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  • FIRST POST
    • 50Twuncle
    • By 50Twuncle 16th Apr 17, 10:01 AM
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    50Twuncle
    BBC Greenwich Time Signal inaccurate
    • #1
    • 16th Apr 17, 10:01 AM
    BBC Greenwich Time Signal inaccurate 16th Apr 17 at 10:01 AM
    Has anyone else noticed that the GTS broadcast by BBC (the clock) is always 10 seconds slow (compared to a radio controlled (atomic) clock ?
    This is supposedly due to the bounce time for a signal on Sky - but I thought that radio signals travelled at almost the speed of light, so should take milliseconds to travel this distance ?
    Whats going on ?
Page 3
    • 50Twuncle
    • By 50Twuncle 18th Apr 17, 8:17 AM
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    50Twuncle
    Well uncle consider a hypthetical download speed so fast that it cannot be measured then the time to load the data bits from source to pc would be nearly instantaneous so high accuracy (ignoring other delays).
    Opposite end of the spectrum one bit a day would tend to cause a bit of a delay......That could be quite important if there wasn't any correction done.

    One way to overcome the time span to transmit and receive a coded time word is first to transmit a marker and then the coded time that refers to that marker. That then enables the time from marker to end of word to be measured and a correction added. Can be done in real time but also attached to records to enable events to be accurately measured on a record.

    It is the not so subtle difference between transmitting a known accurate frequency (a clock frequency) and adding on to that clock, data that gives real time information? By the time the info is read and decoded time has moved on......And other posts intervene...
    Originally posted by Ainsley1
    But that is not "download speed" - that is LAG - ie) the time taken for the signal to arrive - which depends on carrier speed - not download speed !
    The actual data itself will be a tiny amount - which will cross from the sender to the receiver in a fraction of a second - which is what I am trying to say - that the time taken to get the data from the sender to the receiver depends purely on the delay brought about within the receiver itself (the time taken "mid air" will be virtually zero (almost speed of light) )
    • Ainsley1
    • By Ainsley1 18th Apr 17, 10:54 AM
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    Ainsley1
    We agree that the lag is a problem?
    Surely what we are discussing here (off at a tangent a bit) is the cause of any lag producing a delay?
    I might be missing something but I don't follow your argument that it is the carrier speed (only) that is the issue.
    Rather I see that as a potentially contributory factor given that several coded time systems can use different carriers and result in virtually the same or similar accuracy and in other cases cannot; but if you have a download speed that causes a delay to transmit a data packet (that contains coded time) that cannot be accurately quantified (i.e. a variable and unknown lag) then it is impossible to get accurate time from that irrespective of the carrier used. In relative terms the longer the delay compared with the desired accuracy the bigger the problem!
    It would be good if you could expand on your disagreement uncle. Are we just looking at the ' lag ' from a different perspective or am I missing something -apart from brain cells
    • pappa golf
    • By pappa golf 18th Apr 17, 11:22 AM
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    pappa golf
    there is one hell of a delay on my TV , its still showing 70s sitcoms and films
    Have YOU had to walk 500 miles?
    Were you advised to walk 500 more?
    You could be entitled to compensation.
    Call the Pro Claimers NOW.
    • onomatopoeia99
    • By onomatopoeia99 18th Apr 17, 11:25 AM
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    onomatopoeia99
    Running a local ntpd server syncing to multiple pool servers around the world should do a fair job of countering communication latency. I used to do that for my local network until Windows built in the ability to ntp sync the clock.

    My clock radio syncs to the NPL signal but only once, when powered on. It has drifted by about four minutes since then, which means I'm now woken up by the radio 4 weather, not the pips!
    INTP, nerd, libertarian and scifi geek.
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    • iltisman
    • By iltisman 18th Apr 17, 12:59 PM
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    iltisman
    Perhaps the BBC are putting quiet subliminal messages before the pips .
    vote labour pip pip beep
    It's only my cheerfulness that keeps me going
    • googler
    • By googler 18th Apr 17, 6:36 PM
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    googler
    i thought terrestrial tv was the old analog system then they stopped it and now we have digital free view....
    Originally posted by prosaver
    Terrestrial TV - analogue or digital - comes from transmitters on 'terra firma'; on the ground.

    As opposed to satellite, which doesn't.
    • unforeseen
    • By unforeseen 18th Apr 17, 7:00 PM
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    unforeseen
    Terrestrial TV - analogue or digital - comes from transmitters on 'terra firma'; on the ground.

