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/*
* 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 <cstdint>
#include <cstdlib>
#include <optional>
#include "api/units/time_delta.h"
#include "api/units/timestamp.h"
#include "rtc_base/numerics/sequence_number_unwrapper.h"
#include "system_wrappers/include/metrics.h"
namespace webrtc {
namespace {
constexpr int kMinimumSamplesToLogEstimatedClockDrift =
3000; // 100 seconds at 30 fps.
constexpr double kLambda = 1;
constexpr int 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);
}
TimestampExtrapolator::~TimestampExtrapolator() {
if (packet_count_ >= kMinimumSamplesToLogEstimatedClockDrift) {
// Relative clock drift per million (ppm).
double clock_drift_ppm = 1e6 * (w_[0] - 90.0) / 90.0;
RTC_HISTOGRAM_COUNTS_100000("WebRTC.Video.EstimatedClockDrift_ppm",
static_cast<int>(std::abs(clock_drift_ppm)));
}
}
void TimestampExtrapolator::Reset(Timestamp start) {
start_ = start;
prev_ = start_;
first_unwrapped_timestamp_ = std::nullopt;
prev_unwrapped_timestamp_ = std::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_ < kMinimumSamplesToLogEstimatedClockDrift) {
packet_count_++;
}
}
std::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 std::nullopt;
}
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 std::nullopt;
}
return prev_ + diff;
}
if (w_[0] < 1e-3) {
return start_;
}
double timestamp_diff =
static_cast<double>(unwrapped_ts90khz - *first_unwrapped_timestamp_);
TimeDelta diff = TimeDelta::Millis(
static_cast<int64_t>((timestamp_diff - 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 std::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
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