std::vector<std::vector<float> > matrix_; 71 72 // 1D array used as temporary storage during the transformation. 73 std::unique_ptr<int16_t[]> audio_vector_; 74 75 // Number of elements allocated for `audio_vector_`. 76 size_t audio_vector_size_ = 0; 77 78 // Optimization case for when we can simply remap the input channels to output 79 // channels, i.e., when all scaling factors in `matrix_` equals 1.0. 80 bool remapping_; 81 82 // Delete the copy constructor and assignment operator. 83 ChannelMixer(const ChannelMixer& other) = delete; 84 ChannelMixer& operator=(const ChannelMixer& other) = delete; 85 }; 86 87 } // namespace webrtc 88 89 #endif // AUDIO_UTILITY_CHANNEL_MIXER_H_