Film-Tech Forum ARCHIVE: IMAX platter/threading question

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Posted by Thomas Pitt (Member # 4322) on 03-02-2008, 03:44 PM:

A few days ago, I visited the Bradford IMAX and saw Lions 3D. Afterwards I went up to the projection booth viewing area and watched the projectionist change over the platters for the next film. While watching, I noticed a couple of differences in the way the platters and projectors are threaded up as opposed to 35mm.

Firstly, instead of putting the very beginning of the leader through the brain and feeding it as necessary, the projectionist wound the leader around his hand 15-20 times. Then he threaded the brain, leaving a mini-reel of leader ahead of it. Afterwards, he took that mini-reel all the way over to the projector, slowly taking up more film from the platter as he threaded the rollers and projector.
After threading, the mini-reel was taken back over to the platter via the rollers and attached to the takeup ring - the excess leader being manually unwound round the takeup ring on the platter.

My question is, what is the advantage in doing it this way? Why not thread the very beginning of the film through the brain, rollers and projector - taking up more film from the platter as necessary? I have a suspicion it may have something to do with the start frame on the reel, but surely the projector could be manually advanced until the start frame is in position?

Posted by Brad Miller (Member # 2) on 03-02-2008, 05:06 PM:

Here is a video showing what you are talking about.

I am in no way an IMAX guy, but from what I understand the platters cannot stop and start like a 35mm platter is able to. Threading "through the brain" would be quite rough. Also I believe the platters are linked to the automation such that they won't start until the system tells it the projector is starting.

Posted by Mike Olpin (Member # 1115) on 03-02-2008, 06:32 PM:

MPX systems thread through the brain, so I don't think it's a limitation of the platter being unable to keep up.

Posted by Brad Miller (Member # 2) on 03-02-2008, 06:39 PM:

With the same QTRU I would presume?

Posted by Monte L Fullmer (Member # 2797) on 03-02-2008, 06:41 PM:

From what I gathered from when I would visit the IMAX unit here in the Boise area when I was in tenure with EDWS, this leader looping form was to obviously keep the leads off the floor, plus with all of the rollers involved from the platter unit and back, it keeps the tension even from the beginning of the brain to the final wrap around the rewind deck.

Plus, when doing "two eyes=3D" the leads have to go to certain rollers and in a certain layout over the roller trees and it looks like this leader looping make the lace-up procedure more simpler.

Posted by Adam Martin (Member # 641) on 03-02-2008, 07:05 PM:

This practice dates back to the MK-I and MK-II reel units which feed film from the outside of the roll instead of the center. These reel units held the film under tension at both payout and takeup to control platter speed, so that you were required to have the machine completely threaded before it would run.

On the center-feed QTRU, it is still the preferred method for non-MPX machines in order to be able to thread the projector and make it all the way back to the reel unit without the film hitting the floor. (The MPX was designed with a free-wheeling threading mode that allows you to pull slack through the already-threaded projector head.)

It is possible to throw a short print off-center with the QTRU in threading mode (fast-start/hard-brake) if clamps are not installed, making another reason to not pull film through the feed unit when threading.

Posted by Steve Guttag (Member # 268) on 03-02-2008, 07:14 PM:

God...did you see that total loser in the video Brad linked to? Probably working at Taco Bell now!

So which eye got the Kelmar film cleaner, Adam, and did you have any problems with film damage using a media cleaner?

Steve

Posted by Brian Michael Weidemann (Member # 2243) on 03-03-2008, 07:43 AM:

The projector doesn't advance more than a few frames during the projector-head portion of the thread-up, so you'll want enough film spooled up to reach the take-up platter on the reel unit. Only when the film is threaded completely with take-up tension, can the projector be advanced via automation to a Start Picture frame.

In 3D especially, both prints are threaded to some marked frame at the input sprocket (since synch between the two eyes is vital), which is before the Start Picture frame on the film, but after any junk leader that's been added (which likely won't match, from the right print to the left).

Spooling up the film until just before the Picture Start area makes the most sense, and yes, the film never hits the floor! [thumbsup]

Posted by Thomas Pitt (Member # 4322) on 03-03-2008, 03:36 PM:

quote:
Spooling up the film until just before the Picture Start area makes the most sense, and yes, the film never hits the floor!

