src/cobalt/media/filters/audio_renderer_algorithm.cc

// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "cobalt/media/filters/audio_renderer_algorithm.h"

#include <algorithm>
#include <cmath>

#include "base/logging.h"
#include "cobalt/media/base/audio_bus.h"
#include "cobalt/media/base/limits.h"
#include "cobalt/media/filters/wsola_internals.h"
#include "starboard/memory.h"

namespace cobalt {
namespace media {

// Waveform Similarity Overlap-and-add (WSOLA).
//
// One WSOLA iteration
//
// 1) Extract |target_block_| as input frames at indices
//    [|target_block_index_|, |target_block_index_| + |ola_window_size_|).
//    Note that |target_block_| is the "natural" continuation of the output.
//
// 2) Extract |search_block_| as input frames at indices
//    [|search_block_index_|,
//     |search_block_index_| + |num_candidate_blocks_| + |ola_window_size_|).
//
// 3) Find a block within the |search_block_| that is most similar
//    to |target_block_|. Let |optimal_index| be the index of such block and
//    write it to |optimal_block_|.
//
// 4) Update:
//    |optimal_block_| = |transition_window_| * |target_block_| +
//    (1 - |transition_window_|) * |optimal_block_|.
//
// 5) Overlap-and-add |optimal_block_| to the |wsola_output_|.
//
// 6) Update:
//    |target_block_| = |optimal_index| + |ola_window_size_| / 2.
//    |output_index_| = |output_index_| + |ola_window_size_| / 2,
//    |search_block_center_offset_| = |output_index_| * |playback_rate|, and
//    |search_block_index_| = |search_block_center_offset_| -
//        |search_block_center_offset_|.

// Max/min supported playback rates for fast/slow audio. Audio outside of these
// ranges are muted.
// Audio at these speeds would sound better under a frequency domain algorithm.
static const double kMinPlaybackRate = 0.5;
static const double kMaxPlaybackRate = 4.0;

// Overlap-and-add window size in milliseconds.
static const int kOlaWindowSizeMs = 20;

// Size of search interval in milliseconds. The search interval is
// [-delta delta] around |output_index_| * |playback_rate|. So the search
// interval is 2 * delta.
static const int kWsolaSearchIntervalMs = 30;

// The maximum size in seconds for the |audio_buffer_|. Arbitrarily determined.
static const int kMaxCapacityInSeconds = 3;

// The minimum size in ms for the |audio_buffer_|. Arbitrarily determined.
static const int kStartingCapacityInMs = 200;

AudioRendererAlgorithm::AudioRendererAlgorithm()
    : channels_(0),
      samples_per_second_(0),
      muted_partial_frame_(0),
      capacity_(0),
      output_time_(0.0),
      search_block_center_offset_(0),
      search_block_index_(0),
      num_candidate_blocks_(0),
      target_block_index_(0),
      ola_window_size_(0),
      ola_hop_size_(0),
      num_complete_frames_(0),
      initial_capacity_(0),
      max_capacity_(0) {}

AudioRendererAlgorithm::~AudioRendererAlgorithm() {}

void AudioRendererAlgorithm::Initialize(const AudioParameters& params) {
  CHECK(params.IsValid());

  channels_ = params.channels();
  samples_per_second_ = params.sample_rate();
  initial_capacity_ = capacity_ =
      std::max(params.frames_per_buffer() * 2,
               ConvertMillisecondsToFrames(kStartingCapacityInMs));
  max_capacity_ =
      std::max(initial_capacity_, kMaxCapacityInSeconds * samples_per_second_);
  num_candidate_blocks_ = ConvertMillisecondsToFrames(kWsolaSearchIntervalMs);
  ola_window_size_ = ConvertMillisecondsToFrames(kOlaWindowSizeMs);

  // Make sure window size in an even number.
  ola_window_size_ += ola_window_size_ & 1;
  ola_hop_size_ = ola_window_size_ / 2;

