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.h"
   9 #include "media/base/media_export.h"
  10
  11 namespace media {
  12
  13 // Generates timestamps for a sequence of audio sample bytes. 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 bytes per frame and samples_per_second
  19 // information so that it can convert audio sample byte counts into timestamps.
  20 // After the object is constructed, SetBaseTimestamp() must be called to specify
  21 // the starting timestamp of the audio sequence. As audio samples are received,
  22 // their byte counts are added to AddBytes(). These byte counts are
  23 // accumulated by this class so GetTimestamp() can be used to determine the
  24 // timestamp for the samples that have been added. GetDuration() calculates
  25 // the proper duration values for samples added to the current timestamp.
  26 // GetBytesToTarget() determines the number of bytes that need to be
  27 // added/removed from the accumulated bytes to reach a target timestamp.
  28 class MEDIA_EXPORT AudioTimestampHelper {
  29  public:
  30   AudioTimestampHelper(int bytes_per_frame, 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
  37   // Adds sample bytes to the frame counter.
  38   //
  39   // Note: SetBaseTimestamp() must be called with a value other than
  40   // kNoTimestamp() before this method can be called.
  41   void AddBytes(int byte_count);
  42
  43   // Get the current timestamp. This value is computed from the base_timestamp()
  44   // and the number of sample bytes that have been added so far.
  45   base::TimeDelta GetTimestamp() const;
  46
  47   // Gets the duration if |byte_count| bytes were added to the current
  48   // timestamp reported by GetTimestamp(). This method ensures that
  49   // (GetTimestamp() + GetDuration(n)) will equal the timestamp that
  50   // GetTimestamp() will return if AddBytes(n) is called.
  51   base::TimeDelta GetDuration(int byte_count) const;
  52
  53   // Returns the number of bytes needed to reach the target timestamp.
  54   //
  55   // Note: |target| must be >= |base_timestamp_|.
  56   int64 GetBytesToTarget(base::TimeDelta target) const;
  57
  58  private:
  59   base::TimeDelta ComputeTimestamp(int64 frame_count) const;
  60
  61   int bytes_per_frame_;
  62   double microseconds_per_frame_;
  63
  64   base::TimeDelta base_timestamp_;
  65
  66   // Number of frames accumulated by byte counts passed to AddBytes() calls.
  67   int64 frame_count_;
  68
  69   DISALLOW_IMPLICIT_CONSTRUCTORS(AudioTimestampHelper);
  70 };
  71
  72 }  // namespace media
  73
  74 #endif