ITU-R BT 808-1992 Broadcasting of Time and Date Information in Coded Form《编码格式中时间和数据信息的广播》.pdf

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1、Rec. 808 1RECOMMENDATION 808*THE BROADCASTING OF TIME AND DATE INFORMATION IN CODED FORM(Question 29/11)(1992)Rec. 808The CCIR,consideringa) that there are cases, notably in HF and satellite broadcasting, where a transmission is received over a largegeographical area containing many time zones;b) th

2、at there should occur further time differences during the year due to the practice of implementing “daylight-saving” time in some countries;c) that there would be advantages in adopting a standard time reference independent of time zones and daylight-saving discontinuities;d) that the transmission o

3、f the local time offset at the source from a standardized time reference might bedesirable in some cases;e) that there is an increasing interest to convey time and date information in a coded form particularly suitable forcontrolling the operation of equipment;f) that standardized encoding methods f

4、or broadcasting time and date would also permit operating economy andsimplify the calculation of relative timings;g) that common worldwide time and date conventions have already been defined in Recommendations 460 and457,recommends1. that all coded broadcast time and date signals should be expressed

5、 only in Coordinated Universal Time (UTC)and Modified Julian Date (MJD) in accordance with Recommendations 457 and 460;2. that a coded local time offset, expressed in multiples of half an hour with range 12 h to +15 h should beappended when required to indicate the difference between UTC and the tim

6、e currently applicable locally within aparticular time zone.Note 1 Additional information on the conventions for the expression of standard time and date, and a method forconversion between date conventions are given in Annex 1.ANNEX 1A note on standard time and date conventions1. IntroductionThere

7、are already international standards concerning the distribution of time and date information. This Annexindicates how these standards relate to each other, and to the needs of broadcasting._*This Recommendation should be brought to the attention of Study Group 7.2 Rec. 808Time and date information i

8、s used to label the actual or nominal point of origin of material (a document, atelevision or radio programme) or the actual or anticipated point of receipt. The difference represents the propagationdelay. There is also a requirement for indicating local clock-time and date in their own right, or fo

9、r use together withsuch labels for assisting decisions or controlling processes associated with broadcasting.The broadcasting and telecommunications environment provides the possibility of sending signals worldwidewithin 1 s of time. It is therefore necessary to accommodate the variations in local t

10、ime (and date) in any method ofcoding intended to be consistent worldwide. There are also known discontinuities in local time (the duplicated hour atthe end of “summer time”, and the “leap second”) to be taken into account.2. Standard timeThe standard unit of time is the second, obtained by defining

11、 the frequency of the caesium atomic transition as9 192 631 770 Hz. For the purpose of creating a regular time scale these seconds are counted to give days, hours andminutes since 1 January 1958. This is known as International Atomic Time (TAI). This time scale, based on a physicalproperty, drifts o

12、ut of step with a time scale, such as Universal Time (UT) or Greenwich mean time (GMT), obtainedfrom astronomical observation. Since the origin of TAI was set in agreement with UT at the beginning of 1958, TAI hasadvanced by about 21 s with respect to UT. In order to provide a time scale with second

13、s coincident to those of TAI, butwithin a close tolerance ( 0.8 s) of UT, a version of TAI offset by a whole number of seconds is maintained by theBureau international des poids et mesures (BIPM). This is known as Coordinated Universal Time (UTC). The toleranceis maintained by occasionally adding (o

14、r, in principle, deleting) a single second to make a 61 s (or 59 s) minute. Thepreferred occasions are at the end or middle of the year, with at least eight weeks notice. For example, one of these“leap seconds” occurred at 0000 h UTC on 1 July 1982 when the UTC seconds marker sequence was:30 June 19

15、82 23 h 59 min 59 s23 h 59 min 60 s1 July 1982 00 h 00 min 00 sAll of the standard time signals used by broadcasters worldwide are derived from the UTC time scale, and thetimes are often, wrongly, referred to in terms of Greenwich mean time (GMT) and an offset. For reasons given above,the UTC time s

16、ignal known in the UK as “the Greenwich time signal”, will sometimes differ from true GMT by morethan half a second. This confusion of name is of little practical consequence in everyday life, but it is significant toastronomers, navigators and lawyers.Recommendation 460 recommends “that all . . . t

17、ime signal emissions conform as closely as possible toCoordinated Universal Time (UTC) . . . from 1 January 1975”.2.1 Time offsetsIn practice all countries refer their national time or times to UTC with an offset. There are 38 different offsetscurrently in use. Except for Nepal (+5 h 40 min) all the

18、 offsets are multiples of half an hour and range from 11 h(Samoa) to +14 h (Anadyr, Russian Federation, in summer time). Many countries advance their time by one hour duringlocal summer (dependent on hemisphere); exceptionally Cook Islands advance by half an hour. There are various datesand times fo

19、r summer time changes. Within some countries (Australia, Canada) there are some time zones differing byhalf an hour. There are some states (Queensland, Australia; Arizona and Indiana, USA) which, unlike their neighbours,do not adopt summer time.It would probably be sufficient to provide a method of

20、signalling a local time offset with a six-bit code givinghalf an hour steps in a range 12 to +15 h. In some applications, the local offset of the programme source or of thetransmitter would be signalled; in other cases, the local offset applicable to the receiver site would be required.Rec. 808 33.

