1 // Copyright 2014 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_CAST_BASE_CLOCK_DRIFT_SMOOTHER_H_ 6 #define MEDIA_CAST_BASE_CLOCK_DRIFT_SMOOTHER_H_ 7 8 #include "base/time/time.h" 9 10 namespace media { 11 namespace cast { 12 13 // Tracks the jitter and drift between clocks, providing a smoothed offset. 14 // Internally, a Simple IIR filter is used to maintain a running average that 15 // moves at a rate based on the passage of time. 16 class ClockDriftSmoother { 17 public: 18 // |time_constant| is the amount of time an impulse signal takes to decay by 19 // ~62.6%. Interpretation: If the value passed to several Update() calls is 20 // held constant for T seconds, then the running average will have moved 21 // towards the value by ~62.6% from where it started. 22 explicit ClockDriftSmoother(base::TimeDelta time_constant); 23 ~ClockDriftSmoother(); 24 25 // Returns the current offset. 26 base::TimeDelta Current() const; 27 28 // Discard all history and reset to exactly |offset|, measured |now|. 29 void Reset(base::TimeTicks now, base::TimeDelta offset); 30 31 // Update the current offset, which was measured |now|. The weighting that 32 // |measured_offset| will have on the running average is influenced by how 33 // much time has passed since the last call to this method (or Reset()). 34 // |now| should be monotonically non-decreasing over successive calls of this 35 // method. 36 void Update(base::TimeTicks now, base::TimeDelta measured_offset); 37 38 // Returns a time constant suitable for most use cases, where the clocks 39 // are expected to drift very little with respect to each other, and the 40 // jitter caused by clock imprecision is effectively canceled out. 41 static base::TimeDelta GetDefaultTimeConstant(); 42 43 private: 44 const base::TimeDelta time_constant_; 45 base::TimeTicks last_update_time_; 46 double estimate_us_; 47 }; 48 49 } // namespace cast 50 } // namespace media 51 52 #endif // MEDIA_CAST_BASE_CLOCK_DRIFT_SMOOTHER_H_ 53