Ei
Sorry if I have your name reversed. By taking this approach it
eliminates the ability to use wwvb as a frequency reference because it
destroys that traceability.
Thats what we are trying to preserve. Or at least re-establish for the
older phase measuring receivers.
Regards
Paul

On 7/8/2012 12:10 PM, Tofurk Ei wrote:

Any possibility of using the decoded signal to un-do the modulation and
feed the reconstituted signal to the older receiver?

On 7/8/2012 12:56 PM, paul wrote:

Peter indeed there could be
But it should not need to be decoded to undo the psk.
Plus documentation lacks some of the details I think to actually do it.
But that would be a significant project since the formats not been
settled completely yet.
Regards
Paul.

On 7/8/2012 6:40 PM, Peter Gottlieb wrote:

Hi Peter,

That's be the hard way, but yes, if the message BPSK coded is computable
and of a known format. If the message contained more than time, like solar
flux, it gets more complicated very rapidly.

A similar thing was done with the Equatorial system 30+ years ago. In that
case, each data bit was broken into something like 32 or 64 chips (I don't
remember). There were two maximally distant, orthogonal chip patterns,
representing 1 and 0. The incoming BPSK message went through a 0 or 180
degree switch, then the IF stages. The switch was driven from a local
(known pattern) chip generator, so that if everything was synced up the
narrow band IF would put out the 0 or 1 that had been encoded. BTW, this
trick vastly improved the system S/N becaust it narrowed the receiver IF
bandwidth many times.

If the chip pattern is not known (fixed) or computable (like a correct
TOD) things go to pot quickly.

Rather than building such a kludge, it would be easier to use the locked
clock in a newly designed receiver and phase compare that to your local
standard directly.

-John

==================

Hi

In this case the data format and it's contents are highly "computable". If you have a good local clock and an initial lock, the rest of what follows is predictable. That of course assumes we know the real format ….

Bob

On Jul 8, 2012, at 6:58 PM, J. Forster wrote:

A risky assumption, and a cold start could be tricky.

Equatorial took many minutes to lock up, with a much higher data rate, and
it did it by slowly sweeping the local clock.

Aside: That's why military spread spectrum systems like good local clocks.
They lock up a whole lot faster that way.

-John

================

On 07/09/2012 12:46 AM, paul wrote:

I have looked at the PTTI 2011 paper (wwvb.pdf) and much of a format is
being shown. Has anyone established the 14 bit sync-word and verified
the format? It seems that aligning up with the normal AM broadcast
should be possible.

Can someone record it as it has been reduced to say 2 kHz and analyze
the produced audio file? Recoding with 48 kHz sampling rate should allow
almost trivial 2 kHz I-Q demodulation to illustrate phase swaps.

Cheers,
Magnus

Hi

The clocks we would be using are much better than what most military systems use….

I also assume that an initial lock up that takes a hour is perfectly acceptable in this application. You will still need a lot of hours / days / what ever of data to get useful stability off of WWVB, spending an hour or more to acquire from a cold start will have little net impact.

Bob

On Jul 8, 2012, at 7:29 PM, J. Forster wrote:

Hi

The gotcha is that they may change the sync word based on test data. They may also tweak other vague points in the spec based on the troubles they run into in their tests or with their silicon.

Bob

On Jul 8, 2012, at 8:07 PM, Magnus Danielson wrote:

If you have a deep fade every few hours or minutes, as is common, relock
time becomes an issue.

-John

==============

There is not an infinity of good sync words. A typical good sync word has
a high positive autocorrelation when synced, sloping downwards
monotonically.

Thus the cross-correlation of the received word with a locally stored
reference can be used to steer the loop using a small dither and a lock-in
technique.

-John

================

HI

My assumption is that for what we would be doing, one lock is all it's going to take. The local clock will be good enough to allow you to never need to re-aquire. You will indeed steer to maintain phase, but you already have and know what's coming in on the modulation.

Bob

On Jul 8, 2012, at 9:16 PM, J. Forster wrote:

On Sun, Jul 08, 2012 at 09:02:53PM -0400, Bob Camp wrote:

I finally read the wwvb.pdf paper (yes, do so before opening

mouth)...

