On 2014-01-17 04:06 AM, Zefram wrote:
- Leap Seconds don't (theoretically) only "leap" - they could also "drop"
The word "leap" doesn't carry any connotation about direction.
In our world, that of television and media, is certainly does!
I think this is a really important point because it illustrates how
misunderstandings can propagate without good definitions of terms.
In television there is a really important standard, often called for
short hand, "timecode", sometimes "SMPTE" (after the standards body that
created it), sometime "EBU", because SMPTE cooperated with the "European
Broadcast Union" (another standards body) when they created it. Its
current incarnation is SMPTE 12M-1-2008.
Wikipedia has a reasonable explanation of it -
SMPTE timecode
http://en.wikipedia.org/wiki/SMPTE_timecode
"SMPTE timecode" is ubiquitous - it underlies the timing of everything
you watch and most of what you listen to - broadcast television, cable,
YouTube, Windows Media, Netflix, Flash (Adobe) and on and on. It also
heavily used in law enforcement, military, scientific and many
industrial operations. Its been around since the late 1960s.
"SMPTE timecode" handles labeling video frames with hh:mm:ss:frames. In
the US and Japan, the tv standard NTSC is used. The "nominal frame rate"
of NTSC is "30 frames per second", but the *true* rate is 30000/1001
frames per second. This is approximately 29.97002997002997... Its a
nasty number.
Its not "real-time", so "SMPTE timecode" has a 'counting method' called
"drop frame". This "drops" frame numbers from the the incrementing frame
count such that the hh:mm:ss portion of the labels represent
approximately real-time hh;mm:ss. We deal with this all the time.
To us, to "drop" a count and to "leap" a count (which we have to do
sometimes) have inverse meanings.
To me, a "positive Leap Second" is a "leap count", but a "negative Leap
Second" is a "drop count", so "Leap Seconds" always seemed completely
misnamed to begin with. Granted, it may never do a "negative Leap
Second", so I figured it was just a simplification.
So there you see how a simple concept is completely misinterpreted by
someone from another discipline. We've really got to have a proper
"terms and definitions" document from which we're working!
-Brooks
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