Why do some movie theaters do this? - AVS Forum
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post #1 of 16 Old 11-08-2012, 11:41 AM - Thread Starter
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When I go to a movie theater, sometimes the audio track will switch between two different variations. It's like it is going from 7.1 to some other form (2.0 or 3.1?). The switch doesn't occur at "special times" but randomly through the movie. I do know that this is not suppose to happen but I was just wondering why this happens and what exactly is happening when it occurs.
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post #2 of 16 Old 11-08-2012, 03:18 PM
 
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because they didnt leave the microphone data organized enough to keep the rear speakers on the whole time .. or they did and they jst arent adding it to the disc.
and then the front speakers are the only things on.

i say calibrate the gelatin and let the system go at it.
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post #3 of 16 Old 11-08-2012, 03:52 PM
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Are you sure you aren't detecting the momentary out of sync signal that results when the reel is changed?
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post #4 of 16 Old 11-08-2012, 10:09 PM
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There are digital cues encoded on the film to tell the theater sounds system what mode to switch to. Not all theaters have the ultimate in sound control yet so some systems do not detect this command and a manual change must be accomplished. BTW, most theaters in big cities now use fully automated projection booths that have the entire movie spliced together into a single piece of film and the film plays like a giant 8 track. The film is specially lubricated with an optically transparent dry lube and is pulled out from the center of the platten, runs through the projector and then winds around the outside of the film on the platten. Sometimes this film platten is 4 feet in diameter.
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post #5 of 16 Old 11-08-2012, 10:33 PM - Thread Starter
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Quote:
Originally Posted by Gizmologist View Post

There are digital cues encoded on the film to tell the theater sounds system what mode to switch to. Not all theaters have the ultimate in sound control yet so some systems do not detect this command and a manual change must be accomplished. BTW, most theaters in big cities now use fully automated projection booths that have the entire movie spliced together into a single piece of film and the film plays like a giant 8 track. The film is specially lubricated with an optically transparent dry lube and is pulled out from the center of the platten, runs through the projector and then winds around the outside of the film on the platten. Sometimes this film platten is 4 feet in diameter.

Thanks. But why is a switch even required in a movie theater? When we watch movies at home, there is no switching occurring on the audio track. confused.gif
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post #6 of 16 Old 11-09-2012, 10:35 AM
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Hi Kain,
Quote:
Originally Posted by Kain View Post

Thanks. But why is a switch even required in a movie theater? When we watch movies at home, there is no switching occurring on the audio track. confused.gif
Prior to the platter system that Gizmologist described, multiple projectors were used. Each projector would hold 6000 feet of film, which would play for 1 hour and 6 minutes. Then a "changeover" would occur (which was visible by two circles that would flash in the upper-right corner of the screen), which meant the next projector would start, and shutters would simultaneously switch to the next projector.

The "out-of-sync" problem that Prometa described is because the soundtrack is no longed on the film, but on a optical disk. The film contains time-code, and the disc needs to keep synchronized with the time-code. When the projectors switch, there is a discontinuity in the time-code that can cause audible 'glitches', as the disc hardware re-synchronizes with the film.

Another issue is that the movies are shipped in a series of 2000 foot reels, and the projectionist needs to splice these together. Individual frames are sometimes lost in the process of re-splicing, and this can also cause discontinuities in the time-code. These transitions would occur every 22 minutes, and are still an issue with the newer platter system that Gizmologist described.

Of course, all of these problems go away with digital projection that theaters are in the process of adapting, and which is basically what you have in your home.
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post #7 of 16 Old 11-09-2012, 01:08 PM - Thread Starter
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Thanks! Makes sense. smile.gif
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post #8 of 16 Old 11-09-2012, 02:52 PM
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Quote:
Originally Posted by MarkHotchkiss View Post

Individual frames are sometimes lost in the process of re-splicing, and this can also cause discontinuities in the time-code.
Or you can loose a number of frames when the projector jambs or the film was given too much slack at the gate and ends up tangled and broken. The projectionist will have to rebuild the movie after that showing, and a few frames can easily be destroyed. You might not see them, depending on the visual action in the scene, but the audio will still have the time code problem.
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post #9 of 16 Old 11-09-2012, 07:28 PM
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You also have the problm that nowadatys, there is rarely a projectionist on site. Everything is set up when the films are delivered and then manger just pushes a button to start everything or in some cases EVERYTHING is on a timer. If there is a loop problem thefilm may get damaged before anyone can respnsd.