  // |num_candidate_blocks_| / 2 is the offset of the center of the search
  // block to the center of the first (left most) candidate block. The offset
  // of the center of a candidate block to its left most point is
  // |ola_window_size_| / 2 - 1. Note that |ola_window_size_| is even and in
  // our convention the center belongs to the left half, so we need to subtract
  // one frame to get the correct offset.
  //
  //                             Search Block
  //              <------------------------------------------->
  //
  //   |ola_window_size_| / 2 - 1
  //              <----
  //
  //             |num_candidate_blocks_| / 2
  //                   <----------------
  //                                 center
  //              X----X----------------X---------------X-----X
  //              <---------->                     <---------->
  //                Candidate      ...               Candidate
  //                   1,          ...         |num_candidate_blocks_|
  search_block_center_offset_ =
      num_candidate_blocks_ / 2 + (ola_window_size_ / 2 - 1);

  ola_window_.reset(new float[ola_window_size_]);
  internal::GetSymmetricHanningWindow(ola_window_size_, ola_window_.get());

  transition_window_.reset(new float[ola_window_size_ * 2]);
  internal::GetSymmetricHanningWindow(2 * ola_window_size_,
                                      transition_window_.get());

  wsola_output_ = AudioBus::Create(channels_, ola_window_size_ + ola_hop_size_);
  wsola_output_->Zero();  // Initialize for overlap-and-add of the first block.

  // Auxiliary containers.
  optimal_block_ = AudioBus::Create(channels_, ola_window_size_);
  search_block_ = AudioBus::Create(
      channels_, num_candidate_blocks_ + (ola_window_size_ - 1));
  target_block_ = AudioBus::Create(channels_, ola_window_size_);
}

int AudioRendererAlgorithm::FillBuffer(AudioBus* dest, int dest_offset,
                                       int requested_frames,
                                       double playback_rate) {
  if (playback_rate == 0) return 0;

  DCHECK_GT(playback_rate, 0);
  DCHECK_EQ(channels_, dest->channels());

  // Optimize the muted case to issue a single clear instead of performing
  // the full crossfade and clearing each crossfaded frame.
  if (playback_rate < kMinPlaybackRate || playback_rate > kMaxPlaybackRate) {
    int frames_to_render =
        std::min(static_cast<int>(audio_buffer_.frames() / playback_rate),
                 requested_frames);

    // Compute accurate number of frames to actually skip in the source data.
    // Includes the leftover partial frame from last request. However, we can
    // only skip over complete frames, so a partial frame may remain for next
    // time.
    muted_partial_frame_ += frames_to_render * playback_rate;
    // Handle the case where muted_partial_frame_ rounds up to
    // audio_buffer_.frames()+1.
    int seek_frames = std::min(static_cast<int>(muted_partial_frame_),
                               audio_buffer_.frames());
    dest->ZeroFramesPartial(dest_offset, frames_to_render);
    audio_buffer_.SeekFrames(seek_frames);

    // Determine the partial frame that remains to be skipped for next call. If
    // the user switches back to playing, it may be off time by this partial
    // frame, which would be undetectable. If they subsequently switch to
    // another playback rate that mutes, the code will attempt to line up the
    // frames again.
    muted_partial_frame_ -= seek_frames;
    return frames_to_render;
  }

  int slower_step = ceil(ola_window_size_ * playback_rate);
  int faster_step = ceil(ola_window_size_ / playback_rate);

  // Optimize the most common |playback_rate| ~= 1 case to use a single copy
  // instead of copying frame by frame.
  if (ola_window_size_ <= faster_step && slower_step >= ola_window_size_) {
    const int frames_to_copy =
        std::min(audio_buffer_.frames(), requested_frames);
    const int frames_read =
        audio_buffer_.ReadFrames(frames_to_copy, dest_offset, dest);
    DCHECK_EQ(frames_read, frames_to_copy);
    return frames_read;
  }

  int rendered_frames = 0;
  do {
    rendered_frames +=
        WriteCompletedFramesTo(requested_frames - rendered_frames,
                               dest_offset + rendered_frames, dest);
  } while (rendered_frames < requested_frames &&
           RunOneWsolaIteration(playback_rate));
  return rendered_frames;
}

void AudioRendererAlgorithm::FlushBuffers() {
  // Clear the queue of decoded packets (releasing the buffers).
  audio_buffer_.Clear();
  output_time_ = 0.0;
  search_block_index_ = 0;
  target_block_index_ = 0;
  wsola_output_->Zero();
  num_complete_frames_ = 0;

