audio_output.h

// Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
//
// An interface representing a real-time audio output system on a particular
// platform.

#ifndef SPEECH_CLIENT_SYNTHESIS_SERVICE_AUDIO_OUTPUT_H_
#define SPEECH_CLIENT_SYNTHESIS_SERVICE_AUDIO_OUTPUT_H_

#include <stdlib.h>

namespace speech_synthesis {

class Threading;

// This is an interface defining the method that fills an audio buffer
// for real-time audio output.  Typically the TtsService implements this
// method for AudioOutput, so that the service has all of the logic for
// computing the samples to be computed, but AudioOutput has all of the logic
// for dealing with the platform-specific audio hardware abstraction.
class AudioProvider {
 public:
  virtual ~AudioProvider() { }

  // This method must return quickly and not block.  It must always
  // fill all of the samples, with silence padding as needed.
  // Returns false if audio should completely stop.
  virtual bool FillAudioBuffer(int16_t* samples, int size) = 0;
};

// This is the abstract definition of a class that does real-time audio
// output.  You must provide an implementation of an AudioProvider that
// provides the actual samples to output on demand (or silence, if there
// are not enough samples available immediately).  Subclasses of AudioOutput
// implement these methods using various platform-specific methods.
class AudioOutput {
 protected:
  AudioOutput() { }

 public:
  // Static factory method, creates whatever AudioOutput subclass
  // has been linked in.
  static AudioOutput* Create(Threading *threading);

  virtual ~AudioOutput() {}

  // Initialize audio output, getting audio samples from the given provider.
  // Returns true on success.  Safe to call more than once.
  virtual bool Init(AudioProvider *provider) = 0;

  virtual void StartAudio() = 0;

  virtual void StopAudio() = 0;

  virtual int GetSampleRate() = 0;

  virtual int GetChannelCount() = 0;

  // The typical number of samples that will be requested at once.
  // The TTS system may want to generate audio in increments of this
  // many samples, but it's just a hint.
  virtual int GetChunkSize() = 0;

  // The number of samples needed to completely fill the audio buffers.
  // The provider must be able to quickly produce at least this many
  // samples in order to avoid underflow.
  virtual int GetTotalBufferSize() = 0;
};

}  // namespace speech_synthesis

#endif  // SPEECH_CLIENT_SYNTHESIS_SERVICE_AUDIO_OUTPUT_H_