# RC vs GPS



## ronalddheld (May 5, 2005)

Assuming a 15s/y quartz movement and one sync per day. For accuracy purposes does it matter if the sync is done via GPS or RC?


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## gaijin (Oct 29, 2007)

.


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## petew (Apr 6, 2006)

FWIW, Citizen states that ultimately the accuracy of their GPS watch after reception will depend on the reception environment and processing of the signal by the watch.


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## South Pender (Jul 2, 2008)

Good question. I've thought about this. My first thought was that GPS would be inherently better, since I thought that it might result in surer reception. Now, though, the evidence seems to suggest that reception problems analogous to those suffered with RC exist with GPS, and that perhaps reception is, in fact, less certain. So, my current thinking (always subject to new evidence) is that it doesn't matter, and since RC is better-established, it may be the better of the two. Of course, all of this is assuming that one is in an area that can receive the RC signal. If one is not, then clearly GPS is necessary.


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## Sabresoft (Dec 1, 2010)

ronalddheld said:


> Assuming a 15s/y quartz movement and one sync per day. For accuracy purposes does it matter if the sync is done via GPS or RC?


RC would connect more or less directly to the atomic clock (say in Denver), and then there would be some small lag in time from the transmitter to receiver that would impart a small error.

GPS I imagine would still get its time from an earthbound atomic clock and so there might be a greater lag due to the distance from the clock to the satellite and back down to the receiver thereby imparting a slightly larger error.


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## Catalin (Jan 2, 2009)

ronalddheld said:


> Assuming a 15s/y quartz movement and one sync per day. For accuracy purposes does it matter if the sync is done via GPS or RC?


I would say that the difference will not be much in that scenario - and will depend a LOT on the precise hardware and software implementation. Of course that the problem of actual signal strength and signal coverage will still remain. And the worst part is that very often in cheap RC models the quartz used is very low-quality and can drift close to one second from one sync to the next ... or even completely non-calibrated (as in the stock ezChronos) - and that could result in more than 2-3 seconds/day of error !!!


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## MrMcGoo (Jun 28, 2010)

My radio controlled Citizens connect more easily at night. There is about a 3 milisecond delay due to distance from Ft. Collins. The Satelite Wave does not connect as easily due to the lack of an open sky near my house. I have to go to the rear of my lot to get away from the trees around my house to get the 40 degrees of open sky required. I can connect faster with the Sat Wave once I get where I need to be.

There has been no practical difference in the accuracy of the SAt Wave versus RC Citizens. The satelites are about 12,600 miles away which may make the watch off roughly 40 miliseconds unless Citizen has compensated for the distance from the satelite. Those fractions of a second make no difference unless I have some special need for precision.

When there is a leap second adjustment, the Sat Wave may have some reception issues per the manual and may take longer to adjust. When are we due for the leap second adjustment?

Bill


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## Catalin (Jan 2, 2009)

MrMcGoo said:


> My radio controlled Citizens connect more easily at night. There is about a 3 milisecond delay due to distance from Ft. Collins. The Satelite Wave does not connect as easily due to the lack of an open sky near my house. I have to go to the rear of my lot to get away from the trees around my house to get the 40 degrees of open sky required. I can connect faster with the Sat Wave once I get where I need to be.
> 
> There has been no practical difference in the accuracy of the SAt Wave versus RC Citizens. The satelites are about 12,600 miles away which may make the watch off roughly 40 miliseconds unless Citizen has compensated for the distance from the satelite. Those fractions of a second make no difference unless I have some special need for precision.
> 
> ...


Very good first-hand information, thank you :-!

I believe there is certainly no leap second scheduled until at least June 2012.

I wonder if you have seen any practical difference on DST change - my experience with RC models is bad (it usually gets to sync 1-2 days later, but time-radio signal is not very good around my place) and I wonder if on the Appleseed Citizen implemented the 'full auto DST' rules ... as we have on the ezChronos :-d


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## MrMcGoo (Jun 28, 2010)

I have 16 radio controlled Citizens and Casios. All make the change to DST and back to standard time automatically because I have a window with great reception. It's all a matter of signal strength and leaving the watches set to change automatically.

DST changes with the Sat Wave are always manually set. There is no auto change feature. You pull the crown out to the first stop and hit the lower button once. Observe that the 9 o'clock wheel has moved properly, then push the crown in to complete. Most of the time was burned pulling up the manual.