    As opposed to satellite, which doesn't.
    Originally posted by googler
    Well, technically it does as the signal gets sent to the satellite from an SGS (Satellite Ground Station)
    • thescouselander
    • By thescouselander 18th Apr 17, 7:22 PM
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    thescouselander
    Almost all the timepieces I have adjust themselves to the time signal, GPS or via the internet and all of them sinc to the second.

    I'm not really sure what the purpose of the the BBCs clock/pips is these days now there are much better time sources available. Seems like an archaic idea well past its use by date - much like the BBC itself in many ways.
    • 50Twuncle
    • By 50Twuncle 19th Apr 17, 8:50 AM
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    50Twuncle
    We agree that the lag is a problem?
    Surely what we are discussing here (off at a tangent a bit) is the cause of any lag producing a delay?
    I might be missing something but I don't follow your argument that it is the carrier speed (only) that is the issue.
    Rather I see that as a potentially contributory factor given that several coded time systems can use different carriers and result in virtually the same or similar accuracy and in other cases cannot; but if you have a download speed that causes a delay to transmit a data packet (that contains coded time) that cannot be accurately quantified (i.e. a variable and unknown lag) then it is impossible to get accurate time from that irrespective of the carrier used. In relative terms the longer the delay compared with the desired accuracy the bigger the problem!
    It would be good if you could expand on your disagreement uncle. Are we just looking at the ' lag ' from a different perspective or am I missing something -apart from brain cells
    Originally posted by Ainsley1
    No - the time taken for the carrier to travel any set distance is only determined by its speed - its frequency is not a factor - if you think of two different sized / different coloured cars travelling 100 miles from a to b - what is the only factor that will determine how long they will take ?
    Their speed...........
    Similarly with radio signals - the only thing that determines the time taken to travel from a to b (ie the lag) will be the speed of transmission - which should be pretty constant (close to the speed of light) - which for the distance that we are talking about (geostationary orbit (22,236 miles) is virtually negligible - leaving the time in the actual "workings" of the tx and rx as being the only delaying factor and I fail to see how this can be any more than milliseconds - so why the delay in the time signal of 10 seconds ?
    • Ainsley1
    • By Ainsley1 19th Apr 17, 10:56 AM
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    Ainsley1
    Ah I see that you mean the carrier speed is (virtually) constant and very fast (in the case of the electromagnetic spectrum -ignoring minor changes that do take place through various mediums)....And that it is the only factor affecting speed and thus lag comes from another cause such as the turn round time via satellites?

    Now carrier speed is independent of frequency however time information (as distinct from frequency/time period of wavelength that produces a clock frequency) usually is modulated onto the carrier as a word (of differing lengths depending on the coding, complexity of message and housekeeping information) and at a range of modulation rates.
    This as an analogy consider transmissions using light/radio telegraphy or waves-we will for the purpose of the discussion accept a constant speed will we not? - then tell me if the rate of transmission varies and hence the delay with the following methods to send a message:
    Ships flags
    Semaphore
    Aldis lamp
    Telegraph
    AM radio transmission
    Digital signal transmission down lines such as ADSL
    Fibre optic digital comms.
    Satellite transmissions such as GPS and similar?

    Are they all equal?

    Now there are differences in overhead and housekeeping, complexity of information length but all have differing rates at which the data can be transmitted such as how fast one can key/send the data.
    All these are analogous to rate of transmission and upload/download speeds. Therefore it is a factor which is what I maintained and you disagreed with.

    There are many a time code systems that find that speed a problem or irrelevant. Those that do use methods to overcome . From my hazy memory SMPTE code might not but IRIG codes do. These are coded words modulated onto electromagnetic carriers and use correction/markers to compensate for the time to load the information resulting from the various transmission rates (albeit small) that is not different to a download rate.

    Must admit though that I do not have a full answer to the long delays you mention but guess it must be processing and comments error correction times.

    Maybe interesting to note GPS positioning relies on not different modulation rates but time differences receiving the carrier data from a number of satellites so these ever so small delays we think of possibly as irrelevant are put to good purpose!
    Last edited by Ainsley1; 19-04-2017 at 10:59 AM. Reason: ahem.....error correction.....
    • Ainsley1
    • By Ainsley1 19th Apr 17, 11:06 AM
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    Ainsley1
    Oh and I meant to add that frequency can be a factor, however small in terms of Greenwich time signals!!

    If you consider frequency (and hence wavelength) irrelevant then you can postulate a theory an why light refracts differently passing through a prism depending upon colour i.e. frequency/wavelength and disprove that it is owing to speed differences of the various wavelengths crossing the boundaries at the prism?
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