Most recent posts have tried to disect the past. This is about the use
of time now and in the future.
_*UTC and Leap Seconds*_
The basis of my understanding is that UTC is a timescale that:
- progresses at a rate of the second (SI) and has done so since
1972-01-01.
- is expressed as a count in the form of date, hours, minutes and
seconds;
- is continuous other than the discontinuities resulting from leap
second corrections;
- is currently the basis for most local timescales;
- is related to the International Atomic Timescale (TAI) by an
integer leap second offset;
- until further notice, is in force and is not expected to be
revised, at least not before 2015.
My primary interest is not how we got here but how we go into the future.
_*The growing importance of precise time*_
My background, experience and field of interest are primarily in
timekeeping and signal generation for television production and
broadcast. In a broad sense television also includes radio and
distribution of other forms of media.
Precise time is important in television both for the synchronization of
various program elements (for example audio/video lip-sync) and the
quality-of-experience (QoE) to the viewer of program segments which must
flow smoothly from one segment to the next.
Historically television developed as a system that was isochronous
glass-to-glass. Typically, within a television facility time, and
synchronization reference signals are generated at a central point and
distributed within the plant in a star topology. These reference signals
are generated from stable local oscillators and because discontinuities
are disruptive to synchronous operations they are corrected to an
external reference, at a time-of-day that will be the least disruptive.
With the replacement of some synchronous signal technologies by packet
switched signals there is a desire to replace or supplement traditional
reference signal distribution with a system that is based on a global
time reference that can be used to generate timing reference signals at
the point of use. The diminished central control over the management of
discontinuities in the time references places an increasing importance
on the deterministic nature of these time references.
How much resolution and precision is required? The answer is not
universal and depends on the application but for example:
- A leap second shift represents about 30 video frames in North
America or 25 frames in Europe. Most television operations are based on
single frame accuracy.
- Audio can require much higher accuracy when considering spatial
rendition of multi-channel audio.
- Video signals are based on very precise frequencies. Recreating
frequencies requires very accurate and deterministic timestamps.
A peculiarity of the North American television system is the actual
frame rate is 30/1.001 Hz. This non-integer rate creates an additional
complication to generating reference signals and an additional
dependence on the time reference being deterministic. This may require
microsecond accuracy and 24/7 availability.
_*Instantiation of leap seconds*_
In a prior inquiry about leap seconds I had asked about the application
of leap seconds to various local tines in the different time zones. I
have searched for documentation on how the leap seconds are instantiated
in local time zones around the world. There is a need for a standard.
This is a reiteration of the request for information.
This question was raised because TF.460-6 specifies the leap second
adjustment to UTC and the emission of UTC. There does not seem to be any
guidance for the application of leap seconds to local timescales for the
timezones UTC+14 to UTC-12. There is the perception by some that the
leap second should be instantiated globally at the same time as it is
signaled by UTC. I looked in TF.460-6 but could not locate a definitive
statement to this effect.
If this is the case then the leap second changes would be instantiated
just before 9:00 (9 AM) local time in Tokyo (UTC+9) and just before
19:00 (7 PM) local time in New York (UTC-5). These are just examples but
in either case it would probably be disruptive to critical time keeping
systems to have a time shift instantiated at these times. The
broadcaster typically makes these changes at a time early in the
morning. I cannot believe that any other industry that has a time
critical operation does not do something similar.
It's not just for television. The evolving application of packet
switched technologies (for example Ethernet and the Internet) in
television, financial, scientific, telecommunications, power generation
and other industries is creating a growing demand for the availability
of accurate and precise, but most importantly deterministic time
references. Several industries have developed IEEE 1588 PTP Profiles
that address their specific needs.
Notwithstanding the possibility of stopping leap seconds it is more
important to:
- have a deterministic procedure for instantiating leap seconds in
each time zone and;
- similarly for standard time and daylight saving time shifts which
understandably are under local governance is lacking a better method for
dissemination.
I believe there is no particular requirement to eliminate the leap
second if their insertions are well and deterministically defined and
communicated. We can't fix what we don't know. Similarly the need to
define the standard time / daylight saving time shifts, however that is
beyond the scope of this group.
_*Summary*_
Time implementations that have evolved over time are no longer good
enough. New and emerging applications are creating more demanding
specifications for time delivery systems.:
- Availability via wired and wireless delivery;
- Microsecond accuracy or better;
- Consistent results via multiple delivery mechanisms;
- Stability to permit the generation of stable frequency references.
It's not time that needs to be fixed, it's the standards or the lack
thereof.
What can this group do to:
- Document local practices for instantiating leap second corrections
to UTC.
- Promote a standard for instantiating leap second corrections in
local time zones.
- Promote standards for the communication of all UTC time corrections
including leap seconds and standard / daylight saving time shifts.
The difficult can be done today, miracles tomorrow, and the impossible
will take a bit longer and many committee meetings.
REGARDS*
Stephen Scott*
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