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douglas.denny@btopenworld.com wrote:
> Re:  NIST (or other) timesignals over the internet:
>
> I would NOT expect any website time standard to be more accurate than a 
couple of seconds or even worse - as received.  If 0.2 seconds is quoted, I 
would  expect this perhaps be an average time delay expected for most of the 
time at most places; but digital transmission paths vary for the internet. I 
would not trust it for any kind of accuracy and certainly not being less than 
a second.   If you use the 'ping' facility on an internet address you get all 
sorts of return times.
>
> It might well be sent out as accurately as you like, but the digital 
transmission paths from node to node can be varied and long and even delayed 
whilst digital packets are waiting in a queue in the system.
>
> That is why when I used to listen to the 'Proms' (for Americans: a well 
known music concert series from the Albert Hall in London sponsored by the 
BBC) on my VHF radio, and watched the visual programme on digital TV there 
was a difference of a couple of seconds between the VHF radio (instantaneous) 
and the TV picture (two or more) seconds slow with the digital processing 
going on.  I switched the TV off preferring the better quality of the radio 
sound.  Impossible to watch the TV and hear with the radio as the violins 
were being bowed out of synch horribly. It's actually distressing to the 
brain to try to watch.
>
> If you want accuracy for tie pips get the short wave receiver out.
Doug:

www.time.gov (NIST's Internet time service) measures the round-trip 
delay to your web browser and adjusts the time display accordingly.   
Whilst the Internet was designed to accommodate greatly varying delays 
in transmitting data (often via different paths with different delays), 
in practicality these days data is usually transmitted between server 
and browser across a session-invariant path.   In fact, www.time.gov 
also displays the expected error in its display based on the path 
statistics it measures.   I have a  WWVL-synchronized clock and can not 
tell the difference between it and the display from time.gov.

(Side note:  You're right -- if you "ping" a distant Internet server (a 
way of measuring the "path length" to an Internet server) you will 
indeed see widely varying numbers -- among different servers.   But if 
you ping the same server repeatedly, the round-trip time is usually 
consistent to a few milliseconds)

It would be interesting to see what the experience of some one in Europe 
would be with this time source (www.time.gov) -- is it as stable as it 
appears in the USA (excluding our offshore cousins of Hawaii and Alaska) 
or is there more variability?

(Another side note -- I don't know the exact digital TV transmission 
standard used in Europe, but the US standard, DTSC, immunizes itself 
against burst noise by transmitting picture frames out of sequence which 
are then reassembled by your TV.  As a consequence there is a delay of a 
second or two between when the TV camera scans a particular frame and 
when it appears on your TV.  So your experience of detecting a 
difference between the live (analog) radio broadcast and the TV signal 
is to be expected.  There's a very nice explanation of DTSC on Wikipedia.)

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