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» Film-Tech Forum   » Operations   » Large Format Forum   » IMAX platter/threading question (Page 1)

 
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Author Topic: IMAX platter/threading question
Thomas Pitt
Master Film Handler

Posts: 263
From: Leeds, West Yorkshire, UK
Registered: May 2007


 - posted 03-02-2008 03:44 PM      Profile for Thomas Pitt   Email Thomas Pitt   Send New Private Message       Edit/Delete Post 
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?

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Brad Miller
Administrator

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From: Plano, TX (36.2 miles NW of Rockwall)
Registered: May 99


 - posted 03-02-2008 05:06 PM      Profile for Brad Miller   Author's Homepage   Email Brad Miller       Edit/Delete Post 
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.

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Mike Olpin
Chop Chop!

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From: Dallas, TX
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 - posted 03-02-2008 06:32 PM      Profile for Mike Olpin   Email Mike Olpin   Send New Private Message       Edit/Delete Post 
MPX systems thread through the brain, so I don't think it's a limitation of the platter being unable to keep up.

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Brad Miller
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From: Plano, TX (36.2 miles NW of Rockwall)
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 - posted 03-02-2008 06:39 PM      Profile for Brad Miller   Author's Homepage   Email Brad Miller       Edit/Delete Post 
With the same QTRU I would presume?

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Monte L Fullmer
Film God

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From: Nampa, Idaho, USA
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 - posted 03-02-2008 06:41 PM      Profile for Monte L Fullmer   Email Monte L Fullmer   Send New Private Message       Edit/Delete Post 
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.

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Adam Martin
I'm not even gonna point out the irony.

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From: Dallas, TX
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 - posted 03-02-2008 07:05 PM      Profile for Adam Martin   Author's Homepage   Email Adam Martin       Edit/Delete Post 
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.

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Steve Guttag
We forgot the crackers Gromit!!!

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From: Annapolis, MD
Registered: Dec 1999


 - posted 03-02-2008 07:14 PM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
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

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Brian Michael Weidemann
Expert cat molester

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From: Costa Mesa, CA United States
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 - posted 03-03-2008 07:43 AM      Profile for Brian Michael Weidemann   Author's Homepage   Email Brian Michael Weidemann   Send New Private Message       Edit/Delete Post 
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]

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Thomas Pitt
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From: Leeds, West Yorkshire, UK
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 - posted 03-03-2008 03:36 PM      Profile for Thomas Pitt   Email Thomas Pitt   Send New Private Message       Edit/Delete Post 
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.

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Ben Wales
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 - posted 03-03-2008 04:35 PM      Profile for Ben Wales   Email Ben Wales   Send New Private Message       Edit/Delete Post 
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.

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Paul Mayer
Oh get out of it Melvin, before it pulls you under!

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 - posted 03-04-2008 03:02 AM      Profile for Paul Mayer   Author's Homepage   Email Paul Mayer   Send New Private Message       Edit/Delete Post 
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.

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Brian Michael Weidemann
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 - posted 03-04-2008 04:53 AM      Profile for Brian Michael Weidemann   Author's Homepage   Email Brian Michael Weidemann   Send New Private Message       Edit/Delete Post 
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.

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Tristan Lane
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From: Nampa, Idaho
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 - posted 03-05-2008 02:16 AM      Profile for Tristan Lane   Email Tristan Lane   Send New Private Message       Edit/Delete Post 
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.

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Tim Reed
Better Projection Pays

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 - posted 03-27-2008 04:04 PM      Profile for Tim Reed   Author's Homepage     Send New Private Message       Edit/Delete Post 
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.

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Tristan Lane
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From: Nampa, Idaho
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 - posted 03-28-2008 02:27 AM      Profile for Tristan Lane   Email Tristan Lane   Send New Private Message       Edit/Delete Post 
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.

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