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    Re: Choice of timepiece
    From: Lu Abel
    Date: 2009 Nov 13, 15:57 -0800

    douglas.denny{at}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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