On 2016-12-30 12:56 PM, Stephen Scott wrote:
NOT "unintentional"
-S
On 2016-12-30 11:37, Brooks Harris wrote:
In SMPTE standards parlance the first sentence is normative, but the
"Note" is informative. The intention of the note is to inform
implementers that the intention for SMPTE purposes is to interpret
the "PTP Epoch" as integral seconds before 1972-01-01T00:00:00Z (UTC).
Unfortunately, and probably unintentionally, the text leaves some
ambiguity because the IEEE 1588/PTP states - "... which is 31
December 1969 23:59:51.999918 UTC" while the SMPTE "note" says, and
the intention is it be, 1969-12-31T23:59:50 (UTC).
Having been a prime instigator of that note, it was very deliberate
and not unintentional. It says nothing about UTC prior to 1972. It
makes clear the relationship between TAI and UTC at
1972-01-01T00:00:00 (UTC) so that the reader is not misled by the
ambiguity prior to that date that might be caused by statements in
IEEE 1588-2008.
-S
Right, I was there, and yes, the point it should be
Having been involved in these discussions I know the intention is the
latter. The words in a standard matter.
One thing for sure - if we can't agree what a particular timescale's
origin, or "epoch", means and its exact relationship to 1972-01-01
00:00:10 (TAI) = 1972-01-01T00:00:00 (UTC) and we don't implement
them consistently, there won't be interoperability no matter how
exacting all the other details of the counting schemes may be.
-Brooks
The choice of TAI - 10 would put the origin of the time scale at the
intersection of the grey vertical line and the green horizontal line
seen in the second plot on
http://www.ucolick.org/~sla/leapsecs/amsci.html
And for any system using that time scale that puts the origin at
1.999918 SI seconds after the (BIH average of the) radio broadcast
time signals said it was 1970-01-01T00:00:00.
But all of this is pedantic and moot for any currently operational
system because it was not operational then. Only a few things like
astrometry of solar system bodies and spindown of a handful of pulsars
have data and models with enough precision to discern this.
Everything and everyone else can reasonably assert that they do not
have to care and that every day has always had 86400 seconds of
duration equal to what they are currently ticking.
--
Steve Allen<[email protected]> WGS-84 (GPS)
UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat
+36.99855
1156 High Street Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064http://www.ucolick.org/~sla/ Hgt +250 m
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