Fluorescent lights keep smartphones from ten-second drifts

Jul 28, 2012 by Nancy Owano weblog
Fluorescent lamps

(Phys.org) -- One obvious way to reset a mobile device's clock is to check out a master clock online but Zhenjiang Li, a computer scientist and doctoral candidate at Hong Kong University of Science and Technology, and colleagues have another way of keeping devices time-worthy. Li and co-authors from universities in China, Singapore, and Illinois, have demonstrated that any mobile device, including a smartphone, that has a light sensor or camera can use the flicker from fluorescent lights to keep accurate time. They have authored a paper titled, "FLIGHT: Clock Calibration Using Fluorescent Lighting," to explore how smartphone clocks can keep precise time via the flicker of fluorescent lights.

“Starting from static deployment in the laboratory, our measurements validate that fluorescent lighting is able to serve as a viable reference for time ,” they wrote.

Most electronic devices use vibrating crystals to regulate their internal clocks. External factors like humidity and temperature, however, can throw off these frequencies to the point where the clocks “drift,” and that drift may be by nearly ten seconds per day, in some devices.

The team calls their new clock synchronization approach FLIGHT, which makes use of the fact that the light intensity from fluorescent lamps varies with a stable period. FLIGHT does not require any extra hardware and radio operations, noted the authors. By sampling a light sensor or camera, FLIGHT can intelligently extract periods for the clock calibration and retain a common notion of time in the system.

By tuning to the light emitted from indoor fluorescent lamps, FLIGHT can intelligently extract the light period information and achieve network wide time calibration by referring to such a common time reference. The researchers said they implemented FLIGHT in TelosB motes. Their experiments used a 12-node test-bed in static and mobile environments.

“Over one-week measurement suggests that, compared with existing technologies, FLIGHT can achieve tightly synchronized time with low energy consumption,” they stated. The researchers think their scheme is accurate to less than a thousandth of a second per day. Since it needs no network connection, they note that the approach carries the benefit of saving a device’s battery life.

The team is to present this work at the MobiCom conference (the 18th Annual International Conference on Mobile Computing and Networking) in Istanbul, Turkey, from August 22 to August 26. MobiCom is a gathering of professionals focused on both mobile computing and wireless and mobile networking. Talks and presentations will address networks, systems, algorithms, and applications that support “the symbiosis of mobile computers and wireless networks.”

Based on the discussion in the authors' paper, there can be an easy and useful relationship between wireless devices and fluorescent lights. They point out that the or camera has been widely embedded in commodity wireless platforms. This, they suggest, offers a great opportunity for plenty of indoor applications to maintain common time by referring to the light emitted from fluorescent lamps. The fluorescent lights in the home or office are actually flickering at regular intervals, though it may be too rapid a for the eyes to notice.

Reactions by science writers elsewhere have reacted favorably to the concept. “Artificial lighting is all around us, and it is a fantastic keeper or time,” said PopSci. “Alternating current provided by power mains runs at a frequency of 50 or 60 cycles per second, with the power delivered dropping to zero twice per cycle.”

Explore further: Tricorder XPRIZE: 10 teams advance in global competition to develop consumer-focused diagnostic device

More information: www.cse.ust.hk/~lzjiang/2012-MobiCom_FLIGHT.pdf

via Newscientist, Popsci

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User comments : 19

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Eikka
3.7 / 5 (7) Jul 28, 2012
The frequency of the fluorescent tube flicker is not stable, because the grid frequency is allowed to drift depending on production and demand. It's one of the indicators of the state of the grid, and keeping the frequency exact is actually impossible because it would require the exact matching of supply and load all the time.

The reason why clocks synchronized by AC remain accurate is because they're constantly connected to the grid. The actual frequency is off by 2-3% almost all the time, but is kept at the average nominal frequency over the span of about a day or two, so the clocks never drift too far from the correct time.

A cellphone would only see the lights every time you take it out of your pocket, so they can't keep a tally of the correct frequency and will drift off by about five minutes a day, which is much much worse than any crystal device they already have.
antialias_physorg
3.4 / 5 (5) Jul 28, 2012
Probably also will have a problem when you fyl transatlantic/transpacific. Some countries have 50Hz, some countries have 60Hz.

If they take their Illinois-calibrated prototype to the conference in Turkey they're in for a big surprise.
SatanLover
5 / 5 (1) Jul 28, 2012
Probably also will have a problem when you fyl transatlantic/transpacific. Some countries have 50Hz, some countries have 60Hz.

If they take their Illinois-calibrated prototype to the conference in Turkey they're in for a big surprise.

it can be detected.
WirelessPhil
5 / 5 (2) Jul 28, 2012
One problem, the new bulbs coming out are different, so this may no longer work.
Light-bulb standards mean you'll need new fluorescent fixtures, will pay more for halogen spotlights
Published: Monday, July 09, 2012, 6:00 AM
http://www.clevel...y_s.html
chardo137
5 / 5 (1) Jul 28, 2012
This is brilliant! They are elegantly accomplishing the same thing as having a much more precisely tested and tuned quartz crystal, along with an array of sensors to detect and compensate for ambient conditions. The 50 or 60 cycle frequency of the grid really does not change much, but they are using it as a calibration device, not the actual time keeper.
Mike_Massen
Jul 28, 2012
This comment has been removed by a moderator.
Eikka
4.2 / 5 (5) Jul 28, 2012
The 50 or 60 cycle frequency of the grid really does not change much, but they are using it as a calibration device, not the actual time keeper.


