On 2019-01-04 9:14 AM, Jonathan E. Hardis wrote:
On Jan 1, 2019, at 1:03 PM, Brooks Harris <[email protected]
<mailto:[email protected]>> wrote:
Back in the days of analog TV (which is still used in some parts of
the world) the broadcast TV signal was one of the most stable time
sources around. This was necessary because the display of the signal
on a CRT TV set depended critically on the timing of the components
of the signal, the horizontal and vertical scan lines of each frame
(actually two interlaced 'fields').
There were experiments at NIST in the early days of TV to use the TV
signal as a time dissemination source. It worked well, as coordinated
with the NIST radio time signals. But it didn't turn out to be a
practical solution.
More specifically, the idea was to put a character code (like ASCII)
in the VIR (vertical interval reference) portion of the signal that
would be the correct time. There turned out to be little interest in
the technology for this purpose, but an alternate application made it
big---closed captioning. For this, NIST won an Emmy Award.
https://www.nist.gov/node/774286 (Link inactive during lapse in
appropriations.)
A "timecode" was very much adopted by the video industry and remains the
glue that holds professional video operations together. The core
standard from the Society of Motion Picture and Television Engineers
(SMPTE) is SMPTE ST 12-1:2014 Time and Control Code. The earliest
versions of the standard were based on IRIG and appeared in the 1970s.
This defines two flavors, "longitudinal timecode" (LTC), used as a
timelink and synchronization protocol over analog wires, and "vertical
interval timecode" (VITC), placing the timecode in the video signal
itself. Many other timing related standards in the industry are based on
it. I wrote an article about it:
Conversion between SMPTE hh:mm:ss:ff Time Code and Frames
http://edlmax.com/SMPTETimeCodeConversion.htm.
I've been on those SMPTE timecode committees for more than 20 years.
With each revision attempts are made to add new features, like high
frame rates and date and time. But each try goes down to failure because
it becomes impossible to safely modify a standard that has become so
widely adopted and deployed. It is from that experience I suspect no
changes will be made to the specification of UTC and the Leap Second,
one of the most widely deployed standards of all time.
-Brooks
_______________________________________________
LEAPSECS mailing list
[email protected]
https://pairlist6.pair.net/mailman/listinfo/leapsecs