My dad was a projectionist during his stint in WW2 and into the 1960's. He was also the official projectionist for the US Senate for a few years. He HATES to auto systems. Can't say as I blame him. I went into professional AV presentations in the 70's and I like the old school ways.
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post #10 of 16 Old 11-09-2012, 07:45 PM
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Quote:
Originally Posted by Gizmologist View Post

You also have the problem that nowadays, there is rarely a projectionist on site. Everything is set up when the films are delivered and then manager just pushes a button to start everything or in some cases EVERYTHING is on a timer.
Yes, that is how the typical multi-screen cineplex works these days. The projection room is actually one large room that extends to every theater in the place, and one projectionist runs all of the projectors. Typically, he has nothing to do, as everything is on timers. He just needs to be available when something breaks.

But if the film breaks, it usually takes a customer to go out to the refreshment stand to let someone know, as the management will have the projectionist cleaning the restrooms.
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post #11 of 16 Old 11-10-2012, 05:29 AM
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The momentary switching of sound quality can happen at reel changes (every 2000 ft or so), but can also happen mid-reel, unrelated to reel changes.

There are 4 optical sound tracks on 35mm release prints. The standard stereo optical track is the "fall back" track in case any of the digital systems can't read their data, because it will produce sound regardless of print damage, which is not true of any of the digital tracks. The switching you hear is that fall-back. The stereo optical tracks contain LtRt, which is a matrix-encoded 4.0 track contained on two discrete channels utilizing Dolby type A analog noise reduction. The high end is limited to 10KHz or so, possibly much less if the optical reader hasn't been aligned, and the dynamic range is limited also, though much improved by Dolby A. All of those factors cause the optical track to be very different sounding.

The digital tracks on optical film are fairly robust, but not impervious to damage from dirt and wear. The most commonly used track is Dolby Digital, which lands between sprocket holes.



You can see that it's possible to damage any of these digital tracks, though the DTS track shown in the image is used to sync an external CD with audio on it, and should be less prone to damage, though sync-loss issues do happen.

Print damage isn't the only cause for fall-back to optical. Reader head alignment or partial failure can result in random digital drop-outs as well.
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post #12 of 16 Old 11-10-2012, 08:10 AM
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So many individuals aren't aware of how all this works. I appreciate the knowledgeable responses,.... so much appreciated for those contributing.


As an aside;
We went to see Skyfall yesterday, in an IMAX theater,.....absolutely stunningly good in my opinion. Every single element of us seeing the film was outstanding. We sat about the 1/4 point in distance, thus the image and sound were entirely immersive. Nobody sat in front of us, so no visual distractions whatsoever. This helps me deal with the theater experience, and inherent potential distractions. This film was immensely enjoyable, the audio was supeb,...understated for sure but superbly executed. I really only was drawn to analyze the sound a couple times, which for me indicates a strong level of immersiveness in all things A/V cool.gif

We don't take in many films in the theater, but the last three have all been in IMAX rooms. Prometheus in 3D, Dark Knight Rises, and now Skyfall. Typical theaters just don't cut it for me.

Thanks

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post #13 of 16 Old 11-10-2012, 07:15 PM
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Quote:
Originally Posted by MarkHotchkiss View Post

Hi Kain,
Prior to the platter system that Gizmologist described, multiple projectors were used. Each projector would hold 6000 feet of film, which would play for 1 hour and 6 minutes. Then a "changeover" would occur (which was visible by two circles that would flash in the upper-right corner of the screen), which meant the next projector would start, and shutters would simultaneously switch to the next projector.
6000 ft reels on two projectors was actually a somewhat rare combination. More typically a house with a two-projector system would just project from the 2000ft reels, with changeovers every 20 minutes or so. The advantage was, no film "prep", splicing onto larger reels, or one big platter. Any house running archival prints would prefer a standard two-projector system using 2k ft reels, as showing a film reel-to-reel reduces the chances of print damage. You want to avoid cutting a print at all, even if just removing the leaders, to preserve the print condition. Splicing onto even 6000ft reels means trimming leaders off head and tail of each reel, and splicing them together, then once the run is finished, breaking the print back down onto the 2K reels, and re-splicing the leaders back on. That first and last frame of each reel takes a beating that way! If the film is run at many theaters, each time it's shown every reel is spliced and respliced for print make-up and break-down. That's why archival prints are shown directly from the 2K shipping reels.

Platters, if not maintained, can be brutal to a print in many ways, and are never recommended for use with archival prints.

