WTF with my computer clock?

[Sylvia Else]

No - The Bill has never used that. Or rather not in general - it may have
been tried on a 'special'.
The current ones are shot HD using progressive scan.

But IIRC, they suppress one field and repeat the other for this effect?

Certainly, no one in their right mind would deliberately reverse the
interlace ordering - the result is unwatchable.

Sylvia.

 

[Sylvia Else]

Depends - the actual ad break times are pretty accurate between some of
the companies - the idea being to prevent channel hopping when the ads
come on. You'll just see ads on the others. Hence the way they crash into
the break on progs not made with this schedule in mind. And most of ITV
comes from just one playout centre, so should be synchronised across the
country.
Start times for progs have never been accurately published. They've always
been approximate - apart from on some data points in the evening.

Here in Australia I got documentary proof that a station was
deliberately running late. See

http://groups.google.com/group/aus.tv/browse_frm/thread/703f398d4e875bc6/a1a580334e9ff9f2

I had recorded that channel that evening, on a PC that has its clock
synchronized to an accurate clock, and the times given in that schedule
were to within one second of when the material was actually broadcast.

They just weren't the times that had been advertised.

Sylvia.

 

[Sylvia Else]

It's not just between broadcasters, the BBC does it between their channels
as well. Their 'Points Of View' viewer complaints show have done a few
reports on viewers complaining about different times on BBC1 and BBC2, at
least one of which had one of their presenters switching between the 2
channels at programme change to demonstrate the problem.

The problem (which they actually proved was real - surprised they were
allowed to show that on BBC1) is that BBC1 often runs 2 minutes early and
BBC2 is 2 minutes late. Switch one way and you have to wait 4 mins for
programme start, switch the other and you miss the start.

Much as I'd like to be able to support the view that the BBC's standards
are falling, I have to advise that I was already being frustrated by the
BBC's apparent inability to keep to its published schedules back in the
early 1980s. This is nothing new.

Australia's counterpart, the government funded ABC which also doesn't
carry advertisements, is also apparently unable, or unwilling, to
broadcast things when they say they will.

I suspect that, as with the commercial stations, it's deliberate. I'm
just less than clear what the motivation would be for a non-commercial
station.

Sylvia.

 

[David Nebenzahl]

You have to set net time up before you can use it.

Well, duh; that was kinda my point.

So I take it you don't disagree with what I said, or have nothing else
to add?

 

[David Nebenzahl]

http://download.cnet.com/Atomic-Clock-Sync/3000-18512_4-14844.html

Thanks, but I'm happy with the little utility I already use that
contacts NIST (Nat'l Institute of Standards and Technology); see
http://tf.nist.gov/service/its.htm for more info.

 

[David Nebenzahl]

I agree that for most a minute per month is reasonable but I would
expect the same accuracy as my $29.99 Timex wristwatch which is more
like a second a month.

So that kinda begs the question of why computer mfrs. can't (or won't)
include clocks that are at *least* as accurate as a Timex, no? Wouldn't
a computah be a more compelling reason for a more accurate clock? (I
know, $$$ bottom line, right?)

If you use the NIST SNTP server you'll be as accurate as how
frequently your SNTP client updates.

Of course, it would be nice to know one's computer would maintain
accurate time even if, god forbid, it was somehow disconnected from The
Network ...

 

[David Nebenzahl]

Functionally impossible. By adding money, you can reduce the drift rate
but you can't make it zero. Period.

I don't care about zero. One minute a month is plenty accurate enough
for me.

Just use NTP. And *stay away* from the stratum one servers like NIST;
they have better things to do than keep your computer's clock on
time.

You're admonishing me not to use NIST? Why?

After all, they offer this service to me. See
http://tf.nist.gov/service/its.htm:

The NIST Internet Time Service (ITS) allows users to synchronize
computer clocks via the Internet. The time information provided by
the service is directly traceable to UTC(NIST). The service responds
to time requests from any Internet client in several formats
including the DAYTIME, TIME, and NTP protocols.

So why shouldn't I use them?

Keep in mind that I use this service *at most* 3 or 4 times a *year*.

 

[Geoffrey S. Mendelson]

You're admonishing me not to use NIST? Why? .....
Keep in mind that I use this service *at most* 3 or 4 times a *year*.

David, it depends upon how you use it. If you use Windows' or MacOS's
automatic time sync or *NIX's NTPDATE, you only access it occasionaly.
Windows and Mac access it once a week, NTPDATE does it whenever it is invoked,
usually when you boot your computer.