Actually, it DOES hit the floor [Frown]

Once the automation stops, the projectionist presses a button on the console that makes the projectors slowly advance, until the tail of the junk footer is pulled off the platter and hits the floor. Once it's free of the platter rollers, the projectionist can open the IMAX hood and un-thread the film, before spinning the takeup platter by hand to get everything back onto it.

Just out of interest, why not un-thread the projector by simply allowing the film to tail out through the projector, like with many 35mm systems?

Incidentally, the Bradford IMAX uses dual SR projectors, not the single GT projector.

Posted by Ben Wales (Member # 92) on 03-03-2008, 04:35 PM:

quote: Thomas Pitt
Just out of interest, why not un-thread the projector by simply allowing the film to tail out through the projector, like with many 35mm systems?

That is just plain Stupid and bad pratice and the likely (High) risk of a Rota Crash and Cam Damage.

Posted by Paul Mayer (Member # 355) on 03-04-2008, 03:02 AM:

Tail outs in IMAX are a big big no no, since a tail out can cause the timing of the cam pins to be knocked out of alignment. Don't know what the policy is now, but when I was running one, a tail out required running a test loop (for 15 minutes?) to check the cam timing. Bad timing can cause the cam pins to inflict perf damage on every subsequent run of a print through that machine. IMAX operators learn pretty quickly to never ever allow the tail to run through the machine.

The old systems I ran at Caesars Palace or watched at Mitsubishi had no automation. I assume nowadays one can program a stop cue to prevent tail outs.

Posted by Brian Michael Weidemann (Member # 2243) on 03-04-2008, 04:53 AM:

quote: Thomas Pitt
the tail of the junk footer is pulled off the platter and hits the floor.

This is minimal, and forgivable, since the tail will still pass over PTRs every time after it has scraped the floor, before a de-thread. Most of the thread-up leader, however, never gets to touch PTRs.

With the first GT system I worked with, the Mark II platters and film path roller pedestals were close enough to each other that loose tails draped over them during a final jog never hit the floor.

Posted by Tristan Lane (Member # 1169) on 03-05-2008, 02:16 AM:

Agreed on tail out. Since the input sprocket maintains the slack loop within the rotor, when the tail of the film leaves the input sprocket, the loop is lost and the output sprocket just pulls the film regardless of whether there is a loop present. The cam pins or registration pins usually end up destroying the film. If the cam takes the brunt of the pull, it can mess up the timing.

On the threading: I've done it both ways. I've been to a number of SR / QTRU sites that were threaded in a typical 35mm fashion, where the leader is pulled through the brain and the projector is threaded after the takeup platter has been wound. The leader never touched the floor, but it did get drug around the front housing of the projecotor.

On MK2 feed units, this was impossible since the centerfeed unit was not present, and the response of the feed control was not quick enough to compensate for the start-stop nature of pulling the film as needed. The "twiddle" method shown in the video was the only acceptable method on MK1 an MK2 installations, even though the old classic 2D projectors had rollers that were installed outside of the normal film path to accommodate unthreading the projector without dragging the film on the floor. I always thought that if a QTRU was used, threading could be accomplished in much the same manner as 35mm since those extra rollers were there.

Posted by Tim Reed (Member # 137) on 03-27-2008, 04:04 PM:

quote: Tristan Lane
Since the input sprocket maintains the slack loop within the rotor, when the tail of the film leaves the input sprocket, the loop is lost and the output sprocket just pulls the film regardless of whether there is a loop present.

I'm surprised this wasn't allowed for in the design.. what about if there's a film break or wrap? This is easily remedied: film leaves that input sprocket, pins retract.

Posted by Tristan Lane (Member # 1169) on 03-28-2008, 02:27 AM:

I don't think it's as simple as you'd think. It would require a lot of extra work to make the cam pins and the registration pins to retract. Also, If the registration pins were moveable, the mechanism would eventually get sloppy and result in an unsteady picture.

Film rarely breaks or wraps with IMAX equipment. I can think of only one time where a problem arose that wasn't due to operator error during my time as a projectionist.

Posted by Brian Michael Weidemann (Member # 2243) on 03-28-2008, 03:48 AM:

I agree. That's a lot of unnecessarily complicated mechanical design to counteract such a rare possibility.

If the film wraps, the feed unit knows about it before long, and then the console shuts down the projector with a Reel Unit Not Ready fault.

