On Tue, 21 Aug 2012, "John Williamson" writ:

Percy wrote:
On Tue, 21 Aug 2012, "John Williamson" writ:

Radio 4 pips on FM are accurate to a few milliseconds, depending on
how far you are from the radio.
How can this be if the final "pip" is 500 milliseconds long?

The time reference point is the end of the last pip. Technically, the
last zero crossing of the signal. I can set a digital watch to within
less than a frame of video with practice using the pips.

OK, understood. I'm amazed at your reaction time!

For a cheap, quick and accurate time signal, use a GPS receiver's
time. This will be UTC accurate to within the speed of light delay
LEO, which is of the order of 10 milliseconds. Clever software can
compensate for these delays, if you want to be *really* fussy.

How many GPS devices show the time to anything less than a second?

None that I know of, but the time changes on the display and the serial
output at the same time as the reference signal does. You can use a PLL
to synchronise any time point you wish within that second.

OK, thanks for the explanation.

--
P