If you are runnin *NIX NTP deamon (including MacOS's) or a third party Windows
time sync program, your computer is in frequent contact with the time server.
In that case, it would be a good idea not to use those servers as they are
heavily loaded down.

For once in a week sync of one computer, you can use just about any server
without worry about it being overloaded or adding any additional load.

If you have multiple computers networked together, that is a different story.

Geoff.

 

[David Nebenzahl]

Got $10.70? Build your own:
<http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=561-1014-ND>

Interesting.

But once you've picked up WWV, what do you do with that signal to derive
a time base from it? (I guess you gots to know something about the
signal, which I don't.) Pretty simple?

 

[Sylvia Else]

Actually,they were very close.

Western Union clocks all over the country were almost always all synched
to within a second or so. The technique was to use clocks (those big
things with the red sweep hand you may have seen in a broadcast studio)
that were basically pretty good, and to synch them to a remote timebase
from time to time.

The clocks were pendulum timed and electrically wound (couple of big dry
cells inside), and every one of them had a leased-line connection to the
nearest WU office, and from there to a national site.

Every 12 hours (AFAIR), Western Union sent a pulse down the wire that
"jammed" the sweep hands of all those clocks to 12 (and illuminated a
little red light behind the clock face so you could see that your time
was being corrected). I don't think the minute and hour hands were
controlled. It was up to the engineering personnel in each station to
twiddle their clocks' pendulums so the clocks could run within a second
or two in 12 hours -- not at all difficult for a good pendulum clock.

So as long as the accounting department paid the WU bill, you could join
your network or insert a local commercial with almost perfect accuracy.

Isaac

In the UK, or in London at least, the mains frequency was maintained
with a very accurate long term average, so that synchronous mains clocks
just stayed correct.

Sometimes, after short power cuts, the frequency was increased to bring
such clocks back to the correct time. Which was actually a bit of a
nuiscance for us - we had clocks that weren't self starting (a
reflection of the rareness of power outages in those days), so after a
power cut, we'd set the clocks correctly and start them, only to find
them gaining.

It seems a backward step that now, forty or so years later, household
wall clocks are less accurate than they were back then.

Sylvia.

 

[Geoffrey S. Mendelson]

I don't know that much about the code used by WWVB. (I are not a
programmist).

It's a very simple system that is well documented. It's simple and slow
enough that anyone used to pulling apart data streams would be able to
decode it with a Z80 derived embedded processor, the ARM chips in dead iPods,
WiFi routers, etc would be "overkill".

Here in Jerusalem, we don't receive the signals of WWVB, or the German
or UK equivalents here. Someone about 50 miles north and out of the
mountains has a clock that syncs, but he never told me which station it
uses (he may not know), or how often it syncs. This lead me to research
how one would do the opposite, devise a local transmitter with an
ethernet port on one end for NTP sync and a 60kHz transmitter to sync a
clock on the other.

I gave up due to lack of a suitable design for the transmitter, no receiver
and a lack of funds to obtain them. You probably could do it out of one
of the Linux based routers, and blink one of the status LED's to generate
the 50/60kHz signal.

I know by now you must be thinking "why would anyone even think of such
a thing", but a discussion a few months ago about resurecting a Heathkit
Most Accurate Clock, got me going. I think I also read a posting that the
WWVB signals were being phased out.

Geoff.

 

[Geoffrey S. Mendelson]

Building a store and forward repeater for WWVB (or the EU equivalent)
60Khz is a waste of time. The storage delay needed to regenerate the
signal will result in the sync pulses arriving too late. However, a
system that uses GPS, GLONASS, or Galileo as a reference, and
generates a simulated time code format will work. The problem is that
at 60Khz, the necessary antenna farm would be huge and the
transmitters rather power hungry. WWVB runs at an EIRP of about 70kW.

That's sort of what I was thinking of. Get the time from NTP, generate
a fresh time code signal, which would not be accurate enough for someone who
wanted truely accurate time code, but to keep a clock that displays to the
minute, or even to the second on time, it would be good enough.

As for the transmitter, how much power do you need to transmit a signal from
a time code generator to a receiver next to it, connected via a coax cable?

A microwatt? A milliwatt?

limited abilities of a commodity router. Methinks you would be better
off with a SBC (single board computer) or common PC (ITX, Mini-ITX,
etc).

It depends. A cheap router, such as the Linksys WRTG-54L (note the L at the
end it's the enhanced model that runs Linux) would do it. It sells new for
not much money, will be obosolete as the 802.11N routers come into general
useage, has an ARM processor, two ethernet interfaces (one connected to a
4 port hub), a WiFi radio and a bunch of status LEDs. The advantage of it
is that there are several alternate Linux packages for it and you can easily
compile your own programs, build your own "flash" (firmware image) and load it.

