[OSM-talk] Camera and Dictaphone clocks

[Andy Robinson]

On 24/02/2008, David Earl <david at frankieandshadow.com> wrote:
> I mentioned in the thread about continuous audio mapping yesterday that
>  the audio was appearing to drift away from the corresponding waymarks
>  after a while. I have now determined that this is due to the inaccuracy
>  of the clock in my dictaphone. I have implemented a solution for audio
>  in JOSM - see below - but this has implications for using camera images
>  as well.
>
>  I recorded three hours off the radio this morning to include the pips
>  at 9am and noon (for non Britishers: Greenwich time signal, an accurate
>  clock which is broadcast on the hour most hours on BBC Radio 4). A
>  suitable alternative would be to track the (very accurate) GPS clock,
>  but that would have meant going outside.
>
>  I then measured the length of the recording between the start of each of
>  the long pips in Audacity, whose timer is derived from the audio
>  sampling rate and therefore reflects the clock in the dictaphone. Sure
>  enough it came out at a remarkably inaccurate 3h00m17s (0.157% out):
>  that means that my dictaphone loses about 2 minutes day!
>
>  I would not be at all surprised to find that camera clocks are also
>  similarly inaccurate - they probably use the same silicon! A similar
>  test for a camera is obviously to photograph a GPS display several hours
>  apart and compare the time difference the image time stamp says with the
>  GPS time difference shown.
>
>  The consequence is that synchronising the time by saying "NOW" or
>  photographing the GPS clock is _not_ sufficient. If you're using voice
>  or pictures just to annotate waypoints, it's just a minor inconvenience
>  - the picture or sound isn't quite where you thought it would be on the
>  track, but the waypoint is from the GPS, so accurate.
>
>  But if you're using the synchronised timestamp or offset into a
>  recording to imply a feature's location, you could be 50m or more out
>  after two-hours of a bicycle mapping session (10s error @ 5m/s).
>
>  For the new audio facilities in JOSM, I've added a voice recorder
>  calibration box into the Audio Preferences panel (will be in tomorrow's
>  build). You measure the purported length of a long chunk of audio to the
>  true time, and enter the ratio, so in my case this is 1.00157. You could
>  do this specially as I did, or during a mapping session just by putting
>  sync cues at either end of your recording. You can measure it either in
>  Audacity or similar as I did, or in JOSM itself by inserting audio
>  markers at the geographical location of the sync cues and noting the
>  time time difference of labels (dG), sync at the first mark, and then
>  time the difference between the second mark and the second audio cue in
>  the commentary (dA). The calibration is then (dA+dG)/dG. Of course if
>  your device clock runs fast, you might have to go back a few seconds,
>  and your ration would be a bit less than 1.
>
>  Of course, this assumes that the clock runs at a constant, if incorrect,
>  rate both during a session, and, if you don't recalibrate each time then
>  between sessions as well.
>

My canon Ixus 55 before I broke it was spot on during a session and
drifted about a second a day otherwise. I'm now back using my old
Olympus 2.2megapixel monster and that I find looses about 5 seconds
every 30 mins of mapping but none (well effectively none) between
sessions. It seems therefore that some action of the camera, perhaps
during writes to the card or something does affect the clock and I
have to manually sinc to the track several times for a long mapping
session.

Cheers

Andy


-- 
Andy Robinson