media/cast/common/clock_drift_smoother.h

   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_COMMON_CLOCK_DRIFT_SMOOTHER_H_
   6 #define MEDIA_CAST_COMMON_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_COMMON_CLOCK_DRIFT_SMOOTHER_H_