Cinema: two questions

I’ve two questions regarding the moving picture shows.

First, what causes all the noise in the image? Every movie I go to has all this junk flashing in and out on the screen: lines, blobs, what appear to be magnified hairs and lint. I can understand that the projector room may have dust, lint, and hairs in it and that these could get stuck to the reel. But the other junk: what is all that? Is that the best reproduction they can do?

Second, how do they synchronize their new-fangled digital soundtrack to the olde-fashioned analog projector? Is the sound encoded beside the frames of the movie, on the same strip of film, or on a separate medium? If it’s on the same medium, is it read concurrently with the film or is it offset by some number of frames so the sound-reading equipment can be separated from the projector?

How movie projectors work.

Dolby Digital sound is on the film, snuggled in between the sprocket holes as a series of 2-D barcodes.

DTS sound is on a CD, which is synched to the film via a timecode signal on the film, next to, or replacing, the standard analog soundtrack.

SDDS data is placed on the far edges of the film. I’m not sure what the SDDS data looks like.

For the various digital formats, they’re all set up with a known length of film (number of frames divided by 24 = time in seconds) away from the projector shutter / lens.

You can find a picture of how the various formats co-exist on a piece of film at Dolby’s website -

The amazing part is not that digital sound works, but that there can be three different formats on the film at the same time, thanks to how the various companies located their data. If you count the analog soundtrack, that takes it to four sound formats on the film.

As for the junk in the picture, keep in mind that an image smaller than a postage stamp is being blown up to 20-30 feet tall, so pretty much anything that’s on or near the film as it goes through the projector will be visible. Long vertical lines are scratches caused by rough handling or a burr anywhere along the film’s travel. Hairs should be obvious in nature. Blobs can be just about anything else - dust, skin cells, fingerprints, etc.

Blobs can also be holes in the film which are used to signal the projectionist to switch reels. Quite obsolete these days since most every theater assembles their reels onto one giant platter projector nowadays.

Thanks for all your replies. The info about audio, gotpasswords, is really interesting – I hadn’t thought to wonder about compatibility between all the different formats you see in various theatres. The basic workings of a projector I understand; does anyone have a link to more info about the various sound formats and their encoding?

Most of the screen garbage I can understand, like little bits of fibre or hairs. I take it the quality of the movie degrades over time as the film is run through the machine more and more often? Scratches and such are also understandable, as they pervade across many frames. I can see that a bit of hair or fibre could get caught in front of the lens for a second or two, too.

Some of the artifacts I see, though, I still can’t place. For example, at a recent screening of About Schmidt, a black disk with a thin yellow border appeared in the upper right of the screen for a fraction of a second. Sure, if someone spilt coffee on the film it would leave a blob. But that blob wouldn’t last for more than a single frame, and shouldn’t be so obvious for the movie-goer. I’d have thought 1/24 of a second wouldn’t be long enough to register a blob like that; how would it pervade over several frames if it was accidentally added to the film?

I suppose those in the know would have opinions regarding the quality of film used by different shops, and the care of handling offered by various cinemas, but that’s not really a concern for me so much as the general practices of these establishments.

Hmm, looks like the How Stuff Works link provided by Q.E.D. explains that circle I saw as being a cue to the projectionist to change reels – likely a technological throwback, as friedo indicates.

That audio stuff is really neat!

This is the sign to switch reels that friedo mentioned. I know this has been discussed extensively, but I can’t find a thread. Someone will be in to fill in the details, but basically there are two of those at a time, spaced ~10 seconds apart. The reel is (or was) changed when the second one flashes.

And if you’re like me, now that you know about these, you’ll see them every damn time from now on.

Fight Club ruined theater films for me…They point out the little “cigarette burns” as they call them, and ever since, I cannot ignore them…:frowning:

Away from the change of reel marker…I would say that some of the noise is simply due to the huge amounts of magnification required to get the image from something quite small, to a screen thousands of times as big - any imperfections and grainyness is bound to show up.

Columbo did the same thing back in 1978. Damn you, Peter Falk! :mad: