A.C. camera motors.

Paul Bruening · May 20, 2008

I've got this Mitchell A.C. motor for my NC. It says on the Mitchell name plate that it is a synchronous motor (115 V, 1 phase). Where does it get its sync from? Does it come from the 60Hz A.C. current or does it get it from an external source? Keep in mind that this thing does not look like a crystal controlled age of motor. It looks more like it is from around or before WWII.

My principle problem with this motor is that it doesn't keep sync with my XL2 in 24P mode. It cycles one frame out over about 25-30 seconds of run time (it runs ever-so slightly slower than the XL2). Can I micro-control the motor's speed by varying the voltage or is it tied to the 60Hz?

Dan Goulder · May 20, 2008

My principle problem with this motor is that it doesn't keep sync with my XL2 in 24P mode. It cycles one frame out over about 25-30 seconds of run time (it runs ever-so slightly slower than the XL2). Can I micro-control the motor's speed by varying the voltage or is it tied to the 60Hz?

Are you taking the .1% telecine pulldown into account? That will put you one frame out somewhere around the 33 second mark.

Hal Smith · May 20, 2008

I've got this Mitchell A.C. motor for my NC................My principle problem with this motor is that it doesn't keep sync with my XL2 in 24P mode. It cycles one frame out over about 25-30 seconds of run time (it runs ever-so slightly slower than the XL2). Can I micro-control the motor's speed by varying the voltage or is it tied to the 60Hz?

The sync motor is running at exactly 24.000 fps, the XL2 is running at 23.976 fps (really at 59.97 fields per second?). At the end of 30 seconds the NC has run a total of 720 frames while the XL2 has run through 719.28 frames hence your "beat". You're running into one of the sets of problems that created the need for dropframe TC, DAT recorders with sampling rates at 48.048 kHz for subsequent telecine transfer and NLE editing, etc. I've never seen one but I know there are inverter power supplies that furnish 120 VAC power at exactly 59.97 Hz to solve some of problems one runs into when one mixes video/film/non-flickerfree HMI's and Flo's, etc.

If you shine a non-flickerfree fluorescent (or better a small neon lamp) through the front of the NC while looking though the aperture with the gate out of the way there will be no long term pulsation in the light if the NC is running at synchronous 24fps. (60 Hz = exactly 2.5 X 24 Hz)

(It must be nice to live in the 25 fps / 50 Hz power PAL world where everything comes out even, I'm still looking for the SOB who created the NTSC RS-170A Color Sync standard)___ (For the young among us: ( N)ever ( T)he ( S)ame ( C)olor.)

Paul Bruening · May 20, 2008

Mr. Goulder and Mr. Smith,

Thank you for the answers. I was beginning to wonder what was going on with this system. The problem is, obviously, my ignorance. Do you think changing the motor's Hz to the suggested 59.97 Hz is a good solution? There are also choices with the XL2. It can select two types of pull down for recording to tape, 2:3 and 2:3:3:2. It also can record directly to hard drive via firewire. I think I recall something about choosing drop frame and non-drop during recording to hard drive. If I follow Wikipedia's article on Telecine Pulldown, http://en.wikipedia.org/wiki/Telecine#3:2_pulldown, I should change the motor speed as well as manage the pull down rate. Which of these methods do you fellows think might be a "Damn, I need an easy solution" solution?

Thanks again,

Paul

Michael Collier · May 20, 2008

Its possible the 24p/23.98 discrepancy is to blame for the slow running, seems to fit the straight face test, but I wonder how you came to the conclusion it is on average one frame off every 25-30 seconds. I assume you shot some footage, telecined it out to video and compared it to a similarly recorded video from your XL-2. If that is the case, you have already compensated for the 23.98fps. Your telecine would have been slowed ever so slightly so as to fit within the NTSC (dumbest system alive....doesn't fit the acronym, sorry hal, but it does fit the spirit. I think every engeneer was out to lunch in 1953....or they didn't have wicked cool FPGA chips on the cheap)

but I digress. Unless your assumption of one frame every 25-30 seconds is based on observation of a 24fps sync projector compared to a video presentation of 24p with a pulldown, it should be assumed the telecine would add the pulldown/slowdown that NTSC would naturaly present. So it seems (to me from limited information) the motor is running slow. I would assume if it is marked synced and is as old as you say, it would most likely tie its sync to the frequency of the line voltage. After all in the 30s and 40s, the only thing capable of keeping time in AC power was the mains, and even then it was still not as tightly regulated as a crystal sync. The only saving grace was likely your audio recorder was synced by the very same power source, so small discrepancies would be matched between film and audio.

