There are three existing and competing systems that the film industry now uses to record and playback soundtracks digitally; each is designed by a different manufacturer -- Dolby, Sony and Digital Theatre Systems Inc. Put all the abbreviations for their competing systems together -- SRD, SDDS, DTS -- and it really makes one wonder why a single digital system could not be agreed upon....then one remembers Corporate Greed, and the question is answered.
Each system uses one of two methods to allow digital sound to accompany the film image. The first method is an on-board/sound-on-film system that records the digital tracks on the film itself, much the same way that the analog soundtracks are recorded alongside the picture on the same piece of film -- Dolby and Sony have opted for this method. The second scheme records the digital soundtracks on a separate medium apart from the film, and then that medium is electronically locked to the film as the image is being projected -- the DTS system uses this method. This interlock method has been used successfully as far back as 1939 with Disney's Fantasound which was developed for FANTASIA's highly innovative stereophonic sound system. It didn't return until the early 50's when interlocking sound was used in Cinerama, early Todd-AO presentations and finally most recently in the IMAX sound system. Both the interlock and sound-on-film schemes have their own advantages and disadvantages.
The challenge of recording six digital tracks on the film itself (Sony system has the potential of recording 8 tracks) is that there is a very limited amount of available film geography on which additional information can be placed. Both the Sony and Dolby systems deal with this space limitation by compressing the sound and using various compression or "masking" schemes that drop out audio information which the system logarithms determine will not be missed by the human ear when they are subtracted from the audio waveform. There are some in the industry who claim that this manipulation of the original sound produces a degraded reproduction that can be heard in A/B tests. Perhaps, but in practical applications in theatre playback, most patrons when queried, will tell you they hear nothing but clear, well defined sound with exceptional clarity and directionality.
Digital Theatre Systems (DTS) on the other hand uses the outboard/interlock scheme. Recognizing the difficulty in recording six tracks of broad bandwidth sound in the very limited free film space, at least not without lots of data compression, DTS's interlock system abandons the film as the location for the sound entirely. Instead, the soundtracks are recorded on a separate or outboard medium, in this case, CD-ROMs. These discs cannot be played by a standard CD player or computer audio programs; they use proprietary configurations designed by DTS.
Use of the interlocked CD-ROMs allows the soundtracks to be recorded without the need for heavy compression, sound masking or other slight-of-hand (ear?) methods of reducing the original sound information. Many audio purists claim the DTS method is by far the best of the three systems because of this advantage. The six soundtracks on the CD are locked to the film frames in playback by a proprietary DTS timecode which consists only of a series of miniscule dots and dashes which are placed on the film. Physically, this timecode can be very small because it need only to contain the information that tells the computer what sound should be playing with which projected frame; it does not carry actual audio information. The timecode is so small that it can be sandwiched between the edge of the image and the edge of the analog soundtrack (see diagram), without need to change any of the original specifications of these elements.
The advantage of the onboard/sound-on-film scheme is that the soundtrack can never be separated from the film -- it is printed on the film along with the picture and the analog track. With DTS's outboard/interlock scheme, there is always the possibility of the soundtrack CD-ROM being physically separated from the film print, say in shipping or in handling the film cans at the inspection depots -- but there is that same possibility that some of the reels could be separated from the rest of the print. Although this is possible, it has never been a major problem in print distribution.
With an interlock system where the soundtrack is not located on the film itself, a major issue is the question of picture/sound synchronization. We all know how disconcerting it is when the sound looses synchronism with the image, even by a few frames. The ability of keeping perfectly accurate synchronism (called "lip sync") must be part of any sound scheme. With onboard/sound-on-film, the position of the soundtrack is forever married to the frame that corresponds to it by virtue of its physical location on the film itself. Synchronization is already accomplished by this very fact. Outboard/interlock systems are not so lucky. In the past, interlock systems such as the magnetic soundtracks interlocked with Cinerama presentations were susceptible to synchronization problems and required some fairly elaborate and complicated methods of interlocking the sound playback unit to the projector and the corresponding picture frame. With this older technology, reliable sync was problematic; today, with the help of computer control, the DTS system has accomplished lip-sync: perfect, reliable and most importantly, repeatable picture/sound synchronism. In fact, the DTS interlock is so accurate that Reel 7 and Reel 2 of a feature can be spliced incorrectly out of order and at the point were the last frame of Reel 2 is spliced to the first frame of Reel 7, the soundtrack will not be interrupted.
DTS not only tracks the correct picture frame to its corresponding digital sound, but should the timecode be momentarily lost, the DTS computer will seamlessly switch to the analog track while it recovers, then switch back to digital when the timecode becomes readable again. Dolby and Sony systems also use the analog track as a backup in the event the digital tracks fail.
Outboard/interlock also provides the distributor with a convenient and cost-effective method of using the same print for various foreign language releases by simply providing the CDs that have been recorded with the desired foreign language soundtrack. The onboard systems do not have this advantage. Further, DTS also uses it's frame identification code to tell a separate video projector what captions to display so that films can be captioned for those hard of hearing. This same time code information can also be use to lock a DTS CD-ROM containing a descriptive narrative that can be used for the blind.
