chromium/services/audio/snooper_node.h

// Copyright 2018 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef SERVICES_AUDIO_SNOOPER_NODE_H_
#define SERVICES_AUDIO_SNOOPER_NODE_H_

#include <limits>
#include <memory>
#include <optional>

#include "base/synchronization/lock.h"
#include "base/time/time.h"
#include "media/base/audio_parameters.h"
#include "media/base/channel_mixer.h"
#include "media/base/multi_channel_resampler.h"
#include "services/audio/delay_buffer.h"
#include "services/audio/loopback_group_member.h"

namespace media {
class AudioBus;
}  // namespace media

namespace audio {

// Thread-safe implementation of Snooper that records the audio from a
// GroupMember on one thread, and re-renders it to the desired output format on
// another thread. Since the data flow rates are known to be driven by different
// clocks (audio hardware clock versus system clock), the base::TimeTicks
// reference clock is used to detect drift and automatically correct for it to
// maintain proper synchronization.
//
// Throughout this class, there are sample counters (in terms of the input
// audio's sample rate) that are tracked/computed. They refer to the media
// timestamp of the audio flowing through specific parts of the processing
// pipeline: inbound from OnData() calls → through the delay buffer → through
// the resampler → and outbound via Render() calls:
//
//   write position:  The position of audio about to be written into the delay
//                    buffer. This is managed by OnData().
//   read position:   The position of audio about to be read from the delay
//                    buffer and pushed into the resampler. This is managed by
//                    ReadFromDelayBuffer().
//   output position: The position of the audio about to come out of the
//                    resampler. This is computed within Render(). Note that
//                    this is a "virtual" position since it is in terms of the
//                    input audio's sample count, but refers to audio about to
//                    be generated in the output format (with a possibly
//                    different sample rate).
//
// Note that the media timestamps represented by the "positions," as well as the
// surrounding math operations, might seem backwards; but they are not. This is
// because the inbound audio is from a source that pre-renders audio for playout
// in the near future, while the outbound audio is audio that would have been
// played-out in the recent past.
class SnooperNode final : public LoopbackGroupMember::Snooper { … };

}  // namespace audio

#endif  // SERVICES_AUDIO_SNOOPER_NODE_H_