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 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 37 // Adds |frame_count| to the frame counter. 38 // Note: SetBaseTimestamp() must be called with a value other than 39 // kNoTimestamp() before this method can be called. 40 void AddFrames(int frame_count); 41 42 // Get the current timestamp. This value is computed from the base_timestamp() 43 // and the number of sample frames that have been added so far. 44 base::TimeDelta GetTimestamp() const; 45 46 // Gets the duration if |frame_count| frames were added to the current 47 // timestamp reported by GetTimestamp(). This method ensures that 48 // (GetTimestamp() + GetFrameDuration(n)) will equal the timestamp that 49 // GetTimestamp() will return if AddFrames(n) is called. 50 base::TimeDelta GetFrameDuration(int frame_count) const; 51 52 // Returns the number of frames needed to reach the target timestamp. 53 // Note: |target| must be >= |base_timestamp_|. 54 int64 GetFramesToTarget(base::TimeDelta target) const; 55 56 private: 57 base::TimeDelta ComputeTimestamp(int64 frame_count) const; 58 59 double microseconds_per_frame_; 60 61 base::TimeDelta base_timestamp_; 62 63 // Number of frames accumulated by AddFrames() calls. 64 int64 frame_count_; 65 66 DISALLOW_IMPLICIT_CONSTRUCTORS(AudioTimestampHelper); 67 }; 68 69 } // namespace media 70 71 #endif 72