There are also distributions for other routers, I recently bought a $30 EDIMAX
wired router that had a distribution for it.

Actually, a 60KHz xmitter is fairly easy to design. The problem is
that all the components would be huge. There's also the not so easy
problem of getting Ministry of Communications approval. It's not on
the designated ham radio frequency list:

If it is directly connected to the input of the receiver, it needs no
license.

I'm sure that nearby users that went through the trouble of obtaining
large antennas, will not be thrilled with your transmissions. Even a
"local" transmitter can carry a substantial distance at 60KHz.

I also expect there are none. Around here the noise level is so high that
they would never hear it until I got into the "real antenna" type system.
A microwatt with true isotropic radiator antenna (a short wire) would not
leave my apartment, let alone go anywhere. But if it is connected directly,
then it is a moot point.

I was thinking of something simple, such as flashing the LED at 60kHz, and
wrapping a pickup loop around it. That's about the same power level and
frequency of a TV remote control and no one from the MOC has come and
complained about any of the ones I have. I'm talking about the RF leakage
from it, not the optical signal.

The GC-1000 used WWV at 5,10, and 15 MHz, all of which are still on
the air. I'm not sure what a 60KHz system would do for you.

Not here. I have never heard them here, nor have I heard CHU (yes, I know it
moved), any of the European stations, etc. I'm not talking about a cheap
portable shortwave, I'm talking about a Kenwood R-5000 with a 75 foot random
wire, or other equally as sensitive ham receivers either with a 20m resonant
dipole or 1/4 wave vertical, or a 40m resonant dipole.

I'm not actually familar with the clock in question, the discussion (I think
it was on this newsgroup) focused on them using WWVB (VLF) radios.

I've given up asking "why". Some of the strange things I've seen on
the internet defy logic and explanation.

I'll agree with that.

Geoff.

 

[Geoffrey S. Mendelson]

They replaced all the transmitters and towers at WWVB a few years ago
to improve service. It now reaches Central Florida without a long wire
antenna & tuner. Why would they spend millions and take a couple years
to do the update if they were planning to shut it down?

They will shut it down eventually because of the cost. NTP servers cost
almost nothing, GPS is "free" because there are no incremental costs for
providing the time signals.

Eventually someone will figure out that a 75kW transmitter has both a
significant expense and a large "carbon footprint".

The upgrade was thought to be needed because of a satellite time system
that was dropped due to GPS was thought to be unable to fit the needs of
the common user. Now GPS units are almost throw away "toys", being used in
almost every cell phone, and for all sorts of SATNAV devices.

Geoff.

 

[Geoffrey S. Mendelson]

Size?
Temp?

Does a tiny watch xtal garner any more accuracy merely because of its
size?

Does a watch xtal have a different temperature coefficient?

You are confusing the hardware clock and software clock in a computer.

The hardware clock is crystal controlled. It is used at boot time to set
the software clock.

The software clock is incremented by the lowest priority interupts, which
causes it to wander off.

There are various schemes to sync it with the hardware clock, but without
an external source, e.g. NTP, the don't work very well as hardware clocks
are not very accurate.

Geoff.

 

[David Nebenzahl]

Since 1981, I've looked inside literally hundreds of computahs and
SBC's. Not a single one has a tunable clock oscillator. One or two
used replaceable modular oscillators, which could pre purchased as a
TCXO, but which were usually supplied as a commodity clock oscillator.

So I wonder if the lowly SX28, one of my favorite little machines to
program (a PIC-like li'l guy) is an exception to this seeming rule?

I ask because, looking at the specs for this CPU, it has some
configuration bits (marked IRCTRIM0-2) that trim the internal RC
oscillator frequency, supposedly in steps of about 3%, up to a maximum
of +/- 8% (yeah, I know, doesn't add up, but whatever). Is this what you
would call a "tunable oscillator"?

These daze, the way to stabilize a TCXO is to first pre-age (beat-up)
the crystal to reduce long term drift. The crystal oscillator is then
characterized over the required temperature range. A table of
frequency versus temperature is generated and saved in a PROM. A PIC
controller on the oscillator takes the measured temperature, reads the
table, and applies the necessary correcting voltage to a varactor to
stabilize the oscillator over a very wide temp range. With this
method, you can take a really awful crystal, and compensate it to
impressive accuracies.

So presumably what I just described is a varactor built into the SX28.