blob: 783c755cd568cba3202505670393a4dc65c7de3e (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
|
// Copyright 2014 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef CAST_STREAMING_CLOCK_DRIFT_SMOOTHER_H_
#define CAST_STREAMING_CLOCK_DRIFT_SMOOTHER_H_
#include <chrono>
#include "platform/api/time.h"
namespace cast {
namespace streaming {
// Tracks the jitter and drift between clocks, providing a smoothed offset.
// Internally, a Simple IIR filter is used to maintain a running average that
// moves at a rate based on the passage of time.
class ClockDriftSmoother {
public:
using Clock = openscreen::Clock;
// |time_constant| is the amount of time an impulse signal takes to decay by
// ~62.6%. Interpretation: If the value passed to several Update() calls is
// held constant for T seconds, then the running average will have moved
// towards the value by ~62.6% from where it started.
explicit ClockDriftSmoother(Clock::duration time_constant);
~ClockDriftSmoother();
// Returns the current offset.
Clock::duration Current() const;
// Discard all history and reset to exactly |offset|, measured |now|.
void Reset(Clock::time_point now, Clock::duration offset);
// Update the current offset, which was measured |now|. The weighting that
// |measured_offset| will have on the running average is influenced by how
// much time has passed since the last call to this method (or Reset()).
// |now| should be monotonically non-decreasing over successive calls of this
// method.
void Update(Clock::time_point now, Clock::duration measured_offset);
// A time constant suitable for most use cases, where the clocks are expected
// to drift very little with respect to each other, and the jitter caused by
// clock imprecision is effectively canceled out.
static constexpr std::chrono::seconds kDefaultTimeConstant{30};
private:
const std::chrono::duration<double, Clock::duration::period> time_constant_;
// The time at which |estimated_tick_offset_| was last updated.
Clock::time_point last_update_time_;
// The current estimated offset, as number of Clock::duration ticks.
double estimated_tick_offset_;
};
} // namespace streaming
} // namespace cast
#endif // CAST_STREAMING_CLOCK_DRIFT_SMOOTHER_H_
|
