ExifTool Forum

This is yet another traffic camera (for motorcycle this one) with a weird storage of the GPS data. There was no way I was going to install unsigned software from an unknown developer on any of my computers, so I spend some time reverse engineering the data in the video. The MP4 from the device have three tracks: video, sound and a text track. I've extracted the data, and despite being labelled as a text track the content is all binary. I was unable to read the data with any of the tools I tried.

A sample video and the output of exiftool --ee sample.mp4 are available for download at https://drive.google.com/open?id=140YsTjfDC_iCW0a4UbTBjOwSRxc0psqj.

After looking at the data for a while, and with help from some friends, we found out that the first part of the data is simply xored with 0xaa. Once this is done the data appear as a string with a bunch of zeros thrown in. there is more data in there I don't know what it's for, but here is some quick and dirty python code decoding it:


#!/usr/bin/env python
import sys

for line in sys.stdin:
    # cleanup the line
    # Fist by removing the prefix and the carriage return
    line = line[40:-1]
   
    # For some reason the data is not directly readable.
    line = "".join(chr(ord(c)^ 0xaa)  for c in line)
   
    # Extract known data, avoiding the wrath of zeros all over the place.
    date = line[0:14]
    location = line[30:49]
    maybe_compass = line[54:57]
    no_clue = line[165:177]
       
    print date, location, maybe_compass, no_clue


Here is the output I got:


20190820075157 N48515873E002197769 031 -008-019+074
20190820075158 N48515928E002197833 034 -020-026+102
20190820075159 N48515986E002197880 036 -006-025+094
20190820075200 N48516048E002197939 037 -023-025+095
...


First is a timestamp, followed by the position in decimal minutes, then the bearing and finally what I suppose is the accelerometer data.

I was wondering if it would be interesting to integrate this decoder (probably used by other devices) in exiftool. I haven't checked the photos taken by the device yet, I'm wondering how those are tagged.

Try the attached config file with this command:

exiftool -config blueskysea.config -ee -G3 --text FILE

There are a bunch of floating point values that start at offset 0xba.  I'm decoding these as "AccelerometerData".  The first value is 0, and the next 3 match the strings of the tag I'm decoding as "Accelerometer", but these look more like angular accelerations to me.  Then another 0 followed by 3 numbers that look very similar to these accelerations.  Then a bunch more that I haven't looked at in detail.  If you figure out any of these, let me know.

- Phil

Edit:  Here is a plot of the 3 text accelerometer readings divided by 100, plus the first 8 of the floating-point values.  Note that you series 1,2,3 (the accelerometer text) lie exactly underneath the series 5,6,7 lines of the floating point values 2,3,4.

Here are the floating point numbers (AccelerometerData values) plotted in groups of 3.  The first plot shows values 2-4, then 6-8, 10-12, 14-16 and 18-20.  I'm guessing that the accelerometer is sampled at 5 Hz, and these are all x,y,z accelerations in units of g.  I haven't looked at values 22-24 yet.



- Phil

OK, here are values 22-24.  I've plotted these using points instead of lines because there are a lot of zero values (buy why?).  They are definitely related to the accelerations, but I don't know how.



- Phil

Thanks a lot for the config file, very useful to reconstitute a GPX file.

This device tags video when it detects some impact event on the accelerometer. I suppose the values are storing how the device decides what constitute a relevant impact. Plus in that particular video there was no hard acceleration or braking, so everything is probably quite smooth.