At this very moment, where I'm at, the grid frequency is 49.98 Hz. If my cellphone was calibrated with that frequency by assumption that it was 50 Hz, it would be off by 34 seconds in 24 hours.

http://www.dynami...grid.htm

Even with the existing crystal oscillator it has, it will drift less than that in a month. The AC grid as a frequency standard is simply inferior to even the cheapest dollar store digital wrist watch.
trekgeek1
5 / 5 (2) Jul 28, 2012
It would be easier to use GPS I think. The GPS in a phone is already calculating the time from GPS satellites as part of the position determination. You could use it a couple of times a day to correct the time. You could also just have the phone get a time correction from the cell towers. They already read control signals from the towers. You could have the phone grab that info from a web server just like your computer does.
Sonhouse
5 / 5 (1) Jul 28, 2012
They could also use the method developed on clocks they call atomic time, my Casio watch has it: a receiver tuned to 60 Khz that is the time hack of WWV, which is atomically referenced, never off by even a millisecond. That would work in the US anyway, most of the US is covered by the transmitter in Fort Collins Colorado, close to the middle of the USA. My watch listens to that signal 3 times in the morning, 3,4,5 AM I think, and if the signal shows up at any of those times it re-syncs with WWV so the watch is pretty much EXACTLY on time. Its quartz crystal is accurate enough so even if a week goes by, it is less than a second off when it syncs.
Eikka
5 / 5 (1) Jul 28, 2012
You could have the phone grab that info from a web server just like your computer does.


It's already done that way. The network sends the correct time to the phone where available.

The question here is calibrating the oscillator of the clock to a known frequency to set the beat, so that it would keep the time. The problem is that the grid frequency is not a known and reliable frequency standard without polling through the internet and asking what the exact frequency is supposed to be, which will be off by the time the answer gets back to you.

And there's a third problem: electronic ballasts in fluorescent lights and compact fluorescent tubes sometimes upconvert the frequency through an intermediate oscillator precisely to prevent discernible flickering.
Neurons_At_Work
5 / 5 (1) Jul 28, 2012
And there's a third problem: electronic ballasts in fluorescent lights and compact fluorescent tubes sometimes upconvert the frequency through an intermediate oscillator precisely to prevent discernible flickering.

This is the biggest deal, and it's not so much 'sometimes' as 'almost always' in this day and age. I've done facility engineering for roughly 35 years and wherever I've been in the US there's been a major push to install or retrofit lights with electronic ballasts. The old T12 bulbs and magnetic ballasts have been restricted by law for years, and pretty much all the T8 and T5 bulbs use high frequency ballasts for efficiency. They run from 20-60 KHz and that varies by brand, so would be useless as a time base.
Sonhouse
5 / 5 (1) Jul 28, 2012
You could have the phone grab that info from a web server just like your computer does.


It's already done that way. The network sends the correct time to the phone where available.

The question here is calibrating the oscillator of the clock to a known frequency to set the beat, so that it would keep the time. The problem is that the grid frequency is not a known and reliable frequency standard without polling through the internet and asking what the exact frequency is supposed to be, which will be off by the time the answer gets back to you.

And there's a third problem: electronic ballasts in fluorescent lights and compact fluorescent tubes sometimes upconvert the frequency through an intermediate oscillator precisely to prevent discernible flickering.

That would be flickering discernible by human eyes, it would go from 60 or 50 Hz to 120 or 100 Hz, still available as a time hack. Maybe in Japan the grid frequency is more tightly controlled than in the US.
mrlewish
3.7 / 5 (3) Jul 28, 2012
Why the "research"? You phone gets its time set by the local tower it is connected to.
Eikka
3 / 5 (2) Jul 28, 2012
Maybe in Japan the grid frequency is more tightly controlled than in the US


Interestingly enough, Japan has both 50 and 60 Hz grid frequencies in use in different parts of the country.

legonadir2
1 / 5 (3) Jul 28, 2012
To original poster. They could be calibrating it to a particular wavelength of the light the camera can pick up.
Neurons_At_Work
not rated yet Jul 29, 2012
That would be flickering discernible by human eyes, it would go from 60 or 50 Hz to 120 or 100 Hz, still available as a time hack. Maybe in Japan the grid frequency is more tightly controlled than in the US.

On a standard magnetic ballast, the bulb flicker is inherently 100/120 Hz. Each half cycle the bulb lights, then goes out as the voltage passes through zero, then fires again on the next half cycle. So ideally one would not be able to perceive the flicker, although I was often able to and it drove me crazy. I was SO glad when the high frequency ballasts came into general use. They operate above 20 KHz for a reason, also--so as not to assault the ears instead of the eyes. It's technically ultrasonic. Wonder what bats and dogs have to say about that?
DGBEACH
5 / 5 (3) Jul 29, 2012
And once we completely switch over to LED lighting, we could embed the time data right into their driver circuitry, and they would be transmitting the time and myriads of other pieces of information before our very eyes, without us even perceiving it.
The Teacher
1 / 5 (2) Jul 29, 2012
Thanks for reminding me of quantum electrodynamics, Nancy Owano. What a world in discovery of an old fact....
The Teacher
1 / 5 (2) Jul 29, 2012
Thanks for reminding me of quantum electrodynamics, Nancy Owano. What a world in discovery of an old fact....
RankineCycle
2.3 / 5 (3) Jul 29, 2012
Neat concept but no practical use, too many better options not to mention the time accuracy on Joe Sixpack's mobile device isn't terribly vital. It reminds me of when I worked on PC's and quite often the time and/or time zone were wrong and the owner didn't care or didn't notice.

Pretty much everything made in the past 10 years has a switching "electronic" ballast. The oscillation can still be detected as I've seen it interfere with infrared remote controls for ceiling-mounted projectors, but it is basically an LC or 555-esque oscillator and isn't going to have any worth in timekeeping.