Bill


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## South Pender (Jul 2, 2008)

I don't think that the difference in offset in ms. between satellite and RC receivers is of any importance. The question--at least for me--is whether or not the watch will remain within about 1 second of the reference time at all times. Whether it's 1.010 or 1.020 seconds is of no importance. My feeling now is that the RC concept is the better (but, of course, only for those within range of the nearest transmitter) and could likely be improved further with some attention given to upgrading the receiving antennae. I've noticed that my "atomic" clocks are absolutely reliable in getting the signal every day, but, of course, their antennae are larger.


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## ronalddheld (May 5, 2005)

The RC timepieces do not know the distance to the transmitter. The GPS watch should solve for your position and time, so there is no transit delay.


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## McAllan (Apr 22, 2011)

Since a watch is not designed for any precision reading like a signal out for technical purposes that's more or less completely irrelevant. The speed of light is about 7 times around the Earth pr. second so the few 1000 km to the transmitter is of very little significance. It's only 0.00666... seconds for 2000 km.


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## mikeynd (Dec 11, 2008)

McAllan said:


> Since a watch is not designed for any precision reading like a signal out for technical purposes that's more or less completely irrelevant. The speed of light is about 7 times around the Earth pr. second so the few 1000 km to the transmitter is of very little significance. It's only 0.00666... seconds for 2000 km.


That radio signals travel darn near as fast as light it would be hard,but not impossible to measure any delays that you may receive from Ft.Collins.It sure the heck would not matter..Where i live i can sync day or night easily which makes the GPS watch unattractive to me.


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## Sabresoft (Dec 1, 2010)

Of course we could get totally silly and have a TC watch with RC and satellite sync.


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## ronalddheld (May 5, 2005)

Why is that intrinsically silly? For easy of use, the RC function would be of the most use. The GPS option would cover more of the 24 hour day, but one typically would have to go outside with a clear enough sky for a sync. Maybe the way to go is using bluetooth or RF dongle to connect to a server via Catalin's way?


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## mikeynd (Dec 11, 2008)

ronalddheld said:


> Why is that intrinsically silly? For easy of use, the RC function would be of the most use. The GPS option would cover more of the 24 hour day, but one typically would have to go outside with a clear enough sky for a sync. Maybe the way to go is using bluetooth or RF dongle to connect to a server via Catalin's way?


I am not sure about going with the server way,,my watch may end up getting a virus.haha


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## Sabresoft (Dec 1, 2010)

ronalddheld said:


> Why is that intrinsically silly? For easy of use, the RC function would be of the most use. The GPS option would cover more of the 24 hour day, but one typically would have to go outside with a clear enough sky for a sync. Maybe the way to go is using bluetooth or RF dongle to connect to a server via Catalin's way?


Not saying that it is silly per se, only that wishing for all three in one watch is asking for a lot when it's difficult to even get one of these features. So I was just saying lets be "silly" and wish for the moon.


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## McAllan (Apr 22, 2011)

ronalddheld said:


> Why is that intrinsically silly? For easy of use, the RC function would be of the most use. The GPS option would cover more of the 24 hour day, but one typically would have to go outside with a clear enough sky for a sync. Maybe the way to go is using bluetooth or RF dongle to connect to a server via Catalin's way?


I'd not say it's silly. In the coverage of the transmitters the RC functionality is by far of most use / the easiest use but unfortunately they don't cover every part of the world so here's GPS a candidate - at least until Galileo is up to full coverage. And choosing one of the two must be whichever provides fastest sync and the most penetrative signal.
It is however a technical challenge as the antennas for RC and GPS take up quite a significant amount of space in even a large watch.

TC on the other hand shouldn't be a (technical) challenge at all except getting the marketing department to accept the brilliant idea. The sensor in most TC watches are integrated on the chip already and so the space needed for that is practically nil.


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## Catalin (Jan 2, 2009)

Sabresoft said:


> Of course we could get totally silly and have a TC watch with RC and satellite sync.


The antennas would make that a pretty big option :-d

On the other hand I find VERY disappointing that there is no RC watch (that I know of) that is also TC - given the fact that all RC watches already have pretty major electronics inside, increasing that to the point where you also do TC would be a very small effort in exchange for the resulting claim b-)


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## ronalddheld (May 5, 2005)

If ad men would stop stating the actual atomic clock precision for the RC watches and tell that iut is a once/day sync with daily offset up to s/d, then Maybe there is a chance for a TC watch with RC capacities.
Will a current receiver process GPS and/or Galileo data?


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## What? (Nov 11, 2010)

To answer the original question, your RC watch could be off by about .03 seconds when receiving, depending on how far you are from the tower.

A GPS watch will receive several different satellite signals at the same time, and each of these satellites has an atomic clock in it. By measuring the miniscule difference between the times received by each satellite, and by knowing where all the satellites are, your GPS can calculate to within a few feet where its location is. This is what I call awesome math.

I haven't run the numbers or looked it up, but I'd guess that at receipt time, the GPS would be a thousand or a million times more accurate than the radio watch.

But realistically, with the cheap quartz movement deviating by a half second a day (I had an RC that was worse), the RC system is good enough, and uses a lot less power.

So the real advantage of the GPS in telling time is when it is actively receiving you will have a more accurate reference time, and that you can use it anywhere in the world where you can get a view of the sky.


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## South Pender (Jul 2, 2008)

What? said:


> To answer the original question, your RC watch could be off by about .03 seconds when receiving, depending on how far you are from the tower.
> 
> A GPS watch will receive several different satellite signals at the same time, and each of these satellites has an atomic clock in it. By measuring the miniscule difference between the times received by each satellite, and by knowing where all the satellites are, your GPS can calculate to within a few feet where its location is. This is what I call awesome math.
> 
> ...


Interesting points. I have an Arbiter satellite reference clock that I use for accuracy timing of my HAQ watches. This device can track up to 12 satellites, but I've found that, given my location, I seldom can track more than 6 or 7. Often I'm working with satellite times based on 5 satellites. As I understand it, the device somehow averages or aggregates in some way the slightly disparate satellite times to provide the value it presents. I have often wondered what the difference in accuracy is between a readout based on 5 satellites and one based on 7 or 8. Does anyone have any insights on this?


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## MrMcGoo (Jun 28, 2010)

The Citizen Sat Wave instructions state that accuracy of the watch depends on internal processing of the watch. I take this to mean that the watch will produce lower accuracy if it only receices the signal from one satellite. If multiple satellites are received, then accuracy can be much better in terms of miliseconds. The watch does not indicate how it processed the signal. Personally, I am not interested in perfect atomic clock time. Within a few miliseconds is more than accurate enough for me.

This watch is not designed to find location or elevation via GPS; it only tells time. The trade off is that the battery lasts for 2.5 years without sunlight. Other GPS watches typically have a battery that lasts about 8 hours if constant GPS information is produced. It's a good trade-off if all you want is accurate time.

Bill


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## What? (Nov 11, 2010)

MrMcGoo said:


> The Citizen Sat Wave instructions state that accuracy of the watch depends on internal processing of the watch. I take this to mean that the watch will produce lower accuracy if it only receices the signal from one satellite. If multiple satellites are received, then accuracy can be much better in terms of miliseconds. The watch does not indicate how it processed the signal. Personally, I am not interested in perfect atomic clock time. Within a few miliseconds is more than accurate enough for me.
> 
> This watch is not designed to find location or elevation via GPS; it only tells time. The trade off is that the battery lasts for 2.5 years without sunlight. Other GPS watches typically have a battery that lasts about 8 hours if constant GPS information is produced. It's a good trade-off if all you want is accurate time.
> 
> Bill


My guess is that this watch does not calculate, and simply takes the strongest single satellite signal and sets the time with it.

This would mean that since the satellite is farther away than any of the land-based radio atomic clocks, the time at receipt will be less accurate than with an RC watch.


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## What? (Nov 11, 2010)

South Pender said:


> Interesting points. I have an Arbiter satellite reference clock that I use for accuracy timing of my HAQ watches. This device can track up to 12 satellites, but I've found that, given my location, I seldom can track more than 6 or 7. Often I'm working with satellite times based on 5 satellites. As I understand it, the device somehow averages or aggregates in some way the slightly disparate satellite times to provide the value it presents. I have often wondered what the difference in accuracy is between a readout based on 5 satellites and one based on 7 or 8. Does anyone have any insights on this?


An actual GPS does not average or aggregate. It CALCULATES based on known positions of the satellites.

Therefore, it can figure out the time with a much higher precision than if it simply took the times from all the satellites it is receiving, each of which is delayed by a signicficant portion of a second due to distance, and averaged them.

Want an idea of the accuracy? A GPS can figure your position to within 10 feet. Light travels at 186,000 miles/second.

You do the math.


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## What? (Nov 11, 2010)

South Pender said:


> Interesting points. I have an Arbiter satellite reference clock that I use for accuracy timing of my HAQ watches. This device can track up to 12 satellites, but I've found that, given my location, I seldom can track more than 6 or 7. Often I'm working with satellite times based on 5 satellites. As I understand it, the device somehow averages or aggregates in some way the slightly disparate satellite times to provide the value it presents. I have often wondered what the difference in accuracy is between a readout based on 5 satellites and one based on 7 or 8. Does anyone have any insights on this?


