Film-Tech Forum ARCHIVE: Change the speed of playback?

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Posted by Adrian Hauser (Member # 4723) on 04-04-2008, 06:29 PM:

Hi Film Tech,

How does one adjust the speed of playback on a film projector.
We have a Cinemeccanica Victoria 12 projector.

We are performing butterfly tests with 1/2 the screen blanked to the film projector and the other 1/2 our Barco 2k projector in our Digital Grading Theatre. With the Barco set to 24 fps the projector seems to be running around 26 Fps.

Adrian Hauser
Sydney, Australia

Posted by Ron Funderburg (Member # 4561) on 04-04-2008, 06:55 PM:

Cinemeccanica's are belt driven machines it would be in the tension setting on the main drive belt but how you get it precise I'm not sure and if you actually match it the Digital out put I am also not sure. I'm not sure why it would need to be.

Posted by Steve Guttag (Member # 268) on 04-04-2008, 07:31 PM:

The best way to get a film machine to run at a particular speed is use a sync motor with timing belts (or direct drive) and then use a good grade of motor controller (one that has an accurate crystal...generally the cheaper ones don't). At that point you can have the machine run at any speed. I would have a good strobe light too (again with an accurate crystal) and shoot the strobe at say the shutter blade and verify speed.

Now, if you are going to be doing a bit of this, then it would make sense to outfit the machine with a pulse-tack and a proper read out (with at least three digits).

Steve

Posted by Louis Bornwasser (Member # 3063) on 04-04-2008, 08:05 PM:

THe absolute value of capacitors will affect the Cinemeccanica speed. More capacitors and a (large) variable resistor will allow some small adjustment.

Years ago, 30 frame operation of a V8 required a new pulley which needed "half a tooth" more. We used a 1000 hz sound loop threaded under the 70mm film along with a frequency cou nter to get exactly 1000 hz. Louis

Posted by Adrian Hauser (Member # 4723) on 04-04-2008, 09:11 PM:

Thanks for the thoughts on this.

Ill check out our options early next week and initially look at the tension on the
belt drive.
I can change the frame rate of the Playback to the Barco as well so between
the two I should b able to get a decent match.

Louis, which capacitors should I be looking for?

Thanks
adrian.

Posted by Richard Fowler (Member # 893) on 04-04-2008, 09:22 PM:

The V12 was their economy machine (based on the Portacine chassis) so a motor swap / crystal control with feedback encoder as Steve notes should do the trick....would be nice if there is a way to slave to the sync from your video / digital files with some phasing control to lock it in after a reference start point. I have used D.C. motors with 4 digit tach / encoder on a V-5 for playback to music / effects but it still required trimming every few minutes.

Posted by John Hawkinson (Member # 1135) on 04-04-2008, 09:43 PM:

My experience is that 1k tone is often not 1k tone. A batch we have lying around was more like 1200 Hz [Frown]

The tone is intended as a level reference not a frequency reference, I guess.

As for strobes, best bet is to check with a photocell/photodiode and an oscilloscope.

--jhawk

Posted by Tim Reed (Member # 137) on 04-04-2008, 11:29 PM:

While it won't help you change speed, you can make a stroboscopic disc for anything you want to check the rotational speed of. I presently only know how to do this for 60Hz systems, but maybe someone else here can fill in the blanks.

EXAMPLE: This is for a 4-8 frame sprocket (360 or 180 rpm shaft), 24.00 fps, viewed under a 60Hz light. I made this in Adobe Illustrator:

When you place this particular disc on the end of the requisite sprocket and shine a 60Hz florescent light on it, the pattern will appear to be still if the projector is running at precisely 24 fps. If the pattern appears to rotate in one direction or the other, the speed is off.

The formula to make one of these discs is:

Bars = F x 2 x 60/rpm

Where,
F is the frequency of light used to view the bars in Hz.
rpm is the target rotational rate of the object in revolutions per minute.

QUESTION FOR THE MATHEMATICIANS... When you work the formula for 50 cycles, the result isn't an integer. How could a fraction be translated to the bars/spacing in a 360 degree pattern? There has to be a way to make this work on 50Hz.

Posted by David Graham Rose (Member # 1450) on 04-05-2008, 03:04 AM:

Gretings

The easiest way to do digital grading/butterfly comparisons would be to junk the Vic 12. It is not up to the job. Buy a Kinoton ECII reference and lock it to the timcode output of the replay machine.

Simple as that. No point buggering about with a Vic 12.

From snowy Cambridge

Goddnight

David

Posted by Steve Guttag (Member # 268) on 04-05-2008, 06:07 AM:

The ECII? While it is the finest studio projector, without question...for just putting out a picture at an accurate speed it might be a bit pricey. Kinoton also has the EQ series (quality inspection machines that also offer a Sony 9-pin option). The ES series, yet less expensive still have variable speed right out of the box (crate). Even the standard E projector can be outfitted with variable speed with readout.

However, the fact remains, ANY projector with a sync motor and timing belts (or direct drive) can output a precise 24fps. By using the aforementioned motor controller solution, you take line-frequency out of the equation.

Steve

Posted by Mark Gulbrandsen (Member # 72) on 04-05-2008, 07:06 AM:

I agree with Steve... I used to use sync motors and variable drive units alot on my dailies setups. The more expensive units are pretty precise as for speed.

Mark

Posted by Andres Briano (Member # 4621) on 04-05-2008, 09:41 AM:

quote: Tim Reed
There has to be a way to make this work on 50Hz.

As a matter of fact, Imax provides 50 and 60Hz pattern print outs to be placed on top of their platter decks for speed check and calibration.

One cute little trick that a colleague of mine -you know who you are- taught me is to take the damping rollers further out (of allignment) so that your analog head reads perfs. Then, by plugging in an oscilloscope on your CP, you can know exactly at what speed you are running.

NOTE: Since you are reading perfs and not frames, everything is multiplied by four. 24fps=96Hz.

Andrés

Posted by Gordon McLeod (Member # 33) on 04-05-2008, 11:09 AM:

quote "Cinemeccanica's are belt driven machines it would be in the tension setting on the main drive belt but how you get it precise "
Belt tension has nothing to do with it since cinemecanica use timing belts on every machine of theirs I have ever worked with (vic12 I have not used)
Use a tach on the shutter or put a shaft encode on one of the sprocket shafts and use a frequency counter to get it dead on and a variable speed motor drive to adjust the motors speed

Posted by Mark Gulbrandsen (Member # 72) on 04-06-2008, 10:02 AM:

Gord, The Vic-10 was direct gear drive. The only one to have that feature that was built in the 50's/60's as far as I know. Drum shutter aside,it was also their best attepmt at a projector IMHO.

Mark

Posted by Gordon McLeod (Member # 33) on 04-06-2008, 12:46 PM:

I was referring to there belt drive machines

Posted by Mike Olpin (Member # 1115) on 04-06-2008, 06:15 PM:

I don't know what this is, but I suddenly want to do whatever Tim Reed wants me to.

Posted by Tim Reed (Member # 137) on 04-06-2008, 06:53 PM:

[rofl]

THAT'S funny! [Big Grin]