View Full Version : Re: Drop test synchronisation

Allan Carman, X 7455
08-17-2000, 11:33 PM
To All,

Further to the method described by Ton van de Bogert for the
synchronisation of analogue and video data.

We to have a Motion Analysis System (four camera, 60 Hz), and I wanted
to test the synchronisation between force platform and video data. I
too used the method described. (I dropped a reflective coated solid
wooden ball, 3 cm. diameter, on the force plate). Reconstructing the
three dimensional position of the ball during the descent, height,
velocity and acceleration was calculated at three different points
during the decent. An average time to impact calculated and compared
to the time to impact as recorded on the force data (1,000 Hz). This
was repeated for 6 drop trials.

I found a delay of 0.008 sec (stdev = 0.001) in the video data as
compared to the force data. As noted by Ton van de Bogert, this was
approx. 1/2 my video sampling frequency (1/120 = 0.0083 sec).

To get to the point. The older version of Motion Analysis that we have
records the trials on video tape which is later played back
one-by-one through a video processor. Synchronisation is by audio tone
placed on each tape at the time of recording. I suspected at the time,
that on the trigger signal reaching the Motion Analysis system the
audio tone was being placed on the next video frame recorded, hence
depending on when the trigger was pressed the delay could be in the
range of 0 to 1/60 sec but would average out at 1/120 sec (1/2 the
frame rate). However, I do not know the inner workings of the
recording system to confirm this.

Allan Carman
PhD candidate
School of Physiotherapy
University of Otago.

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