On Tuesday, August 21, 2012 8:56:11 PM UTC+1, Harry Bloomfield wrote:
Ramsman explained :
On 21/08/2012 19:52, Harry Bloomfield wrote:
Moonraker used his keyboard to write :
How accurate is the time given on Windows? Also how about digital
radio and freesat, I guess there is a time delay at least on the last
2. I notice that the pips are rarely used these days, and apart from
the Olympics I cannot remember the last time I heard Big Ben broadcast.
Big Ben is not really that accurate, so for the radio they use a
recording which is accurate - or used to be. Now it can be less accurate
due to digital links in the transmission system.
Windows draws it time data from an Internet Time Server, of which there
are several. So assuming you set your PC to sync itself fairly
frequently, it should be near enough. Possible errors are delays in the
time packet delivery, but I understand Windows checks and compensates
for these delays before calibrating its clock.
Your GPS's time should be spot on, assuming it has lock with the
satellites, though there might be delays in the software.
If you want continuous accuracy, then you need to monitor MSF, which
transmits an accurate time signal once per minute. MSF is what is used
for setting radio clocks and watches, but they only usually sync
themselves once per 24 hours, assuming they manage to receive the
signal. MSF transmits (60Khz) not only an accurate time signal, but a
complete set of data for time, date including the year, in every
complete minute via a 1 bit per second, plus a fast code 'croak' set of
date in the final second. I wear a MSF controlled solar powered watch,
have several MSF controlled clocks and have a weather station which also
syncs itself to MSF. In the 70's I wrote software to continuously decode
and continuously display the MSF time, from Rugby. I had to write to GPO
MSF Rugby and beg for a spec for the data format, to be able to decode
it 
My watch (Citizen Eco-Drive) and clocks receive their signals from the German
transmitter at Mainflingen near Frankfurt am Main. I haven't checked them
against any GPS devices, but I shall do so.
I noticed during the BBC Olympic broadcasts that the stadium clock was about
seven seconds behind my watch.
I would not expect the stadium clock to be out, I wonder if there might
have been a deliberate delay loop in the transmission? There is always
some delay in digital broadcasts, then more delays inside the TV set
decoding it. Overall, a second or so.
My radio watch attempts a sync once per 24 hours. If it fails on the
first attempt to get a sync, it tries a second time an hour later. If
successful it shows a little satellite (?) symbol in its LCD display.
Amazingly, I have never known it fail to manage to gain sync in the 7
years I have worn it - thinking back to when I built a large
specialised
MSF receiver and how I struggled to get stable reception. lol
The other morning, my "Rugby" alarm clock was about 6 minutes out. I was listening to the radio and used to seeing the pips and seconds on the clock synchronised.
Since the clock cannot have drifted that far in 24 hours, I'm guessing it must have misread the Rugby time signal. I switched the sync off and then back on, and it corrected the time within a few minutes !
Simon.