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View Full Version : Re: Drop test synchronisation



Thomas Seeholzer
08-22-2000, 02:04 AM
Hello all,

Perhaps this is a little too simple: Did you keep in mind that the NTSC
standard is not 30 frames per second, but only 29.97 frames per second?
This small difference will add up to one frame in about 30 seconds.

Perhaps the video equipment in question is working with multiples of 29.97
and not with exactly 120.0 or 240.0 Hz?

Thomas Seeholzer
SIMI Reality Motion Systems GmbH
Tel: +49 89 321459-0
Fax: +49 89 321459-16
Mail: seeholzer@simi.com
Web: http://www.simi.com

>
> To all,
>
> a couple of years ago I did some experiments with the Motion Analysis
> System (four cameras FALCON HR240 (240 Hz), Software EVa 4.0, National
> Instruments AD-converter AT-MIO-64F-5, Kistler force plate
> 9287A) and I
> also had syncronisation problems.
>
> I tried to check syncronisation dropping a stick (with a reflective
> marker attached to its top end) vertically on the force plate, so that
> it bounced off the plate a couple times before it fell over.
>
> As I remember, I tried different video frequencies (at least 240Hz and
> 120Hz) with force platform data at 960Hz but did not find the
> dis-syncronisation dependend on video frequency.
> What I found was, that the offset between video and force
> data increased
> with recording time which means that either the video or the force
> sampling rate (or both) did not exactly have the value I set them to.
>
> I did least square fits of the video data during the first free fall
> phases of the stick for every trial. Trying to reproduce gravitational
> acceleration as close as possible I found out that the video
> rate error
> was ~1.e-3, so I assumed that the sampling rate of the force data
> significantly incorrect.
>
> Arnim Henze
> Institut f\"ur Astronomie und Astrophysik - Abteilung Biomechanik -
> Auf der Morgenstelle 10, D-72076 T\"ubingen, Germany
> email: henze@tat.physik.uni-tuebingen.de
> Tel.: ++49 7071 29 78654
> Fax : ++49 7071 29 5889
>

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