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Movie theater sound formats & how they work

Movie theater sound plays back much the way your stereo at home does: it's fed a signal, which is puts into amplifiers and crossovers (so high sounds go to tweeters and low sounds go to woofers and subwoofers) and then it's played through speakers. In a movie theater, there are more speakers - usually at least a center channel, a left channel, a right channel, and a mono surround that'll have speakers on both sides of the theater - and there is frequently a lot of wattage going on - the Village theater in Westwood has an astounding 18,000 watts! I think mine at home is 200 watts and if I turn it up more than about half way, I get complaints from the manager. But the technology is the same.

At home, the signal comes from a tape or a CD (or the radio). A tape is magnetic, and the signal is generated by varying strengths of the magnetism on the tape. It's an analog format - there's a direct correlation between the magnetism on the tape and the voltages of the sound - and it's prone to hiss and hum and whatnot. A CD is a digital format - a laser reflects off pits in the CD and that makes a series of ones and zeros that are then converted into a voltage signal by circuits in the CD player. There's no problems related to degrading magnetism or wear, as the laser never touches the disc.

Film frame with mono analog sound
Mono sound - just one channel, standard before about 1977 - was achieved with that wiggly white line to the left of the picture. A light would shine through the wavy line into a photosensor, which would generate varying voltages based on how big the line and thus how bright the light was. Those voltages are sent to the amps. Wear and tear on the film can have an effect on sound quality if that space gets beat up or scratched.
Film frame with stereo analog sound
Dolby Labs came up with two wiggly lines in that space that would add up and play correctly in mono systems, but could be decoded by a complex scheme into four channels - center, left, right and a mono surround - that became a standard: Dolby Stereo. Eventually they came up with Dolby Spectral Recording (SR), which is an greatly improved noise reduction system (another 10-15db over standard [type A]).
Film frame with SR-D encoding
The next big step was Spectral Recording Digital (SR-D), which uses the space between the sprocket holes on the left to put digital information. It's like a grey-green static (with a really, really tiny Dolby Labs logo in the middle - very cool) and like analog sound, an optical sensor shines a light through the film to get the information. Instead of voltages, it reads ones and zeros, like a CD. It's a six-channel system like SR.
Film frame with SDDS encoding
Dolby isn't alone. Sony has Sony Dynamic Digital Sound (SDDS). It's an eight channel format - adding Left Center and Right Center channels - but few films are recorded with eight, and some theaters don't have the extra speakers. It's a blue-grey "static" down both edges of the film, and also requires a special optical sensor and decoder. Both systems have redundant sets to help with data errors from worn or damaged film.
Film frame with DTS time code
A different system is Digital Theater Sound (DTS), which made a big splash with "Jurassic Park". It uses a machine readable time code, encoded down the edge of the picture (in this drawing, the dotted yellow line.) The time code reader controls a special external CD player, which has the digital soundtracks spread over (for most films) two CDs. The CD's aren't prone to wear, but the time code on the film is.
Film frame with all three digital formats
It's possible for film to be encoded with all formats - analog Dolby Stereo or Dolby SR, SR-D, SDDS, and DTS time code - because they all use different parts of the film. Plus, the physical readers can be put in various places above or below the projection unit, allowing installation of all the formats at once. (There was once also Cinema Digital Sound (CDS) but it didn't allow for the backup analog track, and quickly died.)

Because various studios have aligned themselves behind one format or another, you pretty much do need all three to make sure any film you get you can play digitally. It's too bad - SDDS has a lot of technical problems and would have died by now if every AMC theater didn't have one. It's not a great place to put information anyway - the edges of film can end up misexposed due to light leakage, and it's where coding information about the type of film stock is supposed to go, and the two certainly can't coexist there. DTS grew quickly because it's cheap, but it's not a great system technically and it requires that you have those CDs, which cost an extra $100. It's rare that you'll have DTS trailers, as you'd need to have the sound on a CD. DTS defends their system saying that by using CDs it's quite inexpensive to simply make new CDs with dubbing for foriegn markets, and use the same prints. Whatever. All I know is it causes problems here if you can't get the CDs, and it's too easy for the time code reader to get confused.

I'll always pick Dolby Digital over the others, and most people I know who work in projection pick it as well. It seems to have the best sound (all systems are digital, yes, but encode and decode in different ways, which can effect the nature of the sound reproduction) and it's evidently the sturdiest system, less prone to problems and dropouts than the other two. It's also showing up on laserdiscs, and it appears it'll be the type of format used with HDTV transmissions.

There used to be 35mm mag, which had magnetic stripes just like tape on the film, and worked just like tape. It's still the way sound is done on 70mm prints. While it's prone to the same problems tape has, 70mm 6-track mag sound when it's clean is the best thing there is. This fall's 70mm rerelease of "Vertigo" was in DTS, but there was no mag backup on the film; pretty scary.

And there you have this first pass at this. Apologies for any errors, and I'll try to correct them as I find them. But I believe this is a pretty detailed primer on what happens up in that little room at the back of the theater.

Otto Kitsinger 13 Jan 97, rev 26 Jan 97

I can remember a time when where we went to the movies was just as important as the movies we went to see .... From the moment moviegoers arrived to buy their tickets, there was a sense of something special, a feeling that to step inside was to enter another time and place. - Gene Kelly