SMPTE EG 2059-10-2016 Introduction to the New Synchronization System.pdf

1、 Approved July 15, 2016 Copyright 2016 by THE SOCIETY OF MOTION PICTURE AND TELEVISION ENGINEERS 3 Barker Avenue, White Plains, NY 10601 (914) 761-1100 SMPTE EG 2059-10:2016 SMPTE ENGINEERING GUIDELINE Introduction to the New Synchronization System Page 1 of 16 pages Table of Contents Page Foreword

2、2 Introduction 2 1 Scope . 3 2 Conformance Notation . 3 3 Concept of the New Synchronization System . 3 4 IEEE 1588 PTP (Precision Time Protocol) 4 4.1 Synchronization Overview . 4 4.2 Device Types . 6 4.3 Redundancy. 8 5 Time Source 8 5.1 Primary Reference Time 9 5.2 Non-Primary Reference Time 10 5

3、.3 A/V Synchronization Signal . 11 5.4 A/V Synchronization Signal + Non-Primary (or Primary) Reference Time 12 6 Synchronization Ecosystem 14 7 SMPTE ST 12-1 Time Code Generation . 14 Annex A Bibliography (Informative) 16 SMPTE EG 2059-10:2016 Page 2 of 16 pages Foreword SMPTE (the Society of Motion

4、 Picture and Television Engineers) is an internationally-recognized standards developing organization. Headquartered and incorporated in the United States of America, SMPTE has members in over 80 countries on six continents. SMPTEs Engineering Documents, including Standards, Recommended Practices, a

5、nd Engineering Guidelines, are prepared by SMPTEs Technology Committees. Participation in these Committees is open to all with a bona fide interest in their work. SMPTE cooperates closely with other standards-developing organizations, including ISO, IEC and ITU. SMPTE Engineering Documents are draft

6、ed in accordance with the rules given in its Standards Operations Manual. SMPTE EG 2059-10 was prepared by Technology Committee 32NF. Introduction The current methods of synchronization for television, audio and other moving picture signals rely on standards that have been in place for over 30 years

7、. These standards are becoming increasingly inappropriate for the digital age with, for example, networked content sharing and the higher frame rates appropriate to HDTV, UHDTV and other image formats. In order to solve these problems, SMPTE has specified a new synchronization system based on alignm

8、ent to reference time measured from the SMPTE Epoch. SMPTE ST 2059-1 specifies how signals are aligned with respect to the SMPTE Epoch. SMPTE ST 2059-2 specifies a Profile of IEEE Std 1588-2008 Precision Time Protocol for distribution of reference time and synchronization metadata in the professiona

9、l broadcast environment. This guideline describes basic ideas, concepts and Use Cases of the new synchronization system, including SMPTE ST 12-1 Time Code generation. SMPTE EG 2059-10:2016 Page 3 of 16 pages 1 Scope The objective of this document is to introduce the basic concepts behind the use of

10、the new synchronization system and the IEEE-1588 Precision Time Protocol in Professional Broadcast Applications, and to give some use cases. Detailed explanation of the technology is out of scope of this document. 2 Conformance Notation This Engineering Guideline is purely informative and meant to p

11、rovide tutorial information to the industry. It does not impose Conformance Requirements and avoids the use of Conformance Notation. Engineering Guidelines frequently provide tutorial information about a Standard or Recommended Practice and when this is the case, the user should rely on the Standard

12、s and Recommended Practices referenced for interoperability information. 3 Concept of the New Synchronization System Periodic AV signals can be created deterministically if their phase at a particular time is known. In the case of a video synchronization signal, if the start point of the video frame

13、 (phase) and the video clock frequency are known, it can be created deterministically at any given point in time. The new synchronization system takes advantage of this characteristic and shares time information across the system rather than transferring synchronization signals themselves. As time i

14、nformation consists of phase (offset from the certain point in time) and frequency, if the phase relationship between time and an A/V signal has been defined, any A/V synchronization signal can be created deterministically from time information. This concept is illustrated in Figure 1. Figure 1 Conc

15、ept of the New Synchronization System SMPTE EG 2059-10:2016 Page 4 of 16 pages SMPTE ST 2059-1 defines a reference point in time (the SMPTE Epoch) and the alignment of A/V signals to the Epoch. The phase of the video signal is given by time since the Epoch modulo video frame period. 4 IEEE 1588 PTP

