On 2016-12-30 12:56 PM, Stephen Scott wrote:
NOT "unintentional"
-S
On 2016-12-30 11:37, Brooks Harris wrote:
This is the interpretation used in the new SMPTE (Society of Motion
Pictures and Television Engineers) ST 2059-2 and ST 2059-1 standards
for synchronization over network systems. It is based on IEEE
1588/PTP. It states:
---------------
6 SMPTE Epoch and Signal Alignment
The SMPTE Epoch shall be 1970-01-01T00:00:00TAI, which is the same as
the PTP Epoch specified in IEEE
Standard 1588-2008.
Note: The SMPTE Epoch is 63072010 seconds before 1972-01-01T00:00:00Z
(UTC).
---------------
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. The intentional point is implementers should treat
"The SMPTE Epoch" as "63072010 seconds before 1972-01-01T00:00:00Z
(UTC)." Unfortunately its a non-normative clause, and that's the best we
could do at the time. My point in general is that its better to discard,
or ignore, the historical "rubber band era" values.
-Brooks
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
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