  // Reset |capacity_| so growth triggered by underflows doesn't penalize seek
  // time.
  capacity_ = initial_capacity_;
}

void AudioRendererAlgorithm::EnqueueBuffer(
    const scoped_refptr<AudioBuffer>& buffer_in) {
  DCHECK(!buffer_in->end_of_stream());
  audio_buffer_.Append(buffer_in);
}

bool AudioRendererAlgorithm::IsQueueFull() {
  return audio_buffer_.frames() >= capacity_;
}

void AudioRendererAlgorithm::IncreaseQueueCapacity() {
  DCHECK_LE(capacity_, max_capacity_);
  capacity_ = std::min(2 * capacity_, max_capacity_);
}

int64_t AudioRendererAlgorithm::GetMemoryUsage() const {
  return audio_buffer_.frames() * channels_ * sizeof(float);
}

bool AudioRendererAlgorithm::CanPerformWsola() const {
  const int search_block_size = num_candidate_blocks_ + (ola_window_size_ - 1);
  const int frames = audio_buffer_.frames();
  return target_block_index_ + ola_window_size_ <= frames &&
         search_block_index_ + search_block_size <= frames;
}

int AudioRendererAlgorithm::ConvertMillisecondsToFrames(int ms) const {
  return ms * (samples_per_second_ /
               static_cast<double>(base::Time::kMillisecondsPerSecond));
}

bool AudioRendererAlgorithm::RunOneWsolaIteration(double playback_rate) {
  if (!CanPerformWsola()) return false;

  GetOptimalBlock();

  // Overlap-and-add.
  for (int k = 0; k < channels_; ++k) {
    const float* const ch_opt_frame = optimal_block_->channel(k);
    float* ch_output = wsola_output_->channel(k) + num_complete_frames_;
    for (int n = 0; n < ola_hop_size_; ++n) {
      ch_output[n] = ch_output[n] * ola_window_[ola_hop_size_ + n] +
                     ch_opt_frame[n] * ola_window_[n];
    }

    // Copy the second half to the output.
    SbMemoryCopy(&ch_output[ola_hop_size_], &ch_opt_frame[ola_hop_size_],
                 sizeof(*ch_opt_frame) * ola_hop_size_);
  }

  num_complete_frames_ += ola_hop_size_;
  UpdateOutputTime(playback_rate, ola_hop_size_);
  RemoveOldInputFrames(playback_rate);
  return true;
}

void AudioRendererAlgorithm::UpdateOutputTime(double playback_rate,
                                              double time_change) {
  output_time_ += time_change;
  // Center of the search region, in frames.
  const int search_block_center_index =
      static_cast<int>(output_time_ * playback_rate + 0.5);
  search_block_index_ = search_block_center_index - search_block_center_offset_;
}

void AudioRendererAlgorithm::RemoveOldInputFrames(double playback_rate) {
  const int earliest_used_index =
      std::min(target_block_index_, search_block_index_);
  if (earliest_used_index <= 0) return;  // Nothing to remove.

  // Remove frames from input and adjust indices accordingly.
  audio_buffer_.SeekFrames(earliest_used_index);
  target_block_index_ -= earliest_used_index;

  // Adjust output index.
  double output_time_change =
      static_cast<double>(earliest_used_index) / playback_rate;
  CHECK_GE(output_time_, output_time_change);
  UpdateOutputTime(playback_rate, -output_time_change);
}

int AudioRendererAlgorithm::WriteCompletedFramesTo(int requested_frames,
                                                   int dest_offset,
                                                   AudioBus* dest) {
  int rendered_frames = std::min(num_complete_frames_, requested_frames);

  if (rendered_frames == 0)
    return 0;  // There is nothing to read from |wsola_output_|, return.