Looks like from Arbiter's website they have satellite clocks of various accuracies.

At a glance it looks like the accuracy of their clocks range from 40ns to 500ns(typical expected accuracy), depending on the model. "ns" is a billionth of a second.

I suppose for scientific purposes you are going to want to grab as many satellites as possible. But for setting your watch I guarantee you there will be zero noticeable difference between 3 satellites and 12 (the theoretical maximum you could see at one time).


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## South Pender (Jul 2, 2008)

What? said:


> Looks like from Arbiter's website they have satellite clocks of various accuracies.
> 
> At a glance it looks like the accuracy of their clocks range from 40ns to 500ns(typical expected accuracy), depending on the model. "ns" is a billionth of a second.
> 
> I suppose for scientific purposes you are going to want to grab as many satellites as possible. But for setting your watch I guarantee you there will be zero noticeable difference between 3 satellites and 12 (the theoretical maximum you could see at one time).


Probably true. However, it's not for setting my watches that I use this, but, instead, for accuracy testing, by which I track the watch's drift from baseline to some time point weeks or months later. Still, I imagine that the difference between the time I get from 6 satellites vs. that from 12 satellites tracked is probably very small, maybe 1 ms. or less. If that is the case, then it won't make any significant difference in my accuracy work to be picking up 6 satellites at certain times, but only 4 at others.


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## What? (Nov 11, 2010)

South Pender said:


> Probably true. However, it's not for setting my watches that I use this, but, instead, for accuracy testing, by which I track the watch's drift from baseline to some time point weeks or months later. Still, I imagine that the difference between the time I get from 6 satellites vs. that from 12 satellites tracked is probably very small, maybe 1 ms. or less. If that is the case, then it won't make any significant difference in my accuracy work to be picking up 6 satellites at certain times, but only 4 at others.


Could probably pull more exact figures off their website, but I'd guess you'd be within a millionth of a second even with only 4 satellites.


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## ronalddheld (May 5, 2005)

You only need 4 satellites to calculate lat, lon, alt, and time. More satellites give a better solution, but probably not perceivable on a watch.


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## everose (Jan 15, 2010)

It seems RC is getting an upgrade in USA.

If this upgrade works well then i wonder if it could impact GPS timekeeping sales in the US RC area? :think:

From Phys.org:
http://phys.org/news/2013-03-code-boost-reception-radio-controlled-clocks.html#nwlt


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## c_malc (Jan 5, 2013)

According to this article from NASA each of the 24 GPS satellites carries 4 atomic clocks, so one satellite will be good enough to set your watch.


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## everose (Jan 15, 2010)

c_malc said:


> According to this article from NASA each of the 24 GPS satellites carries 4 atomic clocks, so one satellite will be good enough to set your watch.


Yes,....But that would not allow Astron (and/or other activity pieces) to work their positioning/timezone sync magic though.


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## Hans Moleman (Sep 24, 2007)

c_malc said:


> According to this article from NASA each of the 24 GPS satellites carries 4 atomic clocks, so one satellite will be good enough to set your watch.


A satellite is quite far off. It takes a while for the signal to travel to earth.
The distance from the satellite to wherever the listener is located varies upon the location of the listener.
A correction for that delay can't be build in.

If you're after the time accurate to within milliseconds or less you need multiple satellites.

If you don't mind if your time is a fraction of a second out, you're right, one satellite will do just fine.


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## ronalddheld (May 5, 2005)

The motion of each satellite is included in the data sent out.


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## everose (Jan 15, 2010)

I believe GPS Satellites do not rely upon their on board Atomic references only. Afaik their on board references are also compared to other sources including the main Atomic Reference at Boulder.

I found this info very interesting from USNO.about time transfer. I don't remember seeing it posted in the forum before. Apologies if it was.

USNO GPS Time Transfer - Naval Oceanography Portal


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## Eeeb (Jul 12, 2007)

everose said:


> I believe GPS Satellites do not rely upon their on board Atomic references only. Afaik their on board references are also compared to other sources including the main Atomic Reference at Boulder.
> 
> I found this info very interesting from USNO.about time transfer. I don't remember seeing it posted in the forum before. Apologies if it was.
> 
> USNO GPS Time Transfer - Naval Oceanography Portal


I didn't realize they still did voice time announcements ...


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## c_malc (Jan 5, 2013)

Hans Moleman said:


> A satellite is quite far off. It takes a while for the signal to travel to earth.
> The distance from the satellite to wherever the listener is located varies upon the location of the listener.
> A correction for that delay can't be build in.
> 
> ...


