Now that you mention it!

I use the app Emerald Sequoia watch that gives me time from 4 servers but it does not change the phone time.
Seconds to one decimal place.

Raj, vu2zap

At 01/04/2015, you wrote:

Hmm.  I'd say that the time setting accuracy may have improved, but the
timekeeping accuracy still isn't wonderful.

I just checked my iPhone, using the Emerald Time app to display the
difference between iOS time and NTP time.  The local time was 850 ms fast.
Then I went to Settings->General->Date and Time and turned "Set
Automatically" off, then back on again.  This seems to have forced the
phone to resync its local time, and now the error is zero (less than 1 ms).

I also have an iPad which had been configured not to set the time
automatically.  It was 2.5 seconds fast.  When I turned automatic setting
on, it reset its local time to within 2 ms of correct.

So the setting appears to be nice and accurate.  But the iPhone, which has
been in "automatic time set" mode ever since the iOS update 3 weeks ago,
was nearly one second off.  I don't know how often iOS does a time sync, so
I don't know how long it took to drift by that much.  This amount of error
does suggest that it only periodically resets itself to the correct time,
without trying to correct the local oscillator to provide continuously
accurate time (like using NTP would do).

• Dave

On Tue, Mar 31, 2015 at 11:59 PM, Anthony G. Atkielski <
anthony@atkielski.com> wrote:

Hi,

First email to the list here. I've been curious about this for a little
while: does the iPhone get sub-second timing info from GPS?

I did an experiment the other day where my friend and my iPhone clocks had
a fairly large offset, of perhaps 0.3 seconds. We were away from any
windows.

We went to a window, and watched as the Google maps location estimate got
to its most accurate setting. I assume the estimate accuracy is based on
the number of satellites the phone can see. Once we got this lock, the
clocks were then synchronized to within a few ms of each other. I estimate
8 ms, but our method (listening to ticks from a homemade app) was not ultra
accurate.

This seems to imply that the iPhone does get sub-second timing info from
GPS. Can anyone confirm/deny this?

On Wed, Apr 1, 2015 at 8:03 AM, Raj vu2zap@gmail.com wrote:

On Wed, Apr 1, 2015 at 9:33 AM, Reid Oda reid.oda@gmail.com wrote:
... Once we got this lock, the

Sound travels at about 1 foot per ms.  So you can effectively delay
the click on one phone my placing it 8 feet farther away then the
other  phone.  Adjust the distance until  the clicks seem to occur at
the same time.  That said, I doubt your ears are sensitive enough to
work at the single digit ms level.  Some musicians have learned to
keep in time and can hear a 20ms difference.  Most people can hear a
50 ms delay.    But as the clicks get close we hear them as one sound

If you have a computer with a microphone yo can record the clicks at
some high sample rate and then look at the spike on the audio
waveform.  This WILL sub-ms clicks.

--

Chris Albertson
Redondo Beach, California

On Wed, Apr 1, 2015 at 12:33 PM, Reid Oda reid.oda@gmail.com wrote:

As of iOS N where I believe N == 5 it does the equivalent of calling
ntpdate every few hours if the network is available.  I assume it uses the
mobile system if the network is unavailable and you have a cellular radio
because the devices (can) have a lot of drift.

On Wed, Apr 1, 2015 at 2:29 PM, Chris Albertson albertson.chris@gmail.com
wrote:

Thanks for the idea of using distance to determine time offset. That's
pretty clever. :)

As for perceiving ms-order offsets of audio events (if you're interested):
it depends on the sound. The sharper the tick the easier it is to perceive
a timing offset. The tick I use is extremely sharp and I can (and I bet you
could too) be able to tell the difference between a 2 and 8 ms offset. 2 ms
sounds like a slightly phasey version of a single tick while at 8 ms you
can clearly hear 2 sounds.

My research is about audio synchronization, which is what got me interested
in clocks.

On 4/1/2015 2:31 PM, Paul wrote:

The phone has to keep synch within a few microseconds of the network.
There was a time when operators were very sloppy about clock time and
really only worried about network frequency, but most operators are now
maintaining 50 ns or so at the base stations and have to maintain within
5 microseconds to meet the LTE specs.  How much of this is preserved
through to the user interface is anyone's guess.

--
mailto:oz@ozindfw.net
Oz
POB 93167
Southlake, TX 76092 (Near DFW Airport)

On Apr 1, 2015, at 1:29 PM, Reid Oda reid.oda@gmail.com wrote:

I use an iPhone app called "Atomic Clock". It syncs with an NTP server of your choice, but it also has a particularly sharp "tick" on the seconds, and a different-toned tick at the top of the minute.

The clock also has an outer circle that can be configured to complete one rotation every second, which is great for comparing between clocks.

--
David Glover-Aoki | https://david.gloveraoki.net/contact
PGP key 5518C7DE  | Amateur Radio KJ6TLX