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Re-syncronising data from multiple devices
#1
Dear all, 
I was experimenting in the Lab with a SDoF system to see if we might be able to make measurements with multiple devices (using the remote control functionality) and re-align after export using the system time data in the meta folder. Both devices (an iPhone and a Samsung), were sitting on the vibrating mass. 
I did the necessary post-processing in Excel, using the delta between the two but I cannot get the time realignment as close as I had hoped/expected. Here you can see the data for the two phones (iPhone first, Samsung second) when they were both on top of each other on the mass. I determined the delta (-8.440 s) using the system time values for each device and adjusted the time trace for the iPhone based on that to align the data.
As you can see below, they are still a little bit misaligned (but SO frustratingly close!). I noticed there may be some DC offset on both so detrended and subtracted the mean but, even tho' a bit better (and you can see they are measuring the same vibration), it is not good enough.
   
It would be possible to "fix" the issue with cross-correlation to find the additional time realignment required but I had hoped to get closer than this.
I wonder if anyone has any thoughts or comments on this.
Thanks, Ben


Attached Files
.xlsx   Mass Acceleration with g Samsung 15-06-16.xlsx (Size: 480.58 KB / Downloads: 109)
.xlsx   Mass Acceleration without g iPhone 15-06-10.xlsx (Size: 323.71 KB / Downloads: 93)
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#2
Unfortunately, system times on smartphones can be quite out of sync, up to several seconds. Sites like https://www.time.gov display how much the clock is off. You should take the result with a grain of salt: the rather simple algorithm is described on the “about” page…

Utilising these offsets, you might get a better time synchronisation. We might include something like this in future phyphox versions, however, we certainly need to do some research on the best way – and privacy protection.

There also appears to be an offset in the amplitude that could have many causes, hardware and software. Please note that “linear acceleration” is a virtual sensor that typically includes data from gyroscope and magnetometer, so-called sensor fusion.
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