src/media/audio/audio_core/mixer/mixer.cc

// Copyright 2016 The Fuchsia 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 "src/media/audio/audio_core/mixer/mixer.h"

#include <trace/event.h>

#include "lib/media/cpp/timeline_rate.h"
#include "src/media/audio/audio_core/mixer/linear_sampler.h"
#include "src/media/audio/audio_core/mixer/no_op.h"
#include "src/media/audio/audio_core/mixer/point_sampler.h"
#include "src/media/audio/audio_core/mixer/sinc_sampler.h"

namespace media::audio {

constexpr uint32_t Mixer::FRAC_ONE;
constexpr uint32_t Mixer::FRAC_MASK;
constexpr uint32_t Mixer::Bookkeeping::kScaleArrLen;

Mixer::Mixer(uint32_t pos_filter_width, uint32_t neg_filter_width)
    : pos_filter_width_(FractionalFrames<uint32_t>::FromRaw(pos_filter_width)),
      neg_filter_width_(FractionalFrames<uint32_t>::FromRaw(neg_filter_width)) {}

//
// Select an appropriate instance of a mixer based on the user-specified
// resampler type, else by the properties of source/destination formats.
//
// With 'resampler', users indicate which resampler they require. If not
// specified, or if Resampler::Default, the existing selection algorithm is
// used. Note that requiring a specific resampler may cause Mixer::Select() to
// fail (i.e. return nullptr), even in cases where 'Default' would succeed.
std::unique_ptr<Mixer> Mixer::Select(const fuchsia::media::AudioStreamType& src_format,
                                     const fuchsia::media::AudioStreamType& dest_format,
                                     Resampler resampler) {
  TRACE_DURATION("audio", "Mixer::Select");
  // If user specified a particular Resampler, directly select it.
  switch (resampler) {
    case Resampler::SampleAndHold:
      return mixer::PointSampler::Select(src_format, dest_format);
    case Resampler::LinearInterpolation:
      return mixer::LinearSampler::Select(src_format, dest_format);
    case Resampler::WindowedSinc:
      return mixer::SincSampler::Select(src_format, dest_format);

      // Otherwise (if Default), continue onward.
    case Resampler::Default:
      break;
  }

  // Use SampleAndHold if no rate conversion (unity 1:1). Otherwise, use WindowedSinc (with
  // integrated low-pass filter). Unless explicitly invoked, we do not use LinearInterpolation.
  TimelineRate src_to_dest(dest_format.frames_per_second, src_format.frames_per_second);
  if (src_to_dest.subject_delta() == 1 && src_to_dest.reference_delta() == 1) {
    return mixer::PointSampler::Select(src_format, dest_format);
  } else {
    return mixer::SincSampler::Select(src_format, dest_format);
  }
}

}  // namespace media::audio