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unknown user
12-06-1994, 11:57 AM
The recent message by Ariel about his system contained some
interesting notes about large variations of videocamera speed (fields/s).

Ariel stated that the speed of the videocamera (and not that
of the tape recorder) is initially lower than the nominal speed (60Hz
for NTSC standard), then after a given period of time reaches a
value that, anyway, does not at all stay stable (+- 2Hz!).

That was completely new to me, and contrary to what I assumed true.
Unfortunately, Ariel wrote his message mainly with the purpose of
describing the evolution of his system during the past years, and
did not fully explain the reasons of the unstability
(or variability) of camera speed.

1) I know that the signal recording starts some seconds after
the recording button is pressed, and I believed that the delay
was electronically controlled by a system able to monitor tape speed,
and that the system did not activate the recording until the tape
would go up to the required speed.

2) I have always assumed that the camera "SAMPLING" frequency
was controlled by a quartz, threfore as much accurate as the speed of
a digital watch. Of course, the video signal from the camera is recorded
on a tape that may run in front of the recording heads at a variable
velocity, because of mechanical reasons. The above conditions imply that
the magnetic signals representing the sampled images (fields)
are physically placed on the tape at variable distances from each other.
However, kinematic analyses are performed digitizing one field at a time,
and the fact that reciprocal distances between fields are variable
should by no means affect the accuracy of the kinematic analysis.
For obtaining accurate results (if we assume constant camera speed)
we only need that the electronic shutter of the camera is opened
at a constant rate (and stays open for a very short interval, to freeze
the image of fast moving objects..., but that's not the key point here).

Where am I wrong? The info given by Ariel might
imply that somehow the camera electronic shutter is controlled by the video
tape recorder, and its rate of image sampling varies with the physical speed
of the tape. If this is true, what's the reason?
(A second unlikely alternative would be that the camera speed
is indeed given by the camera itself, and the electronics of the camera cannot
maintain a constant speed; however, this seems to me a not realistic
hypothesis)

Is there anyone among the subscribers (either Ariel himself, if
this meets his commercial policy, or others) who can help me to better
understand what are the reasons of the videorecording speed variability
reported by Ariel?

With regards,

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