1 /* 2 * Copyright (c) 2016 The WebRTC Project Authors. All rights reserved. 3 * 4 * Use of this source code is governed by a BSD-style license 5 * that can be found in the LICENSE file in the root of the source 6 * tree. An additional intellectual property rights grant can be found 7 * in the file PATENTS. All contributing project authors may 8 * be found in the AUTHORS file in the root of the source tree. 9 */ 10 11 #ifndef RTC_BASE_TIMESTAMP_ALIGNER_H_ 12 #define RTC_BASE_TIMESTAMP_ALIGNER_H_ 13 14 #include <stdint.h> 15 16 #include "rtc_base/constructor_magic.h" 17 #include "rtc_base/system/rtc_export.h" 18 19 namespace rtc { 20 21 // The TimestampAligner class helps translating timestamps of a capture system 22 // into the same timescale as is used by rtc::TimeMicros(). Some capture systems 23 // provide timestamps, which comes from the capturing hardware (camera or sound 24 // card) or stamped close to the capturing hardware. Such timestamps are more 25 // accurate (less jittery) than reading the system clock, but may have a 26 // different epoch and unknown clock drift. Frame timestamps in webrtc should 27 // use rtc::TimeMicros (system monotonic time), and this class provides a filter 28 // which lets us use the rtc::TimeMicros timescale, and at the same time take 29 // advantage of higher accuracy of the capturer's clock. 30 31 // This class is not thread safe, so all calls to it must be synchronized 32 // externally. 33 class RTC_EXPORT TimestampAligner { 34 public: 35 TimestampAligner(); 36 ~TimestampAligner(); 37 38 public: 39 // Translates timestamps of a capture system to the same timescale as is used 40 // by rtc::TimeMicros(). |capturer_time_us| is assumed to be accurate, but 41 // with an unknown epoch and clock drift. |system_time_us| is 42 // time according to rtc::TimeMicros(), preferably read as soon as 43 // possible when the frame is captured. It may have poor accuracy 44 // due to poor resolution or scheduling delays. Returns the 45 // translated timestamp. 46 int64_t TranslateTimestamp(int64_t capturer_time_us, int64_t system_time_us); 47 48 // Returns the translated timestamp without updating the states. This is to 49 // allow TimestampAligner to translate capturer time into system clock based 50 // on earlier observations. It won't guarantee monotonicity. 51 int64_t TranslateTimestamp(int64_t capturer_time_us) const; 52 53 protected: 54 // Update the estimated offset between capturer's time and system monotonic 55 // time. 56 int64_t UpdateOffset(int64_t capturer_time_us, int64_t system_time_us); 57 58 // Clip timestamp, return value is always 59 // <= |system_time_us|, and 60 // >= min(|prev_translated_time_us_| + |kMinFrameIntervalUs|, 61 // |system_time_us|). 62 int64_t ClipTimestamp(int64_t filtered_time_us, int64_t system_time_us); 63 64 private: 65 // State for the timestamp translation. 66 int frames_seen_; 67 // Estimated offset between capturer's time and system monotonic time. 68 int64_t offset_us_; 69 70 // State for the ClipTimestamp method, applied after the filter. 71 // A large negative clock drift of the capturer tends to push translated 72 // timestamps into the future. |clip_bias_us_| is subtracted from the 73 // translated timestamps, to get them back from the future. 74 int64_t clip_bias_us_; 75 // Used to ensure that translated timestamps are monotonous. 76 int64_t prev_translated_time_us_; 77 // Offset between |prev_translated_time_us_| and the corresponding capturer 78 // time. 79 int64_t prev_time_offset_us_; 80 RTC_DISALLOW_COPY_AND_ASSIGN(TimestampAligner); 81 }; 82 83 } // namespace rtc 84 85 #endif // RTC_BASE_TIMESTAMP_ALIGNER_H_ 86