16、(Precision Time Protocol) The new synchronization system uses IEEE Std 1588-2008 PTP as a means of sharing precision time across a network. PTP is a time distribution protocol which runs on a network, such as a LAN. It allows for time synchronization with sub-microsecond accuracy. The PTP defined in

17、 IEEE 1588 has many possible parameters and options. A Profile is used to define sets of attributes and their values in order to optimize its application to specific industries or for particular purposes. SMPTE ST 2059-2 specifies the SMPTE PTP profile which is optimized for professional broadcast a

18、pplications. The following is a simple explanation of the PTP operation. See IEEE Std 1588 and SMPTE ST 2059-2 for further detail. 4.1 Synchronization Overview As illustrated in Figure 2, PTP performs time synchronization by exchanging PTP messages periodically between a PTP master and PTP slaves. T

19、hese messages include timestamps corresponding to time of transmission or reception of the message or a related one. PTP slaves perform time synchronization using the timestamps. Figure 2 Concept Diagram of the PTP Synchronization Control SMPTE EG 2059-10:2016 Page 5 of 16 pages In order to synchron

20、ize time at the PTP slave to the PTP master, both the frequency and phase of the time information have to be controlled. The principle is shown in Figure 3. Figure 3 Time Synchronization: Frequency and Phase The PTP slave has to extract both frequency and phase information from PTP messages. Figure

21、4 shows one example of how frequency information may be extracted and calculated. The Sync message is a PTP message which the PTP master sends to PTP slaves periodically. Assuming that the network delay is always the same between the PTP master and a PTP slave, the difference between Sync message in

22、tervals is measured by the PTP master (m) and the Sync message intervals measured by PTP slave (s) is the frequency difference. Therefore frequency can be synchronized by a control system in the PTP slave to remove the difference (m-s). In order to achieve precise synchronization it is essential to

23、minimize the impact of variations in the network delay. Also in order to speed up the synchronization time, it is effective to reduce the Sync message intervals. Figure 4 Example of Frequency Control SMPTE EG 2059-10:2016 Page 6 of 16 pages Figure 5 shows an example of phase control. The Delay_Req m

24、essage is a PTP message that a PTP slave sends periodically to the PTP master. Assuming that the network delay between the PTP master and the PTP slave is always same, and is the same in each direction, the time difference between when the Sync message is sent and when the Sync message is received (

25、t2-t1) = Network Delay + Time difference (between PTP master clock and PTP slave clock) and time difference between when Delay_Req message is sent and when Delay_Req message is received (t4-t3) = Network Delay - Time difference. Therefore time difference (phase) can be calculated as follows: Time Di

26、fference = (21)(43)2 Again as it is assumed the network delay is always same, in order to achieve precise synchronization it is essential to minimize the impact of variations in network delay. Also, in order to speed up the synchronization time, it is effective to reduce the Delay-Req message interv

27、als. Figure 5 Example of Time Phase Control 4.2 Device Types PTP is a distributed protocol that specifies how the real-time clocks in the system synchronize with each other. These clocks are organized into a master-slave synchronization hierarchy. PTP executes within a logical scope called a domain.

28、 All PTP messages are always associated with a particular domain. IEEE Std 1588 defines three kinds of clock: an ordinary clock which is an end device with one port on the network described above; a transparent clock, and a boundary clock both of which have more than one port on the network. Note: M

29、ore precisely, there are two types of transparent clocks: an end-to-end transparent clock and a peer-to-peer transparent clock. In this document, only the end-to-end transparent clock is described. SMPTE EG 2059-10:2016 Page 7 of 16 pages 1. Ordinary Clock The ordinary clock can be a grandmaster clo

30、ck (PTP GM) in a system, or it can be a slave clock (PTP slave). The PTP GM is at the top of the master-slave hierarchy and determines the reference time for the entire system within a domain. 2. Transparent Clock As described above, reducing the effect of network delay variation is essential in ord

31、er to perform precision time synchronization. One option to reduce the impact is to use a transparent clock. The transparent clock, illustrated in Figure 6, is a special switch for PTP and embeds the time taken to pass through the switch (residence time) into PTP event messages. PTP slaves can compe

32、nsate for varying network delay by subtracting residence time and thereby reduce its impact. Figure 6 Concept of Transparent Clock 3. Boundary Clock The maximum number of PTP slaves that a PTP GM can control is limited by several conditions, including the rate at which it can process and respond to

33、Delay_Req messages. A boundary clock is designed to increase the maximum number of PTP slaves connected to the same PTP GM by establishing hierarchical structure. Figure 7 illustrates a boundary clock. This includes both a PTP master and a PTP slave, and conveys the time synchronized by the PTP slav

