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

Jonathan Buzzard jonathan at uk.me.buzzard
Mon May 1 09:33:01 UTC 2006


On Tue, 25 Apr 2006 17:09:33 -0700, Max Power wrote:

[SNIP]

> LF time services are OK, and are necessary over large transnational 
> regions -- like Sub Saharan Africa, Australasia and South America ... but 
> any new LF service needs to be more technologically advanced than WWVB, MSF 
> or DCF77 and its Swiss twin. In these regions 10 LF frequencies need to be 
> allocated, but the signal to be transmitted needs to be more modern than 
> WWVB or DCF77 -- maybe using some form of low complexity PSK or low 
> complexity QAM and 240 hz to 480 hz of bandwidth. The signal must be 
> futureproofed -- as above.
> 

OK? What other technology can provide such accurate signals at such a low
cost? The simple amplitude modulated nature of the LW signals makes for
such low cost implementations. A PSK or QAM modulation is going to put the
cost of hardware to decode the signal up to much to be useful.

If you need greater accuracy than a LW signal can provide then
GPS/Glonass/Galileo is the best way to go.

Further more since 1983 the DCF77 signal has been phase modulated in
addition to amplitude modulated further improving the time signal while
being 100% backwards compatible.

What I will admit is that all LW time signals are hugely wasteful of the
available bits as they all seem to used BCD to encode the time. It would
be much better to use straight binary, as then you need just 30 bits to
encode the date/time to the nearest minute for the next 1000 years while
keeping the time/day/year separate for simple decoding (11bits for the
minute of the day, 9 bits for the day of the year and 10 bits for the
year). Throw in another five bits for DUT, another bit for daylight
savings and something for signalling leap seconds any you have still have
20+ bits for error checking and correction and future growth.

What advantages do the SW time signals have over the LW ones? As far as I
can see between LW signals and satellite systems there is only a tiny
market left for any other time signal system.


JAB.

-- 
Jonathan A. Buzzard                 Email: jonathan (at) buzzard.me.uk
Northumberland, United Kingdom.       Tel: +44 1661-832195




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