media/base/audio_timestamp_helper.h

   1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef MEDIA_BASE_AUDIO_TIMESTAMP_HELPER_H_
   6 #define MEDIA_BASE_AUDIO_TIMESTAMP_HELPER_H_
   7
   8 #include "base/time/time.h"
   9 #include "media/base/media_export.h"
  10
  11 namespace media {
  12
  13 // Generates timestamps for a sequence of audio sample frames. This class should
  14 // be used any place timestamps need to be calculated for a sequence of audio
  15 // samples. It helps avoid timestamps inaccuracies caused by rounding/truncation
  16 // in repeated sample count to timestamp conversions.
  17 //
  18 // The class is constructed with samples_per_second information so that it can
  19 // convert audio sample frame counts into timestamps. After the object is
  20 // constructed, SetBaseTimestamp() must be called to specify the starting
  21 // timestamp of the audio sequence. As audio samples are received, their frame
  22 // counts are added using AddFrames(). These frame counts are accumulated by
  23 // this class so GetTimestamp() can be used to determine the timestamp for the
  24 // samples that have been added. GetDuration() calculates the proper duration
  25 // values for samples added to the current timestamp. GetFramesToTarget()
  26 // determines the number of frames that need to be added/removed from the
  27 // accumulated frames to reach a target timestamp.
  28 class MEDIA_EXPORT AudioTimestampHelper {
  29  public:
  30   explicit AudioTimestampHelper(int samples_per_second);
  31
  32   // Sets the base timestamp to |base_timestamp| and the sets count to 0.
  33   void SetBaseTimestamp(base::TimeDelta base_timestamp);
  34
  35   base::TimeDelta base_timestamp() const;
  36   int64 frame_count() const { return frame_count_; }
  37
  38   // Adds |frame_count| to the frame counter.
  39   // Note: SetBaseTimestamp() must be called with a value other than
  40   // kNoTimestamp() before this method can be called.
  41   void AddFrames(int frame_count);
  42
  43   // Get the current timestamp. This value is computed from the base_timestamp()
  44   // and the number of sample frames that have been added so far.
  45   base::TimeDelta GetTimestamp() const;
  46
  47   // Gets the duration if |frame_count| frames were added to the current
  48   // timestamp reported by GetTimestamp(). This method ensures that
  49   // (GetTimestamp() + GetFrameDuration(n)) will equal the timestamp that
  50   // GetTimestamp() will return if AddFrames(n) is called.
  51   base::TimeDelta GetFrameDuration(int frame_count) const;
  52
  53   // Returns the number of frames needed to reach the target timestamp.
  54   // Note: |target| must be >= |base_timestamp_|.
  55   int64 GetFramesToTarget(base::TimeDelta target) const;
  56
  57  private:
  58   base::TimeDelta ComputeTimestamp(int64 frame_count) const;
  59
  60   double microseconds_per_frame_;
  61
  62   base::TimeDelta base_timestamp_;
  63
  64   // Number of frames accumulated by AddFrames() calls.
  65   int64 frame_count_;
  66
  67   DISALLOW_IMPLICIT_CONSTRUCTORS(AudioTimestampHelper);
  68 };
  69
  70 }  // namespace media
  71
  72 #endif