Path: utzoo!attcan!utgpu!watserv1!watcgl!imax!dave
From: dave@imax.com (Dave Martindale)
Newsgroups: comp.graphics
Subject: Re: digitizing images
Message-ID: <1989Dec7.134309.11032@imax.com>
Date: 7 Dec 89 13:43:09 GMT
References: <129@tobler.UUCP>
Reply-To: dave@imax.com (Dave Martindale)
Organization: Imax Systems Corporation, Oakville Ontario
Lines: 28

In article <129@tobler.UUCP> iony@tobler.UUCP (Ion Yadigaroglu) writes:
>
>The idea is to keep the disk with its tracking info. While the laser
>is over the film, you freeze the positioning and focus system.
>Of course you have a little less time for doing both, but it seems
>to me the feedback system might be flexible enough to handle this.
>The biggest problem is avoiding that the circuits "panic" as they
>are designed to handle only small breaks.

The servo loop probably has a low-pass filter between the error
amplifier and the motor driver.  A few judiciously-placed FET switches
and maybe an extra opamp to act as a buffer would allow you to "freeze"
the servo drive for reasonable times.

However, locking the drive current at some particular current is no
guarantee that the pickup will remain motionless.  In fact, if it was
moving at the time you went into "freeze" mode, it is guaranteed to
keep moving for a while.

And then there is the disc itself.  Watch the servo circuits in a CD
player sometime - they are constantly making small adjustments in focus
and tracking because your average CD is not flat, and does not have its
"centre" hole punched in the centre.  The player NEEDS those servo
loops operating all the time just to stay on track.  If you "froze" the
pickup for any significant distance, it is just about guaranteed to be
out of focus and out of alignment when the spiral tracks re-appear.
If you only want to scan images the size of 8mm film frames, you might
just get away with it.  But I doubt anything larger could work.