I think I read the "Binary Phase Shift Keying Modulation"

paragraph on page 10 to indicate they are using ABSOLUTE, not
differential BPSK.

They refer to the "baseband waveforms s0(t) and s1(t)".   To me

this is the absolute I vector... and this clearly says  that a 0 is
always upward (or by convention in phase), and a one is always downward
(180 out)...  They clearly say the phase shift is 180 degrees...

I would think this clearly could be phrased better...

It appears the data format they propose is quite well defined in

the paper, though they clearly indicate that a proposed extension is
changing the barker code sync word for frames every so often so as to
indicate a different frame type that might contain highly entropic (eg
volatile and unpredictable) information of  undefined character
including a possible mechanism for sending arbitrary and completely
apriori unpredictable bitstreams, though doubtless constrained by the
hamming codes used for FEC/error detection and the barker code sync
word.

On a quick read it appears the complete 60 second time frame

format is defined unambiguously.  There are somewhat unpredictable DST
bits and leap second bits in there... but in practice those change VERY
infrequently from 60 second frame to frame or even from week to week
or year to year. (Yes Congress likes to muck with DST every decade or
so...).

I am still reading more carefully, but I think this means that

the entire phase and amplitude sequence of the signal is defined for the
current initial version if you know the time of day and date and the
current leap second and DST settings (which change VERY infrequently).
And I THINK I understand this means the absolute phase sequence
relative to the 60 KHz going into the modulator at the transmitter....

Thus the initial signal phase modulation could be removed by

some comparatively simple itty bitty microp software driving a balanced
modulator  BUT future signal extensions might not have that property.

As for acquiring bit sync with the signal, both the amplitude

and phase information should allow a micro to do this easily and
relatively quickly if the I vector were provided to the micro somehow.
This would presumably be possible by either sampling the 60 KHz directly
with an A/D (at 240 Ksample/sec) or by using an external balanced mixer
driven by local synthesized 60 KHz.  Even just an envelope detector
would work with strong signals because of the AM  component, and this
might be enough to acquire adequate bit sync for some purposes.

Software PLLs at 1 second rate are duck soup for even a SLOW

micro... and frequency errors are tiny so tracking can be tight. And
acquisition for these is also very fast given reasonable SNR. Only takes
forever if SNR is so low it takes that many seconds correlation to see a
reliable tick.

I admit as I think about this that if one synthesized the clock

for a itty bitty simple micro from say a local DUT 10 MHz whose phase
relative to WWVB one is monitoring one could do much of the entire job
by using programmable timers on the micro and its internal A/D.  This
includes phase error versus WWVB output and of course TOD output.

One would almost certainly want to either use external balanced

mixers (FET switches ?) and produce an analog I and Q (low pass
filtered) for processing by a really slow micro or use a fast enough one
to take a stream of actual real 60 KHz input samples at 240 KHz and
compute filtered I and Q)  (and LP filter/decimate it) (yes, with
accurate A/D clocking from suitable microp output pin interval timers
you might well be able to subsample by a lot and not actually ever deal
with even any close to a  240 KHz sample stream with the micro).

This would of course allow computation of the vector positions

of the WWVB signal modulation in I and Q space relative to the 10 MHz
clock from the DUT.  And from that one should be able to compute
the various moments of 10 MHz DUT clock drift and do a decent job
of compensating for it (better and better as the DUT clock gets more
stable/predictable) and ride out fairly long fades and outages without
losing a pretty  good idea of the expected WWVB phase.

Presumably most standards whose phase one is tracking with such

setups are very stable, thus the holdover should be considerable
if one uses a good error and drift estimate to adjust ones local
idea of WWVB phase relative to local clock derived from the standard
to compensate.  And guess what, determining a local error and drift
estimate is precisely what such a system is doing...

And yes one could also drive a balanced modulator with the micro

to generate a de-psked signal for legacy receivers in the museum, but
in fact the micro would already know all that those things typically
provided by legacy equipment (phase of local DUT standard versus WWVB
and time of day and possibly SNR and or signal strength).

