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