34、e to the downstream PTP slaves as a PTP master. SMPTE EG 2059-10:2016 Page 8 of 16 pages Figure 7 Concept of Boundary Clock 4.3 Redundancy IEEE Std 1588 defines the Best Master Clock Algorithm (BMCA) as the default PTP redundancy mechanism. BMCA The BMCA is used to determine which of the available c

35、locks is the best clock and which should therefore be used. It is also used to determine whether a newly discovered clock is better than the local clock itself. When the current best PTP master fails, BMCA is initiated to find the best clock to be a new PTP master. When using a BMCA, SMPTE ST 2059-2

36、 specifies use of the IEEE 1588 default BMCA mechanism. IEEE Std 1588 also defines several PTP redundancy mechanisms as options. The following alternate master option is permitted in SMPTE ST 2059-2 and could be an appropriate redundancy mechanism for a video system: Alternate Master This option all

37、ows alternate PTP masters that are not currently the best PTP master to exchange PTP timing information with PTP slaves. This will allow a PTP slave switchover to an alternate PTP master with a small phase excursion when the current best PTP master fails. 5 Time Source The new synchronization system

38、 can be applied to various use cases according to system requirements. The essential point is that in each case PTP slaves create a synchronization signal using exactly the same mechanism. SMPTE EG 2059-10:2016 Page 9 of 16 pages Aside from use cases related to the network configuration such as incl

39、usion of transparent clocks or boundary clocks, the main distinguishing factor between use cases is the type of time source used by the PTP GM. There are two types of timescale supported by IEEE Std 1588: PTP and ARB (arbitrary). With the PTP timescale, the Epoch is the SMPTE Epoch. In the case of t

40、imescale ARB, the Epoch is set by an administrative procedure, may be reset during normal operation, and does not necessarily correspond to the SMPTE Epoch. The type of time source used by the PTP GM is indicated by the attribute timeSource. In the case of the SMPTE ST 2059-2 profile, two additional

41、 values for timeSource have been added for cases when the frequency reference is derived from a legacy synchronization signal. The following four types of time source are assumed, and the features and typical use cases of each are described. 1. Primary reference time The PTP GM is synchronized to a

42、primary reference time, for example by means of GPS (Global Positioning System) reception. The timescale is PTP. 2. Non-primary reference time The PTP GM is synchronized to time source that is not referenced to a primary reference time. The timescale is PTP. 3. A/V synchronization signal A/V synchro

43、nization signal such as black burst. Timescale is ARB. 4. A/V synchronization signal + non-primary (or primary) reference time Combination of A/V synchronization signal such as black burst and non-primary (or primary) reference time. Timescale is ARB or PTP. 5.1 Primary Reference Time Figure 8 illus

44、trates an example of a use case where GPS is used as a time source. By sharing a primary reference time, synchronization can be achieved between PTP slaves that are not necessarily connected to the same PTP GM. Synchronization can even be achieved for devices not connected to the PTP GM if the prima

45、ry reference time can be obtained independently. Also, as illustrated in Figure 8 as Facility 1, a legacy synchronization signal based facility can also co-exist if the master generator is locked to the primary reference time and as a result the output legacy synchronization signals are Epoch aligne

46、d. Therefore a distributed synchronization environment can be easily established. This could be the optimum solution for new green field facilities. SMPTE EG 2059-10:2016 Page 10 of 16 pages Figure 8 Use Case where Time Source is a Global Reference Time 5.2 Non-Primary Reference Time Figure 9 illust

47、rates an example of use case where a non-primary reference time such as a wall clock is used. As with current conventional operation using a master sync generator, synchronization can be achieved between PTP slaves connected to the same PTP GM. The precision of the shared time depends on the precisi

48、on of the GM time reference. This could be the solution for a closed island system. Figure 9 Use Case where time source is non-primary reference time SMPTE EG 2059-10:2016 Page 11 of 16 pages 5.3 A/V Synchronization Signal Figure 10 illustrates an example of use case where a free-running legacy A/V

49、synchronization signal such as color black alone is used as a source of PTP time, without any reference to an actual time source. An example of such an application is where it is wished to replace a small part of an established system based on a legacy synchronization signal such as black burst. Figure 10 Use Case where time source is A/V synchronization signal Figure 11 shows the concept. In this case, the PTP GM extracts frequency and phase information from the A/V synchronization signal and this