[ntp:questions] Re: Clock accuracy & auto setting : digital television does a crap job of providing time services...

G-squared stratus46 at yahoo.com
Sun May 7 22:37:59 UTC 2006

 Alan Larson wrote:
 > Hi Dave,
 >   Since the application for ATSC broadcasting seems to be mostly
 > the clocks in the receiver's equipment, the 232 picoseconds of NTP
seems to
 > rather far beyond what is needed.  After all, 232 picoseconds is
just under
 > 7 centimeters at light speed.  Thus, broadcasting the time of day
(to set
 > the clock in your TV recorder) to that resolution seems a bit
 >   0.01 second seems adequate to me to not miss the program.
 >   The problem as I see it is that the sources are way bad (as you
 >   Clearly, NTP would be a useful and available way of keeping a
 > transmitting time on a broadcast signal in good sync.
 >   With ATSC, there seems to be some difficulty with daylight saving
time --
 > when the time advanced this spring, I noticed that the electronic
 > guides apparently did not advance.  Attempts to set the clock on my
 > receiver from over the air signals got various times which did not
 > local time.  I eventually gave up and went back to the
recommendation in
 > the addendum to the manual to manually set it to local non-DST time,
 > the timezone set.  That gets the program guide to match, but the
clock of
 > course reads an hour slow.
 >   It is unclear whether the Asians who programmed the thing didn't
 > understand time, or whether the standard is that completely flawed
that it
 > doesn't account for properly dealing with transferred times in UTC
 > converting for display.  (Though, transmitting daylight saving time
info in
 > the WWVB style seems a good idea, given the government's willingness
 > fiddle with it and almost certainly break lots of things with built
 > advance/fallback calculations.)
 >   Precision is not a solution for lack of accuracy.
 >   Neither precision nor accuracy will make up for implementation
 > 	Alan
 > p.s. Doesn't CBS news still do the ping at the top of the hour?  I
 > they are still coming out from KCBS here.  (I don't know what they
do about
 > satellite delay of the feed (signal travel time, and any digital
 > times).  I have no idea how they will deal with the variable time
delays of
 > IBOC digital processing.

Be aware that, at least in LA, some stations put the 'ping' ahead of an
8-10 second delay. Realistically, do you even need to be more accurate
than a frame of video?

I think 0.01 seconds is 10-100 times better than it needs to be. We're
not trying to pinpoint the position of the news helicopter, just start
your DVR without missing the head of the show. The phone company time
is sufficient for that.

I suspect the times are 'way bad' because it isn't automatically loaded
yet and the stations aren't getting any complaints and those guys have
plenty to do withouit setting a clock almost nobody checks.

Personally, I set the times and record times manually with no problems.


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