Yes you're right, I stand corrected, thank you. It's this very fact of signals from different satellites arriving at slightly different times that allows position to be calculated. Plus its far from an insignificant delay (over 1/10th of a second)



ronalddheld said:


> The motion of each satellite is included in the data sent out.


This is a good point but I think I was still wrong: 
Each satellite must send it's own position included in its data transmission or how else could positional calculations be triangulated ? But if your watch doesn't know where *it *is then it's still not possible to calculate the delay from just one satellite.


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## everose (Jan 15, 2010)

Hans Moleman said:


> A satellite is quite far off. It takes a while for the signal to travel to earth.
> The distance from the satellite to wherever the listener is located varies upon the location of the listener.
> A correction for that delay can't be build in.
> 
> ...


According to the links below it would seem the "delay" between the satellite/s and the receiver-device is apparently a vital part of the process in order for the receiver to calculate very accurate location and/or time.

Ironically it seems updating the display/interface may be one of the most common causes of time errors for GPS receivers, (for those receivers which do not put a high priority on updating the display) ......which would seem to be somewhat of an "own goal" o|


More detailed info here:
The role of GPS in precise time and frequency dissemination: http://ilrs.gsfc.nasa.gov/docs/timing/gpsrole.pdf


...And from Wikipedia:
Atomic clock - Wikipedia, the free encyclopedia

"
*Global Positioning System*

The Global Positioning System (GPS) provides very accurate timing and frequency signals. A GPS receiver works by measuring the relative time delay of signals from a minimum of four, but usually more GPS satellites, each of which has four onboard caesium or rubidium atomic clocks. The relative times are mathematically transformed into three absolute spatial coordinates and one absolute time coordinate. The time is accurate to within about 50 nanoseconds. However, inexpensive GPS receivers may not assign a high priority to updating the display, so the displayed time may differ perceptibly from the internal time. Precision time references that use GPS are marketed for use in computer networks, laboratories, and cellular communications networks, and do maintain accuracy to within about 50ns.
*[edit]Time signal radio transmitters*

A radio clock is a clock that automatically synchronizes itself by means of government radio time signals received by a radio receiver. Many retailers market radio clocks inaccurately as atomic clocks;[SUP][27][/SUP] although the radio signals they receive originate from atomic clocks, they are not atomic clocks themselves. They are inexpensive time-keeping devices with an accuracy of about a second. Instrument grade time receivers provide higher accuracy. Such devices incur a transit delay of approximately 1 ms for every 300 kilometres (186 mi) of distance from the radio transmitter. Many governments operate transmitters for time-keeping purposes.*"*


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## Hans Moleman (Sep 24, 2007)

everose said:


> Ironically it seems updating the display/interface may be one of the most common causes of time errors for GPS receivers, (for those receivers which do not put a high priority on updating the display)


Too right, this incredible accurate time is in every GPS receiver and chip. The only hard part is getting to it.

And if a smart-phone user claims they have the accurate time on their phone they're probably right. Only they don't get to see it and use it!

Oh the joy of half truths.


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## everose (Jan 15, 2010)

Hans Moleman said:


> .........if a smart-phone user claims they have the accurate time on their phone they're probably right. Only they don't get to see it and use it!
> 
> Oh the joy of half truths.


Hans you really have a talent for encapsulating the essence of a (sometimes) complex point in a practical, interesting, sometimes amusing and easy to understand way for those of us who are less technically inclined.

In the past i have thanked you ((in my mind)) many times for this admirable quality of yours. I thought i should perhaps get around to actually thanking you
in a post.... *"Thank You !"*


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## Hans Moleman (Sep 24, 2007)

everose said:


> Hans you really have a talent for encapsulating the essence of a (sometimes) complex point in a practical, interesting, sometimes amusing and easy to understand way for those of us who are less technically inclined.
> 
> In the past i have thanked you ((in my mind)) many times for this admirable quality of yours. I thought i should perhaps get around to actually thanking you
> in a post.... *"Thank You !"*


Wow! Thank you.


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## ronalddheld (May 5, 2005)

isn't the display delay fixed and could be compensated for?


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## Andrew McGregor (Dec 27, 2011)

ronalddheld said:


> isn't the display delay fixed and could be compensated for?


It is, and it could. You simply calculate what it should say when it is finally visible, and draw that. In fact, the clock widgets on some computers actually do this (the main one I know does, because I read the code, is xclock, widely available on Linux and unix variants, including MacOS X).


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