It has occurred to me that it might be necessary to either

squelch (eg turn off) the de-psked output to the legacy receivers on
signal fades or perhaps BPSK modulate it with a several times the 1
second bit rate pseudo random bit stream to ensure that the legacy
receivers never saw anything they thought was good lock until the time
of day lock allowed determination of absolute WWVB signal phase reliably
(and the new code with FEC seems to make this VERY reliable).

Enough drunken ramblings late at night...

--
Dave Emery N1PRE/AE, die@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."

There is an advantage to BPSK switching at the zero crossings in that the
spectrum is significantly narrowed.

-John

==============

On 07/09/2012 03:32 AM, Bob Camp wrote:

Except for leap second announcements.

Another thing is to monitor for cycle-slips.

Cheers,
Magnus

Hi

Leap seconds may never happen if PHK gets things organized … :)…

Cycle slips are the main thing you would be watching for. The "slow steer" to maintain synch is to catch or correct cycle slips.

Bob

On Jul 9, 2012, at 6:15 PM, Magnus Danielson wrote:

David
Read your comments and have been traveling. So finally a chance to email.

I read the document also and walked away with what I shared.
In your reading would you believe the following.
Its an absolute phase and that when it switches to 0 there is 1
transition at the beginning of the second to 180 degrees staying that
way to the next bit or flipping again to 0 degrees if its a 1 at the 1
sec tic???
Is there a way to sense from the document that there is a bias towards 0
lets say.
I could not figure that out.

What I have seen on the scope is that I believe a 0 could be multiple
flips during the 0 bit and thats perplexing.
Regards
Paul.

On 7/9/2012 1:53 AM, David I. Emery wrote:

On Wed, Jul 11, 2012 at 03:48:52PM -0400, paul swed wrote:

What I mean by absolute phase is that a 1 is always 180 degrees

and a zero always 0 degrees.  In your example this would imply that the
two ones in a row would result in two seconds of 180 degree phase
without a flip after the first 1.

The document is confusing, but the best I can do with its

language is to conclude they are talking about absolute phase.  Normally
when one talks about baseband waveforms one is referring to absolute I
and Q components relative to an unchanging carrier phase, not relative I
and Q with respect to the last bit phase.  So I take their language to
mean a zero is 0 degrees and a 1 180 degrees relative to an unchanging
carrier.

Differential encoding is the opposite, a 1 is always the

opposite phase from the last bit, a zero always the same phase as the
last bit (or sometimes  the inverse where a zero is the transition and a
one is not).

Differential encoding tends to have little "DC" component or

bias toward either one or zero or one phase or the other, absolute
encoding does if the data it encodes does.

--
"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."

On 07/14/2012 01:49 AM, David I. Emery 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.

Cheers,
Magnus

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

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@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."

I received the following message from John Lowe at NIST.
I thought it might be of interest to you.

-----Original Message-----
From: John Lowe [mailto:lowe@boulder.nist.gov]
Sent: Monday, July 09, 2012 12:55 PM
To: Ron Ward
Subject: Re: Phase-locking 60 kHz timing and frequency standard
receivers

We are changing the format to improve our reception capability.
Frequency standard comparisons will still be possible with new or
modified equipment.

On 7/5/2012 9:39 AM, Ron Ward wrote:

Hi John:

Why is WWVB changing to BPSK?  For cheaper clocks?

What about frequency standard comparisons with WWVB?

How am I going to monitor GPS and ensure that it's working correctly and
not being played with by DOD?

What am I going to do with our phase-locking 60 kHz timing and frequency
standard receivers?

Why did you not make the new data format backward compatible with
existing phase-locking 60 kHz timing and frequency standard receivers,
like they did with black and white to color TV transition in the 1950's?

What about +- 45 degree modulation?

We use to have LORAN, OMEGA, and WWVB.

Thanks,

Ron

-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of David I. Emery
Sent: Friday, July 13, 2012 8:35 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Phase modulation detection/NIST plan

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

of

so

either

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@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."

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