And film doesn't just "break". Any decent enough staff knows to fix worn or bad splices. I've seen mid-show tail-outs only two or three times in the last ten years. I only remember the cause of one of them, and it was operator error. (The guy who moved the film onto the platters, for the show that was playing when I came in, had nicked the top of the print, about half-an-inch, on the outer edge ... just enough to rip the full width of the film as it went through, and it crashed, 30 seconds of end credits left to go. It took a lot of splices to repair the subsequent nicks, located about every 15 feet of the remaining film, to get it to playable status.) We were back on screen, only lost one show, as the tail-out didn't misalign the cam ... just tore up 5 feet of film.

Posted by Tim Reed (Member # 137) on 03-31-2008, 05:00 PM:

quote: Tristan Lane
It would require a lot of extra work to make the cam pins and the registration pins to retract.

You mean, more work than engineering multiple lamphouses and associated optics, moving 70mm film horizontally at blinding speeds, using compressed air for mechanical control, effecting double-system sound, and dual-strip projectors -- all under computer control and monitoring?

Something as simple as a gate that falls back or pins that retract as a safety measure shouldn't be a major hurdle. How much work did it take to do IMAX Magic Carpet? Or Omnimax? For what, one or two installations?

And, they get how many millions of dollars for these?

Posted by Tristan Lane (Member # 1169) on 04-03-2008, 07:42 PM:

Regardless of the complexity of the modifications required to do what Tim wants, one simple fact remains: There's no reason to do it. Someone should be there to make sure the film stops...regardless of automation.

Projecting IMAX is a job that requires an attentive operator that is present at all times. I believe it is the last mainstream film format that still requires competent operators in every booth. The equipment is expensive, the film is expensive, and minor mistakes cost thousands. Tail out is always avoidable if someone is there to make sure the projector stops.

Posted by Scott Norwood (Member # 30) on 04-09-2008, 09:25 AM:

quote: Tristan Lane
I believe it is the last mainstream film format that still requires competent operators in every booth.

So there is nothing wrong with incompetent operators (or no operators at all) for 16mm, 35mm, and 70mm?

Posted by Tristan Lane (Member # 1169) on 04-10-2008, 01:59 AM:

quote: Scott Norwood
So there is nothing wrong with incompetent operators (or no operators at all) for 16mm, 35mm, and 70mm?

It appears so.

Posted by Charles Greenlee (Member # 3856) on 04-28-2008, 12:20 AM:

Sadly, yes. Or even no operators, as continual loop platters and automations that automatically start on times would prove. If your projectors start by themselves and don't need to be rethreaded every show, then you wouldn't need a projectionist. [bs]

Just another way to replace a competent person who cares for the quality of their job with some minimum wager, or even a computer.

As far as the winding of the leader as such. I can't help but notice the platter rings don't have the button and split. I'm guessing they require the friction of the leader itself to hold it onto the ring? I can imagine having to bring that much leader around to the rewind platter would cause it to end up all over the floor. It looks like it takes a good amount of leader to wrap the ring enough to lock onto it. Actually sounds like a good concept even for 35mm, that way you're not losing leader to that little fold, to slip into the split in the ring, that always wears out. That and keeping it off the floor would make your leader last longer. I like that.

Posted by Mark Gulbrandsen (Member # 72) on 05-03-2008, 04:54 AM:

quote: Tristan Lane
Also, If the registration pins were moveable, the mechanism would eventually get sloppy and result in an unsteady picture.

Indeed... the reg pins can't retract they are on both sides of the field flattner which can only move up and down. The fact that they are stationary is part of what makes the system so good. On the Iwerks 15/70 clones the only part that goes bad every few thousand operating hours is the cam unit bearings... Nothing else ever really fails in the projector itself unless... One Iwerks system near me that we are involved with originally had no UPS system at all and a power bump would cause all sorts of problems and damage. Talk about inertia keeping things moving! One severe power bump caused the loss of about 300 feet of L eye and R eye picture much of which was wrapped around the lower inside of the mechanisms and all caught up in the drive belts and stuff... it had to all be cut apart to get it out. You don't just grab these platter systems to stop them... you could loose an arm or be severly injured. After that mishap a massive UPS system was installed capable of running the entire dual projector system for close to 30 min.

Mark

Posted by Monte L Fullmer (Member # 2797) on 05-03-2008, 05:03 AM:

(...not to drift this to a side or off-topic here...)

quote: Charles Greenlee
Actually sounds like a good concept even for 35mm, that way you're not losing leader to that little fold, to slip into the split in the ring, that always wears out.

