RE: Accurate Time - A very important other issue..

From: Derek C Breit (breit_ideas@poyntsource.com)
Date: Mon Feb 14 2011 - 00:35:16 UTC

  • Next message: Bill Arnold: "Obs for Feb. 13"

    (Remember this thread is generally for discussion only. Do not let it stop
    you from observing..)
    
    
    Re : The clock on a cell phone
    
    I have just boxed up all my stuff, as a week of rain is expected here on the
    West Coast of CA.. Next time I have the stuff out, I will have to compare
    the phone to the KIWI.. 
    
    I can assure you already that there is no basis for the assumption that the
    cell phone will have anything resembling accurate timing.. 
    1) Absolute time is unimportant for most operations and is therefore not
    provided for in most applications, even when it could be.. 
    2) We tend to observe when it is 'cold' and LCD latency is also an issue in
    timing. Latency, and therefore accuracy, would be temperature sensitive.
    3) Even if it is 'accurate', matching it to a video field or frame is non
    trivial.. 
    4) Even if you can 'stamp' the time, from an accurate time base, onto a
    video frame, you still have to verify one way or the other that there are
    not any dropped frames / duplicated frames (or fields as the case may be).
    Etc..
    
    Easy to test roughly, generally impossible to test "accurately".. (Dismantle
    the phone, use an oscilloscope....). 
    I would be very surprised if anyone has done this.
    
    Another simple test is to go to your local Astronomy Society meeting, and
    gather everyone's cell phone together and watch the displays on all of them
    and see if they all change (apparently) at the same time.. Try it again
    after a couple hours outside when it is 40F or colder when the phones are
    cold..
    
    
    Of course yet another underlying issue with "accurate timing" (a subject you
    may have noticed that I enjoy) is this.. 
    "Do satellite observations need accurate absolute timing (and to what
    precision)?" No.. All the 'usual suspects' in this group have told me
    essentially that I could do a lot more if I was to get away from such high
    precision, but I am an 'old dog and new tricks' don't come easily. A perfect
    analogy is would you measure a 4x4 post with a micrometer?? Of course not..
    I do because I know the underlying time is accurate.. 
    
    
    My whole point is primarily a semantic one about "My timings are accurate to
    +/- (whatever).." My response is *ALWAYS* the same.. "HOW DO YOU KNOW??
    Accurate time to one person isn't necessarily accurate to another.. When it
    involves a manufacturer of a cell phone or a computer, it creates a problem
    for us users. 
    +/- 0.5s is fairly easy. 
    +/- 0.1s isn't too involved, but does take a little thought and effort. 
    Going below +/- 0.008s (which is the center point of an NTSC / EIA video
    field)?? That's a Project..
    
    Derek
    
    
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