I can only assume, not knowing much about older sync motors, is that somewhere your drivetrain has more drag than the tolerances allow. So if an AC motor turns a certain amount per phase, this motor occasionally slows due to drag, and at a certain point in a 25-30 second cycle, it lacks the momentum to turn, even though the phase would theoretically be able to move the motor the requisit amount.

This is all just a guess though. I have no practicle knowledge of motors that old. take it with a grain of salt. Others can chime in. Hal is an engineer and dp. he can tell me if I am way off base or on to something.

Paul Bruening · May 20, 2008

Hey Michael,

I didn't provide you with enough info. The Mitchell NC is the projector. Bruce McNaughton converted it. The XL2 is the telecine recorder pointed through an EF converter and macro lens into the gate of the NC. I'm having trouble getting them to sync up with each other. I assumed, ignorantly, that the XL2's 24P mode would match the NC motor's speed.

Also, I only assumed the NC was slower than the XL2. But, I don't actually know which one is slower or faster. As well, my assessment of the cycling of blurred frames is only approximate at 25-30 seconds. It's difficult to determine exactly where the same point of blur is in each cycle since they pass rather gradually.

On top of all this, this is the third motor I've tried. The stepper motor unit provided by Bruce was set to 24fps and was off in rather the same way. Bruce said to ship it back to him but I can't afford it at the moment. I had a Cinema Products motor for it but it blew out its bearings after about 30 minutes of sporadic use. I suspect it was a bum unit when it was sold to me. It ran out of sync like these other two. This Mitchell motor is the third I've tried. I actually have two of these but the other one is bum. It was sold to me by the same guy that sold me the bum CP motor. That's why I'm asking for informational help, here. I'm running out of solutions and long since ran out of money.

Hal Smith · May 20, 2008

I can only assume, not knowing much about older sync motors, is that somewhere your drivetrain has more drag than the tolerances allow. So if an AC motor turns a certain amount per phase, this motor occasionally slows due to drag, and at a certain point in a 25-30 second cycle, it lacks the momentum to turn, even though the phase would theoretically be able to move the motor the requisit amount.

This is all just a guess though. I have no practicle knowledge of motors that old. take it with a grain of salt. Others can chime in. Hal is an engineer and dp. he can tell me if I am way off base or on to something.

Michael: It could be a slow sync motor, that's why I suggested the poor man's stroboscobe check of the camera speed.

Paul: If you know of a source of a 59.97 Hz inverter (hopefully in rental somewhere or see if Ken Stone knows of one) you might try it. Someone probably made a 59.97 crystal motor for Mitchells, it wouldn't be a huge deal to take something like a working CP motor and purchase a custom crystal to run it at 59.97, not 60 Hz. I've purchased a few custom crystals from International Crystals here in OKC over the years. They're a bit slow to get around to custom jobs and charge around $150 for a special but they can do it. Also, Clive Tobin might be of some help in convincing a CP motor to run at 59.97.

Paul Bruening · May 20, 2008

Hey Hal,

I sent you a return PM. I'm pretty sick now that I've chucked the CP motor away. I didn't realize there were options available on it. I looked at it and thought to myself, "Well, who the hell would bother with something like this?" I can't really blame my wife. She just asked about something that was under foot. I made the decision.

So, the inverter option is the remaining choice. I confess, I don't know how an inverter can change the Hz of an AC power source. I get voltage converters, inverters, and rectifiers. I can only guess how the cycle can be changed.

I couldn't find anything on Google. All of the 59.97 Hz searches led up to video monitor related links. I'm guessing that monitor power supplies might not run a motor this big so ripping one out of a scrap unit doesn't seem viable. It apparently isn't something you can get at a retail web store, either.

I'll set the rig up again and see if I can select other XL2 settings and if they make a difference. Whatever I come up with, it will have to be cheap. The local job market has thinned out due to the crapped out economy. Places like Monster.com list nothing locally in my field at all anymore. I may find myself joining the prestigious Society of Motion Picture Burger Flippers (SMPBF) soon.

Hal Smith · May 20, 2008

So, the inverter option is the remaining choice. I confess, I don't know how an inverter can change the Hz of an AC power source. I get voltage converters, inverters, and rectifiers. I can only guess how the cycle can be changed.

Geez! I understand the wife's motivation but obviously she's never heard of Murphy's Principle of Cold Storage: You throw something out the day before you discover a REAL need of it.

Inverters have to have a frequency generator of some sort inside. Normally it's a pretty uncontrolled circuit running somewhere around 60 Hz but there's no reason the frequency generation circuit can't be controlled by a crystal, just like a crystal generator or crystal motor. There used to be a fair number of crystal inverters around to run professional sound gear with sync motors inside from 12 volt DC vehicular power sources. If you find one, it would be the same idea as changing the crystal inside of a motor to convert the inverter to 59.97 Hz.