View Full Version : Re: tech-request 16mm-video transfer

Jim Walton
05-21-1995, 02:04 PM

In response to your request for film-to-video transfer for data analysis
purposes. There used to be an outfit in San Francisco called One-Pass
Video (1 China Basin Building), but I know they changed their name
recently. I think they were bought-out by someone. I've used their
services and they certainly have (had) the capabilities you need. I'll
make a couple of calls in the morning and try to find their new identity
for you. Meanwhile, you've brought up an interesting topic which may
have implications for others on the list, so I'll post this to the group
as a whole ...

A word of caution, if you plan to analyze video tapes produced by a
service bureau, you MUST tell them that you want a "one-to-one frame
transfer". If you don't, they will automatically use the so-called 3-2
pull-down method to copy the film, and it will play havoc with you. In
case you're not familiar with "3-2 pull-down", here's the logic ...

Typically, 16-mm film is shot at 24 f.p.s., but (standard) video requires
60 fields in the same amount of time. Let's consider the "normal" case,
where someone wants to make a copy of a film for documentary purposes--or
for qualitative analysis. If your service bureau were to copy frames of
film to fields of video, one-for-one, the video recording would be too
short--that is it, would play in 2/5ths of the time of the original film.
So how do they compensate?

The standard practice is to take the first frame of film and make three
video FIELDS from it. Then they take the second frame of film, and make
two video FIELDS from it, interlacing the fields as they go. Similarly,
they make three copies of the third frame, and two copies of the fourth
frame, and so on. If you continue this process to its logical conclusion
(over the first second), you'll find that by the time they've copied the
24th frame of film, they'll have produced 60 video FIELDS or, after
interlacing, 30 video FRAMES. (I'll let you figure out which film frames
are meshed to produce the various video FRAMES. It's fairly logical, and
I'm sure it's unecessary to lay it all out here.) The net result is a
video recording that takes the same amount of time to play as the original
film. If they didn't do this, "Gone With the Wind" would live up to its
name when you tried to watch it on your VCR/TV at home!!

Clearly, a tape that has been made using the "3-2 pull-down" method is
worthless (or close to it) for analysis purposes--unless you have the
ability to extract FIELDS from the video in a 3-2 fashion, which I doubt.
And I can tell you from experience that doing this "manually" is a real
pain in the you know what. Yes, your video data analysis equipment will
blithely read the video tapes produced by the "3-2 pull-down" method, but
your displacement data will contain well-structured steps, and velocities
will keep diving to zero. By now I'm sure the reasons for this will be
obvious to you.

If you ask for a "one-to-one frame transfer", you'll get one video FIELD
for each frame of film. You won't be able to watch the video at the
original rate--unless you can use your VCR to slow things down, but you'll
have what you need for analysis purposes.

Well, I hope this has been helpful. Again, I'll try to find out what
happened to "One-Pass" for you in the morning. If I can't find them,
I'll locate someone else in the area for you.

Jim Walton.

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* JAMES S. WALTON, Ph.D. * *
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* 4D VIDEO * *
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On Sun, 21 May 1995, Rodger Kram wrote:

> Biomch-L Community:
> I have had a request from a collegue to borrow some of my high speed 16mm
> films. I no longer have a way to analyze them myself (moved to a new
> institution), nor does this collegue. We both have video analysis
> equipment.
> I am looking for suggestions as to how to transfer from 16mm film to VHS
> video in a frame by frame registered manner.
> The films in question are of kangaroos hopping and thus not easy to
> re-collect and of course over time chemical film decays to dust. (as we all
> do eventually)
> thanks for your two seconds of attention,
> Rodger Kram
> Assistant Professor
> Human Biodynamics Dept.
> 103 Harmon
> University of California
> Berkeley, CA 94720-4480
> phone 510 643-9370
> FAX 510 642-3355
> e-mail rkram@garnet.berkeley.edu