By the way, people may not realize that theatrical projectors cannot rewind. The reels are removed and rewound using a rewind device, either geared-manual, or motorized, on a sort of work-bench like system. A guy running a two-projector changeover system is busy! Constantly lacing up the next reel, then rewinding the last, then timing his changeovers to be precise and unobtrusive. Not as easy as it sounds. Thus the birth of the platter...no changeover, no rewinding. Just potential print damage.
Quote:
Originally Posted by MarkHotchkiss View Post

The "out-of-sync" problem that Prometa described is because the soundtrack is no longed on the film, but on a optical disk. The film contains time-code, and the disc needs to keep synchronized with the time-code. When the projectors switch, there is a discontinuity in the time-code that can cause audible 'glitches', as the disc hardware re-synchronizes with the film.
The system described is DTS only. Both of the other digital audio systems are printed on film. DTS uses a tiny time-code track for sync with an external CDROM drive, and there can be sync issues at changeover, though frankly, anybody using DTS is probably operating from a platter system, and hopefully, the splices between reels have been accurately made, which wouldn't cause a sync issue. But, what happens is, if a print has already been shown at another theater, there may be lost frames when the print is either assembled to the platter or broken down to the 2k ft reels for shipping. It's a result of sloppy film handling, or print damage, but with DTS it can cause sync slippage until the system corrects. DTS has been less favored in general because of the dual-media problem, but technically the tracks on the CDROM are of a higher bit rate than Dolby Digital on film, though they use a totally different codec. DTS for theatrical and DTS as it appears on a consumer disc are not at all similar.
Quote:
Originally Posted by MarkHotchkiss View Post

Another issue is that the movies are shipped in a series of 2000 foot reels, and the projectionist needs to splice these together. Individual frames are sometimes lost in the process of re-splicing, and this can also cause discontinuities in the time-code. These transitions would occur every 22 minutes, and are still an issue with the newer platter system that Gizmologist described.

All true, but the lost-frame issue stems from sloppy film handling only. There is no good reason a frames need be lost if splicing and handling is done carefully. You can always fault a projectionist somewhere for this.
Quote:
Originally Posted by MarkHotchkiss View Post

Of course, all of these problems go away with digital projection that theaters are in the process of adapting, and which is basically what you have in your home.
We can argue the comparative pros or cons of digital projection, but in this world of budget-cutting, automated systems operated by unskilled and under-paid labor, automated all-digital systems will probably result in better projection. Just a few years ago I almost never got through the first reel of a picture without leaving my seat to complain about something...focus, sound level, blown drivers, framing, lack of anamorphic lens, or anamorphic when it shouldn't have been, lamp trim, you name it. But since digital projection, I don't seem to get up to gripe as much. Either I'm more tolerant in my old age, or things have gotten better.
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post #14 of 16 Old 11-10-2012, 09:19 PM
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Hi Has,

Thanks for your insight. You have closed a few holes in my knowledge with regard to the audio formats. I did not realize that only DTS used CDs, although I should have, since I've seen the picture you posted many time before.

Also, I suspect what Kain is hearing is the fail-back to the optical stereo track, as you said.

I think your comment about using 2000 foot loads instead of 6000 foot loads was true up until the advent of the cineplex. As you said, swapping 2000 foot loads meant that the projectionist was always busy, and you would therefore need one projectionist per screen. When you put 6000 feet on each of two projectors, and then have them coupled with an auto-changeover mechanism, the projectionist only needs to start the film, and then rewind & reload the film after it has finished. The splicing is done once on Friday when the film arrives, and is cut apart again when the film's run ends. It allows one projectionist to service multiple screens. It also allows for a less competent projectionist.

By the way, that was the reason studios and theater owners were so adverse to movies much over two hours (remember "Brazil"?). Movies over 12,000 feet just wouldn't fit on the two projectors. Platters alleviated that problem, and also allowed for the addition of more trailers and commercials.
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post #15 of 16 Old 11-10-2012, 11:26 PM
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In many cineplexes with the latest gear, there is no rewinding as I explained earlier. The film pulls out of the center on the platter, runs through the projector and returns to te outside of the platter. When the contracted show dates are completed the film splice at the beginning is undone and the packaged again for transport. to the next venue or the warehouse.
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post #16 of 16 Old 11-11-2012, 07:14 AM
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Quote:
Originally Posted by Gizmologist View Post

In many cineplexes with the latest gear, there is no rewinding as I explained earlier. The film pulls out of the center on the platter, runs through the projector and returns to te outside of the platter. When the contracted show dates are completed the film splice at the beginning is undone and the packaged again for transport. to the next venue or the warehouse.

FWIW I can confirm this.

When they are showing the same movie in multiple rooms, there is only one pancake, and there is film running back and forth along the walls and just under the ceiling, up and down the corridor where the projectors are situated.

Has to be seen in action with 3 different movies being shown concurrently in multiple rooms to be believed!
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