View Full Version : video and force synchronization

Kieran Moran
10-03-1996, 11:08 PM
Dear fellow subscribers,

As I am sure you are all aware accurate quantification of joint moments
and powers requires synchronized kinematic and actor-environment
reaction force data sets. When the source of kinematic data is from
video images the problem exists of how to perform this synchronization

A general strategy employed is to record the 'video out signal' on a
spare analogue channel simulatneously with the force data. Then by
identifying certain characteristics of the video signal (i.e the
vertical blanking pulses, VBP) a more exact temporal location of when
the recording of a video image field is being initaiation can be
determined. Finally, in order to identify which video field corresponds
to the start of the force-time data a pulse can be sent from the
computer at the start of data collection to light an LED. This LED is
located in the video camera view.

Employing a similar approach to this O'Conner et al. (1995) presented a
method whereby a simple computer program identified vertical blanking
pulses which correspond to the start of a video field.

********* I am trying to find other researchers (or the original
authors) who have implimented their approach. I wish to know what
limitations people have found with this method, if any, and whether they
correspond with mine (see below). If the limitations are substantiated I
will continue with a small project aimed at overcoming these problems.

LIMITATIONS (as I see them):

(1) the video signal must be sampled at extremely high rates to allow
successful recording of two successive vertical blanking pulses which
are of very short duration (1.2 ms).
(2) the sampling rate must NOT be multiples of the frame rate, resulting
in rates that complicate the subsequent kinetic calculations. This is
because the frame rate and force sampling rate will be different.
(3) the collected digital data of the video signal must be exported to
apply the computer program. However, not all commercially available
software analysis systems have this facility (i.e. Kistler's Bioware).
(4) the program may theoretically identify horizontal blanking pulses as
vertical blanking pulses, resulting in erroneous synchronization.

Any comments you send will be most appreciated and summarised for all

Kieran Moran (Ph.D student)

P.s. I am aware of Kistlers own 'video synch' device.

Kieran Moran ka.moran@ulst.ac.uk
Biomechanics Section
Northern Ireland Bioengineering Centre
Northern Ireland
00 44 1232 368996