System Clock Apparently Gaining One Second Every 30 Minutes [Telecom]

System Clock Apparently Gaining One Second Every 30 Minutes [Telecom]

Post by Robert Bono » Mon, 11 Jul 2011 09:01:25






>      [ ... ]

>> The NIST facility is _amazingly_ accurate.  They have 'smarts' on
>> the server end that measure the round-trip latency to the user, and
>> advance the 'tick' so that it _arrives_ at the user's location at
>> the proper instant.  With quality software, one can set the system
>> clock with sub- millisecond accuracy.  If memory serves, the
>> 'jitter' is around 15 _micro- second_.

>     This must be the one that you reach at www.time.gov.  When you
>click on your time zone it tells you to wait, which appears to be
>while it is measuring the round trip latency.  Then in a few seconds
>the digital time (hours, minutes, seconds) appears in the box.

>     If it doesn't want to come up at www.time.gov there is also a
>button to click at www.nist.gov, which brings up the same page.

"Accuracy", over the Internet, is orders of magnitude worse than the direct
dial-up connection.  The Internet NTP protocol is good for getting within
about 100 milliseconds.

Quote:>     The is also a timing pulse in the interstitial information on PBS
>stations, as the instructions on video recorders often spell out in
>detail.

"Most" PBS stations, when broadcasting 'network' programming, to be precise.
Since this is a one-way protocol, derived from a 'synchronized' source at
the network head-end, one can easily have 600-800 (or more!) millisecond
'delay' at the receiving end.  "Good enough" for a VCR, but absolutely
dreadful if you need a high-precision/high-accuracy timestamp.
 
 
 

System Clock Apparently Gaining One Second Every 30 Minutes [Telecom]

Post by Harold Hallikaine » Tue, 12 Jul 2011 03:49:38


DST is complicated! I designed a system that logs events in UTC. For
user convenience, I wanted to display in various local times,
including the time where the unit is located and the time where the
user is located. Standard time is simple enough. I just add an offset.
DST is a lot more difficult. It applies in some locations and not
others. It changes based on local time, not UTC. The date and time of
the change varies by country. Countries change the date and time of
DST changes. I read that some countries may delay the change one year
to put an election in DST. I finally gave up and let the user select
whether to apply a DST offset to the displayed time or not.

A list of various DST change time and dates is located at
http://www.timeanddate.com/time/dst/2011.html .

Harold

 
 
 

System Clock Apparently Gaining One Second Every 30 Minutes [Telecom]

Post by Adam H. Kerma » Wed, 13 Jul 2011 00:14:56



>     The is also a timing pulse in the interstitial information on PBS
>stations, as the instructions on video recorders often spell out in
>detail.

The clock in my cable-ready VCR with NTSC tuner hasn't found the signal
since the analog broadcast went off air. They never bothered to find a
way to add it to the digital broadcast so that the signal was present
after digital-to-analog conversion by set-top boxes from the cable or
satellite company or over-the-air converter box.