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

Max Power mikehack at u.washington.edu
Mon May 1 12:42:07 UTC 2006


>> 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.

For consumer purposes (save cellular telephony) GNSS technology is too 
costly.
NTP is OK for computers, but has a learning curve.
===============
> If you need greater accuracy than a LW signal can provide then
> GPS/Glonass/Galileo is the best way to go.

I am unaware of WWVB doing this.
If WWVB does this, its signalling must be very quaint.
Typical 1983 signalling medthods (bitfields transmitted) would probably not 
pass modern muster.
==============
> 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.

BCD is the universal flaw of LW time stations.
NTP could fix the problem, but NTP is not perfect for this [without 
tweaking].
BCD could be said to use odd signalling in not using {seconds} within a day, 
albeit seconds are not suitable for LW signalling rates. I assume both NTP 
and BCD signalling medthods are aimiable to 'bit averaging' for LW time 
service's slow signal rate and error correction condtions.
==============
> 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.

DRM on MW and SW only sends the Julian date and time.
There is some BCD coding similar to WWVB and DCF77 -- for time and date.
However, there is no [univeral] NTP or UNIX time packet in DRM.
SW signals do cover areas LW cannot reach (like Antartica) -- and with 
adiquate frequency diversity provide a higher time resolution vs LW. DRM can 
cope with ionspheric delay via indicating TX (and at the reciver RX) coords.
LW time stations are trapped in late 1960s style signalling.
============
> 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. 





More information about the questions mailing list