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gideon
12-08-1994, 11:56 AM
Dear Biomch-L Subscriber:
After corresponding with few of the Subscribers I can clear my
statement about the video fields/second problem.
1. The crystal frequency of the video cameras is extremely stable.
2. In order to digitize manually or automatically 60 Hz video signal
there is a shift due to the Interlace signal . This can result in an
error of 1 field.
3. If data is collected with 2 video cameras, as long as gen lack
is not used, another discrepancy of one field can be occurred.
4. If you rely on the VCR counter, it seems that the counter is not
consistent with the Panasonic 7350 and lack on the even or add fields at
random. This can result in additional one field discrepancy.
5. If the system grab in RAM multi frames, and then after the
digitizing this frames, lets say 16 frames, needs to find the position
on the VCR again, there is another discrepancy of one to two fields.
6. If the user intended to collect analog data such as Fore Plate,
EMG etc. with the video, synchronization may be another source of error
up to 2 fields.
All the above potential problems can be resolved by grabbing the
fields digitally to the hard disk and then process digitally the frames
to correct for potential errors in synchronization. Then synchronization
of external signals such as force plate, EMG, and other transducers, has
to be carefully evaluated and tested.

Enclosed is two of the correspondences I received from Dr. De Leva:

To: ariel1@ix.netcom.com, deleva@risccics.ing.uniroma1.it
Subject: Re: videorecording speed variability
Cc: deleva@risccics.ing.uniroma1.it
Status: U

Thanks, Dr. Ariel.

It is still not clear to me how your system works, but it's
now clear that you were referring to synchronization problems, and not
unstable sampling frequencies. That's a completely different topic,
and if you have developed a cheap method to evaluate and correct the
phase shift between the video signals, and provide a common
"time base" to put together video signals with other analog inputs,
such as those from force plates, etc., I believe you did something
important.

Paolo de Leva

To: ariel1@ix.netcom.com, deleva@risccics.ing.uniroma1.it
Subject: Re: videorecording speed variability
Cc: deleva@risccics.ing.uniroma1.it
Status: RO

Dear Dr. Ariel,

regarding your question about odd and even fields, I am
afraid I don't have any solution. Until now, I just accepted the
error, and hoped that the smoothing algorithm would solve the
problem. Contrary to what you wrote, I am not an expert at all.

The solution of using either odd or even fields works,
but then your sampling frequency decreases to 30Hz, while if you use
both odd and even fields the frequency is twice as much.

I guess your special grabbing card is capable to provide
information such that the software can tell odd from even fields.
Then you probably add one dot to the vertical coordinates of the
points digitized on the odd fields. Is that right? I think this is
a very good solution.

Paolo de Leva

I think this issue is very important to discuss since the raw
digitizing data set the tone for the accuracy of any biomechanical
analysis of movement.

Respectfully,
Gideon Ariel, Ph.D.