chromium/services/audio/delay_buffer.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_DELAY_BUFFER_H_
#define SERVICES_AUDIO_DELAY_BUFFER_H_

#include <memory>

#include "base/containers/circular_deque.h"

namespace media {
class AudioBus;
}  // namespace media

namespace audio {

// Records and maintains a recent history of an audio signal, then allows
// read-back starting from part of the recording. While this looks a lot like a
// FIFO, it is not because Read() will typically not be reading from one end of
// a queue.
//
// The audio format is the same throughout all operations, as this DelayBuffer
// does not resample or remix the audio. Also, for absolute precision, it uses
// frame counts to track the timing of the audio recorded and read.
//
// The typical use case is the loopback audio system: In this scenario, the
// service has an audio output stream running for local playback, and the
// stream's audio is timed to play back in the near future (usually, 1 ms to 20
// ms, depending on the platform). When loopback is active, that audio will be
// copied into this DelayBuffer via calls to Write(). Then, the loopback audio
// stream implementation will Read() the audio at a time in the recent past
// (approximately 20 ms before "now," but this will vary slightly). Because of
// clock drift concerns, the loopback implementation will slightly compress/
// stretch the audio signal it pulls out of this buffer, to maintain
// synchronization, and this will cause it to vary the number of frames read for
// each successive Read() call.
class DelayBuffer { … };

}  // namespace audio

#endif  // SERVICES_AUDIO_DELAY_BUFFER_H_