[time-nuts] Phase modulation detection/NIST plan

[David I. Emery]

On Sat, Jul 14, 2012 at 02:38:34AM +0200, Magnus Danielson wrote:

> I think the PTTI article isn't as much documentation as presentation of 
> general principle, showing details more as to present how it can be 
> done, but not necessarily guarantee it will be done that way. Knowing 
> the synchronisation sequence, polarity should not be ambiguous. Also 
> note that other data such as hours would be known from the AM signal, so 
> we can reverse engineer it. A receiver knowing this sequence will either 
> bootstrap from the AM or attempt straight lock. It's not too hard to 
> build a maximum likelihood receiver for it.

	I read the article as not a definitive specification frozen in
stone, but as a complete and relatively fully specified proposed design
with perhaps some details subject to adjustment or revision.

	The question of absolute versus differential phase shift keying
is, of course, rather fundamental to being able to decode the signal at
one level but at another not terribly central to the core of the design
for a coding and modulation scheme that works at much lower C/N levels
than the AM version did while preserving the legacy AM and its coding
for existing hardware.

	SOME place in the design of a differentially coded signal there
has to be a decision whether or not to structure the data encoding  so
some specific bit (or more properly symbol) in each frame (or at least
some known frames relative to the time of day) (in this case I mean 1
minute long TOD frame) is of a known absolute reference phase.

	If this is done than it becomes possible in a reasonable time to
determine an absolute  60 KHz carrier phase after a fade, if it is not
done and every single bit of data is not absolutely predictable (the
current TOD coding would be absolutely predictable given knowledge of
the time and date and  of leap seconds and DST settings, but they make
clear future extensions would probably not have this property as
additional messages are added including emergency messages and the like
which are never predictable) there is no way to reliably decide after a
fade which phase is which as this depends on knowing the number of ones
and zeros in all the frames transmitted since one last saw the signal,
something that is in the general case impossible if the signal has faded
and the bits were not observed.

	An absolute encoding has no ambiguity - if one knows the time of
day within a second one knows the transmitted phase except for during
bits that might vary with unknown data (eg emergency messages,
extensions to the standard and newly changed DST and leap second
settings and FEC bits based on them) and MOST bits are always known
phases, especially of course the sync code words.   So even with
terribly poor C/N one should be able to relatively quickly resolve the
phase ambiguity after a period of signal loss... and in many cases if
one still has a good idea of the time, within a couple of seconds
(symbols) of signal reacquisition.

	On another point, I am not of the school that providing much
better weak signal performance for simple, low power, and cheap LF time
of day clocks using  WWVB is somehow a minor improvement that primarily
benefits China because they make most cheap self setting "atomic"
clocks.   There are innumerable applications for low cost low power
human level 1 second accurate time of day in modern electronic systems -
examples are traffic lights and school crossing signs and water
sprinklers and street lights and other outdoor lighting and many
others... these systems are not normally network connected  and there is
no current wide area technology short of power hungry GPS with its weak
signals and relatively high cost and difficult reception from many
locations to do this.

	And with minimal effort to ensure compatibility, there should be
no conflict with use of the same carrier signal as a frequency reference
too... the problem of several decades old antique time and frequency
gear being incompatible seems very minor, and of course we have already
discussed ways to handle this if needed.

	And as long as the existing frequency reference use of the
carrier continues to work as a backup to GPS with modern updated gear
that capability hasn't been lost - except maybe if your particular
variety of tin foil hat requires vacuum tube VLF reference gear because
of EMP fears or something similar.

	I think the new WWVB proposal seems sensible and a reasonable
design...that should serve the public well.

-- 
  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."