21、DateThe change of date varies, of course, with local time, so a common broadcast standard for date would bereferred to UTC and it would be corrected, if necessary, by the operation of the local offset.There are several calendars in use world-wide but a simple common reference, the Modified Julian Da

22、te(MJD), has been defined for this purpose. This is a five-digit decimal number increasing by one at midnight UTC. Theorigin of the count is 17 November 1858, because at midday on that date the Julian Day (used by astronomers to givecontinuity from 4713 BC) reached the figure 2 400 000. A more conve

23、nient reference is 31 January, 1982 when theMJD was 45 000. It is a simple matter to calculate time intervals, even over many days, by use of MJD and UTC(provided the occasional leap second is known or can be neglected).Recommendation 457 recommends that for modern timekeeping and dating requirement

24、s, a decimal day countshould be used wherever necessary; the calendar day should be counted from 0000 h TAI, UTC or UT, and be specifiedby a number with five significant figures.Although not defined in any standards, it is convenient to use the idea of a local day number which isadvanced or delayed

25、by the local time offset and which changes at local midnight.3.1 Week numberFor many commercial purposes, and for planning broadcast programme schedules, it is convenient to work interms of day of the week, week number and year.There is an international standard (ISO 2015) for the numbering of weeks

26、. This can be summarized by sayingthat weeks begin on Mondays, and that week 1 of a year contains the first Thursday of January. The week number can beassociated with a day of the week (conventionally, Monday = 1 to Sunday = 7) and a year to specify a particular date.Note that occasional years (abou

27、t 5 in 28) have 53 weeks, and that the “week-year” of a date in the inclusive range 29December to 3 January may differ from the “calendar” year. The relation between week number and MJD is given in 4.Although the ISO week numbering system is in general use worldwide, other week number systems remain

28、within certain organizations. In some cases, the week number of Monday accords with ISO but the week is taken to runfrom Saturday to Friday for example. In other cases, even the years containing 53 weeks are different.3.2 Calendar dateThe various calendar systems in use are well known and, in most c

29、ases, well defined. In these cases, it ispossible to generate a formula for conversion between calendar systems, the convenient intermediate standard being theMJD. The information for conversion between MJD and the Gregorian calendar is given in 4.Certain calendar systems depend on a suitably qualif

30、ied person witnessing an event (e.g. the first sighting of acrescent moon, or the sighting of a particular type of fish off a Pacific Island) and these can only be related to the MJDafter the event.4. Conversion between time and date conventionsThe types of conversion which may be required are summa

31、rized in the following diagram.The conversion between MJD + UTC and the local day number + local time is simply a matter of adding orsubtracting the local offset. This process may of course involve a “carry” or “borrow” from the UTC affecting the MJD.The other five conversion routes shown in the dia

32、gram are as follows:-Note 1 These formulae are applicable from 1 March 1900 to 28 February 2100 inclusive.4 Rec. 808D01-scFIGURE D01 = 6,5 cmSymbols used: MJD : Modified Julian DateY : year from 1900 (e.g. for 2003, Y = 103)M : month from January = 1 to December = 12D : day of month from 1 to 31WY :

33、 “week number” year from 1900WN : week number according to ISO 2015WD : day of the week from Monday = 1 to Sunday = 7K, L, M, W, Y : intermediate variablesINT : integer part, ignoring remainderMOD 7 : remainder (0-6) after dividing integer by 7* : multiplicationA: To find Y, M, D from MJD:Y = INT(MJ

34、D 15 078.2)/365.25)M = INT(MJD 14 956.1 INT(Y*365.25)/30.6001)D = MJD 14 956 INT(Y*365.25) INT(M*30.6001)If M = 14 or M = 15 then K = 1 else K = 0Y = Y + KM = M 1 K*12B: To find MJD from Y, M, D:If M = 1 or M = 2 then L = 1 else L = 0MJD = 14 956 + D + INT(Y L)*365.25) + INT(M + 1 + L*12)*30.6001)C:

35、 To find WD from MJD:WD = (MJD + 2) MOD 7) + 1D: To find MJD from WY, WN, WD:MJD = 15 012 + WD + 7*(WN + INT(WY*1 461/28) + 0.41)Rec. 808 5E: To find WY, WN from MJD:W = INT (MJD/7) 2 144.64)WY = INT (W*28/1 461) 0.0079)WN = W INT(WY*1 461/28) + 0.41)Example:MJD = 45218 W = 4 315Y = (19)82 WY = (19)82M = 9 (September) WN = 36D = 6 WD = 1 (Monday)_