Today, many features are released with all three digital tracks on the film -- tri-D prints -- (see diagrams below) and therefore can be sent to a theatre having any one of the three playback systems. This is possible because all three systems use different areas of the film to record the digital information. Dolby uses the spaces between the sprocket holes. Sony uses the space on the outside edge of the film and the outer wall of the sprocket hole. Since this area is subject to a great amount of wear by the projector sprocket drive wheels and rollers, Sony also uses the same area on the opposite side of the film as an identical, redundant track. DTS, as mentioned, uses the septum between the picture and the analog soundtrack to carry its time-code. With a print that contains all three digital formats, the producer doesn't have to worry about which print needs to go to which theatre. And of course, all three digital formats still retain the standard bilateral, analog stereo soundtrack -- 5 channels, usually recorded in Dolby SR. Thus, even a theatre that has none of the digital equipment -- and there are lots of them out there, especially in the outer boroughs -- such a theatre can still play a digital film quite acceptably using the analog surround stereo tracks.
Darth Vader image © 1976 LucasFilm Ltd.
Detail - actual print magnification:
Note: Sony's SDDS places an identical track on the opposite edge of the film (not shown), in the same location -- between the edge of the film and the edge of the sprocket hole.
Parenthetically, as a lesson in backward compatibility, it should be mentioned that a few years before the three current systems appeared, a collaboration between Kodak and Optical Radiation Corporation pioneered the first commercial digital sound system for theatrical release prints. It recorded 6 tracks of audio as do all three of the existing systems, but it placed the digital datastream in the area that was originally occupied by analog soundtrack. It was as much a technical success as any of the current systems, but like all things technical, the economic aspects of it must work as well. A few films were released with this system called Cinema Digital Sound (CDS) and they sounded and played back every bit as well as today's digital systems. Unfortunately, prints made in CDS contained no analog track, that area on the film being occupied by the new digital datastream. This meant that the distributor needed to not only maintain a double inventory of both analog and digital soundtrack prints, but had to make sure the right print was shipped to the theatre that could play that format.
One of the key factors in the success of all three current digital systems is that a tri-D(igital)/analog print -- one containing all four formats -- DTS, Dolby-D, SDDS and an analog Bi-lateral stereo track -- can be sent to any theatre and it will play correctly, no matter what equipment the theatre owns. Imagine the chaos if the distributor needed to supply separate prints for all three digital formats as well as a fourth for theatres that could only play analog tracks! With some features, while the prints do contain all three digital formats, because of contractual agreements (or disagreements), the theatre may be instructed that they are only licensed to use only specific tracks and may not use one or two of the others.
Even though we feature DTS, our system has built-in flexibility; we can incorporate any or all of the other systems within a matter of a few hours setup time should the occasion arise when DTS tracks are not available.
Brooklyn Center Cinema plays digital features with the DTS system. Coupled with our THX-type 6 Channel Total Surround MegaSound, it produces incredibly life-like stereo reproduction, from the faintest whisper of a breeze to the thunder of an earthquake; it is sound you feel as much as hear.
With the DTS system, we find that the sound quality is nothing short of breathtaking. Our playback system uses point-source left-right surrounds (massive ElectroVoice systems in either corner of the theatre), a unique configuration favored in THX specifications over wimpy, multiple sidewall surrounds that are used in the typical multiplex shoebox theatres. This, and our unusually powerful, earth-shaking subbass bins constitute our signature 6 Channel Total Surround MegaSound ®
We often relate the story of how when we first installed the 4 subbass bins with the 18 inch long throw drivers, we wanted to stress the system to see where distortion began and how much headroom we had. For the test source we chose a CD featuring a recording of Pictures at an Exhibition. We slowly increased the level to over 130db with no sign of stress to the system. However as the kettle drums hammered out their famous finale, a frantic call to the booth was made by our resident Lighting Designer, Steve Bailey. He was screaming over the music that was playing in the theatre. At first it was thought that he was trying to say we needed to turn off the sound and turn the house lights out, which made no sense. What he was actually saying was to turn off the sound system because the house lights were blowing out. The sound pressure was actually vibrating the bulb filaments and breaking them with each pound of the drums. It was clear that the system was able to produce clean and powerful sound without any hint of stress; we were ecstatic. Our Lighting Designer was not.
Because our booth incorporates an Allen-Heath 24 channel mixer, and an extensive patch bay feeding the amplifiers, we can also accommodate any new processing scheme that the industry may develop in the future without needing to do any extensive rewiring. For example, we obtained the only existing multitrack magnetic print of TOMMY from John Mosley, the sound designer who developed Quintaphonic Sound that was used on that film so it was not a standard configuration as well as with our special engagement of LADIES AND GENTLEMEN -- THE ROLLING STONES; it was simply a matter of inserting the necessary matrix decoders into the system to extract the unique tracks and configurations for these unique mixes.
If the industry moves to standardize the center channel surround, sometimes called the EX surround and sometimes called "Circle Surround" which adds an additional center channel in the rear of the theatre between the Left and Right surround channels, we are ready. We already have the processor necessary to derive the center surround channel as well as the speakers and amps...all the other wiring in the booth and in the theatre is in place. Trouble is, you can count on one hand the number of films that have been released which are actually mixed to include this additional channel.