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
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
|
/*
* Copyright (c) 2011 The WebRTC project authors. All Rights Reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
#include "modules/video_coding/timing/timestamp_extrapolator.h"
#include <algorithm>
#include "absl/types/optional.h"
#include "rtc_base/numerics/sequence_number_unwrapper.h"
namespace webrtc {
namespace {
constexpr double kLambda = 1;
constexpr uint32_t kStartUpFilterDelayInPackets = 2;
constexpr double kAlarmThreshold = 60e3;
// in timestamp ticks, i.e. 15 ms
constexpr double kAccDrift = 6600;
constexpr double kAccMaxError = 7000;
constexpr double kP11 = 1e10;
} // namespace
TimestampExtrapolator::TimestampExtrapolator(Timestamp start)
: start_(Timestamp::Zero()),
prev_(Timestamp::Zero()),
packet_count_(0),
detector_accumulator_pos_(0),
detector_accumulator_neg_(0) {
Reset(start);
}
void TimestampExtrapolator::Reset(Timestamp start) {
start_ = start;
prev_ = start_;
first_unwrapped_timestamp_ = absl::nullopt;
w_[0] = 90.0;
w_[1] = 0;
p_[0][0] = 1;
p_[1][1] = kP11;
p_[0][1] = p_[1][0] = 0;
unwrapper_ = RtpTimestampUnwrapper();
packet_count_ = 0;
detector_accumulator_pos_ = 0;
detector_accumulator_neg_ = 0;
}
void TimestampExtrapolator::Update(Timestamp now, uint32_t ts90khz) {
if (now - prev_ > TimeDelta::Seconds(10)) {
// Ten seconds without a complete frame.
// Reset the extrapolator
Reset(now);
} else {
prev_ = now;
}
// Remove offset to prevent badly scaled matrices
const TimeDelta offset = now - start_;
double t_ms = offset.ms();
int64_t unwrapped_ts90khz = unwrapper_.Unwrap(ts90khz);
if (!first_unwrapped_timestamp_) {
// Make an initial guess of the offset,
// should be almost correct since t_ms - start
// should about zero at this time.
w_[1] = -w_[0] * t_ms;
first_unwrapped_timestamp_ = unwrapped_ts90khz;
}
double residual =
(static_cast<double>(unwrapped_ts90khz) - *first_unwrapped_timestamp_) -
t_ms * w_[0] - w_[1];
if (DelayChangeDetection(residual) &&
packet_count_ >= kStartUpFilterDelayInPackets) {
// A sudden change of average network delay has been detected.
// Force the filter to adjust its offset parameter by changing
// the offset uncertainty. Don't do this during startup.
p_[1][1] = kP11;
}
if (prev_unwrapped_timestamp_ &&
unwrapped_ts90khz < prev_unwrapped_timestamp_) {
// Drop reordered frames.
return;
}
// T = [t(k) 1]';
// that = T'*w;
// K = P*T/(lambda + T'*P*T);
double K[2];
K[0] = p_[0][0] * t_ms + p_[0][1];
K[1] = p_[1][0] * t_ms + p_[1][1];
double TPT = kLambda + t_ms * K[0] + K[1];
K[0] /= TPT;
K[1] /= TPT;
// w = w + K*(ts(k) - that);
w_[0] = w_[0] + K[0] * residual;
w_[1] = w_[1] + K[1] * residual;
// P = 1/lambda*(P - K*T'*P);
double p00 =
1 / kLambda * (p_[0][0] - (K[0] * t_ms * p_[0][0] + K[0] * p_[1][0]));
double p01 =
1 / kLambda * (p_[0][1] - (K[0] * t_ms * p_[0][1] + K[0] * p_[1][1]));
p_[1][0] =
1 / kLambda * (p_[1][0] - (K[1] * t_ms * p_[0][0] + K[1] * p_[1][0]));
p_[1][1] =
1 / kLambda * (p_[1][1] - (K[1] * t_ms * p_[0][1] + K[1] * p_[1][1]));
p_[0][0] = p00;
p_[0][1] = p01;
prev_unwrapped_timestamp_ = unwrapped_ts90khz;
if (packet_count_ < kStartUpFilterDelayInPackets) {
packet_count_++;
}
}
absl::optional<Timestamp> TimestampExtrapolator::ExtrapolateLocalTime(
uint32_t timestamp90khz) const {
int64_t unwrapped_ts90khz = unwrapper_.PeekUnwrap(timestamp90khz);
RTC_DCHECK_GE(unwrapped_ts90khz, 0);
if (!first_unwrapped_timestamp_) {
return absl::nullopt;
} else if (packet_count_ < kStartUpFilterDelayInPackets) {
constexpr double kRtpTicksPerMs = 90;
TimeDelta diff = TimeDelta::Millis(
(unwrapped_ts90khz - *prev_unwrapped_timestamp_) / kRtpTicksPerMs);
if (prev_.us() + diff.us() < 0) {
// Prevent the construction of a negative Timestamp.
// This scenario can occur when the RTP timestamp wraps around.
return absl::nullopt;
}
return prev_ + diff;
} else if (w_[0] < 1e-3) {
return start_;
} else {
double timestampDiff = unwrapped_ts90khz - *first_unwrapped_timestamp_;
TimeDelta diff = TimeDelta::Millis(
static_cast<int64_t>((timestampDiff - w_[1]) / w_[0] + 0.5));
if (start_.us() + diff.us() < 0) {
// Prevent the construction of a negative Timestamp.
// This scenario can occur when the RTP timestamp wraps around.
return absl::nullopt;
}
return start_ + diff;
}
}
bool TimestampExtrapolator::DelayChangeDetection(double error) {
// CUSUM detection of sudden delay changes
error = (error > 0) ? std::min(error, kAccMaxError)
: std::max(error, -kAccMaxError);
detector_accumulator_pos_ =
std::max(detector_accumulator_pos_ + error - kAccDrift, double{0});
detector_accumulator_neg_ =
std::min(detector_accumulator_neg_ + error + kAccDrift, double{0});
if (detector_accumulator_pos_ > kAlarmThreshold ||
detector_accumulator_neg_ < -kAlarmThreshold) {
// Alarm
detector_accumulator_pos_ = detector_accumulator_neg_ = 0;
return true;
}
return false;
}
} // namespace webrtc
|