I used to do it all the time with slipwinding the film on the rings, esp with the STRONG rings and managed to teach a few this trick. It became faster to slipwind than to fight that obnoxious overlap clamp style of this certain rings. The Christie rings are tons better on both slipwinding or slotloading.

Slipwinding came from my carbon arc/changeover days when I would slipwind the film on the takeup reel in the lower magazine.

But, when others began to train downstream, that slipwinding on the ring trick faded away quickly.

Posted by Tristan Lane (Member # 1169) on 05-03-2008, 04:43 PM:

quote: Charles Greenlee
I'm guessing they require the friction of the leader itself to hold it onto the ring? I can imagine having to bring that much leader around to the rewind platter would cause it to end up all over the floor.

The leader never touched the floor. You only pulled as much as you needed. We wound the rings by holding the film in place for a full revolution and by the next half turn, it was bound to the ring. We then spun the takeup platter until the auto-load foil tape was positioned where we wanted it and proceeded to thread the projector. There's so much surface area with 70mm, that only a half turn more after one full wrap secured the film to the ring nicely.

This method is undeniably rougher on the leader than the "twiddle". To overcome this, we added scrap film to the front of the print.

Mark, you see where I'm coming from. The registration pins are so reliable because they are stationary. If the added retraction to them, I would imagine picture steadiness would be greatly affected, even if the tolerances were tight.

EDIT: To add to my comments at the top, I worked in a booth before extended play platters came out. The diameter was small enough that most people could reach the far side of the takeup ring and continue holding the leader to it. I'm guessing this would be difficult with the huge platters in use now.

Posted by Charles Greenlee (Member # 3856) on 05-04-2008, 05:21 AM:

Not really. I could hold on to the leader, during its entire revolution.

And my observations about the loads of leader ending up all over the floow, wasn't regarding the "slip winding" as you put it, but more with the slot loading. As with most places around here, and what I was taught, had you pulling the leader out until the 1st splice lined up with the top roller of the platter stand (which was spaced appropriately away for the 7-sec delay), then thread the projector (while probably stepping all over your spooled out leader), then thread back to the rewind platter. Wrapping it around your hand, and carrying it with you, and using the slip winding on the ring seems much much more elegant.

Go IMAX (courtesy of Film-tech) for showing me a better way. Lets hope I find somewhere I can use it. I may end up back at one of the local Carmikes. Though they mostly run digital, some film occasionally, but that's for a different thread. I would love to learn to run large format, if not an IMAX itself. Seems like a bit of work, but well worth it.

Posted by Mark Gulbrandsen (Member # 72) on 05-04-2008, 08:11 AM:

quote: Tristan Lane
Mark, you see where I'm coming from. The registration pins are so reliable because they are stationary. If the added retraction to them, I would imagine picture steadiness would be greatly affected, even if the tolerances were tight.

Exactly! There are a number of variations on the cam units though in the IMAX stuff. The Iwerks (CDC) cam units are all pretty much the same. When the bearings in the cam unit which are ABEC 7 bearings begin to go out of tolerance you can actually see the problem on the screen as a very slight horizontal jitter. The cam unit operating within tolerance is actually just as important as the fixed pins are. Retracting the pins wouldn't be all that hard to do(like reg pins in a Mitchell camera movement) but keeping them within tolerance and lubricated complicates it immensly. Camera movements have felt pads around each reg pin that hold just enough oil for 1000 feet of film at sound speed.

Mark

Posted by Tristan Lane (Member # 1169) on 05-04-2008, 09:02 PM:

In addition to the tolerance issue, there really wouldn't be a good way to keep the pins lubricated without said lubricant ending up on the film, and then ending up on the field flattener during subsequent runs.

I miss working with those projectors.

Posted by Jesse August (Member # 1798) on 05-07-2008, 11:10 PM:

Tristan which theatre did you work at?

Posted by Chris Slycord (Member # 4239) on 05-08-2008, 08:45 AM:

He's got to be at the OMNIMAX in Portland as there aren't any imax theaters in Vancouver, WA unless a new one's been built and the imax people didn't update their site.

Posted by Tristan Lane (Member # 1169) on 05-08-2008, 05:41 PM:

I haven't operated IMAX in over 5 years. I started as a projectionist at Edwards Boise IMAX, where we had an SR / QTRU setup. I worked for a short time in Spokane at the city's IMAX theater. It was a single Classic projector with a Mark II reel unit.