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A
APPLICATION NOTE 3: TRANSMITTING TIME OF DAY USING XDS
PACKETS
A.1
OVERVIEW
This application note describes how to properly encode Time of Day information using EIA-608-B
Extended Data Services (XDS) packets. In the United States, the Public Broadcasting System (PBS)
Television network and its affiliates have been providing this information to the viewing public since the
mid 1990s.
The application note is written in a tutorial format in order to give Engineering and
Operations personnel an understanding of time, time zones and daylight saving time concepts, as well
as how this information is encoded into the XDS packets. The application note also provides step by
step procedures for setting up the Evertz 8084 and 8084AD Caption/XDS encoders (referred to
generically as 8084AD throughout this document) and the Evertz MetaCast 2 software to accomplish
this.
A.2
WHAT IS THE CORRECT TIME ANYWAY?
A.2.1. UTC AND TIMEZONES
Prior to the late nineteenth century, time keeping was a purely local phenomenon. However, once
railroads began to operate and move people rapidly across great distances, time became much more
critical. In the early years of the railroads, the schedules were very confusing because each stop was
based on a different local time. The standardization of time was essential to efficient operation of
railroads.
In 1878, Canadian Sir Sanford Fleming proposed the system of worldwide time zones that we use
today.
He recommended that the world be divided into twenty-four time zones, each spaced 15
degrees of longitude apart. Since the earth rotates once every 24 hours and there are 360 degrees of
longitude, each hour the earth rotates one-twenty-fourth of a circle or 15° of longitude. Sir Fleming's
time zones were heralded as a brilliant solution to a chaotic problem worldwide.
In 1884 an International Prime Meridian Conference was held in Washington D.C. to standardize time
and select the Prime Meridian. The conference selected the longitude of Greenwich, England as zero
degrees longitude and established the 24 time zones based on the Prime Meridian. Though most U.S.
states began to adhere to the Pacific, Mountain, Central, and Eastern time zones by 1895, the U.S.
Congress made the use of these time zones mandatory by the Standard Time Act of 1918.
There are six time zones in the fifty United States as shown in Table B-1. The local time of day in a
specific time zone is calculated with respect to Universal Time, Coordinated (UTC) sometimes also
referred to as Greenwich Mean Time or (GMT). UTC is the world's time standard and is essentially the
standard time at the Prime Meridian that runs through Greenwich, England. All other time zones have
an offset with respect to UTC, which represents the number of hours (and sometimes minutes) that
they are ahead or behind the UTC time.
APPLICATION NOTES
8083XDS-AD Analog and SD-SDI XDS Encoder
Revision 1.0.2
AN-1
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