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Re: Drop test synchronisation

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  • Re: Drop test synchronisation

    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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