  wsola_output_->CopyPartialFramesTo(0, rendered_frames, dest_offset, dest);

  // Remove the frames which are read.
  int frames_to_move = wsola_output_->frames() - rendered_frames;
  for (int k = 0; k < channels_; ++k) {
    float* ch = wsola_output_->channel(k);
    SbMemoryMove(ch, &ch[rendered_frames], sizeof(*ch) * frames_to_move);
  }
  num_complete_frames_ -= rendered_frames;
  return rendered_frames;
}

bool AudioRendererAlgorithm::TargetIsWithinSearchRegion() const {
  const int search_block_size = num_candidate_blocks_ + (ola_window_size_ - 1);

  return target_block_index_ >= search_block_index_ &&
         target_block_index_ + ola_window_size_ <=
             search_block_index_ + search_block_size;
}

void AudioRendererAlgorithm::GetOptimalBlock() {
  int optimal_index = 0;

  // An interval around last optimal block which is excluded from the search.
  // This is to reduce the buzzy sound. The number 160 is rather arbitrary and
  // derived heuristically.
  const int kExcludeIntervalLengthFrames = 160;
  if (TargetIsWithinSearchRegion()) {
    optimal_index = target_block_index_;
    PeekAudioWithZeroPrepend(optimal_index, optimal_block_.get());
  } else {
    PeekAudioWithZeroPrepend(target_block_index_, target_block_.get());
    PeekAudioWithZeroPrepend(search_block_index_, search_block_.get());
    int last_optimal =
        target_block_index_ - ola_hop_size_ - search_block_index_;
    internal::Interval exclude_iterval =
        std::make_pair(last_optimal - kExcludeIntervalLengthFrames / 2,
                       last_optimal + kExcludeIntervalLengthFrames / 2);

    // |optimal_index| is in frames and it is relative to the beginning of the
    // |search_block_|.
    optimal_index = internal::OptimalIndex(
        search_block_.get(), target_block_.get(), exclude_iterval);

    // Translate |index| w.r.t. the beginning of |audio_buffer_| and extract the
    // optimal block.
    optimal_index += search_block_index_;
    PeekAudioWithZeroPrepend(optimal_index, optimal_block_.get());

    // Make a transition from target block to the optimal block if different.
    // Target block has the best continuation to the current output.
    // Optimal block is the most similar block to the target, however, it might
    // introduce some discontinuity when over-lap-added. Therefore, we combine
    // them for a smoother transition. The length of transition window is twice
    // as that of the optimal-block which makes it like a weighting function
    // where target-block has higher weight close to zero (weight of 1 at index
    // 0) and lower weight close the end.
    for (int k = 0; k < channels_; ++k) {
      float* ch_opt = optimal_block_->channel(k);
      const float* const ch_target = target_block_->channel(k);
      for (int n = 0; n < ola_window_size_; ++n) {
        ch_opt[n] = ch_opt[n] * transition_window_[n] +
                    ch_target[n] * transition_window_[ola_window_size_ + n];
      }
    }
  }

  // Next target is one hop ahead of the current optimal.
  target_block_index_ = optimal_index + ola_hop_size_;
}

void AudioRendererAlgorithm::PeekAudioWithZeroPrepend(int read_offset_frames,
                                                      AudioBus* dest) {
  CHECK_LE(read_offset_frames + dest->frames(), audio_buffer_.frames());

  int write_offset = 0;
  int num_frames_to_read = dest->frames();
  if (read_offset_frames < 0) {
    int num_zero_frames_appended =
        std::min(-read_offset_frames, num_frames_to_read);
    read_offset_frames = 0;
    num_frames_to_read -= num_zero_frames_appended;
    write_offset = num_zero_frames_appended;
    dest->ZeroFrames(num_zero_frames_appended);
  }
  audio_buffer_.PeekFrames(num_frames_to_read, read_offset_frames, write_offset,
                           dest);
}

}  // namespace media
}  // namespace cobalt