A SPROCKETFUL OF DIGITALS
Our current economic woes not withstanding, the market for surround-sound home theater systems continues to flourish. Now, even the most hidebound audiophiles must concede that the marriage of audio and video has been well and truly consummated. In the not too distant future, the union will see yet more inter action-with digital technology playing a major role.
Whatever the degree of technical sophistication of tomorrow's home theater systems, they still must depend on the movie business for program material. After all, movie videocassettes and videodiscs are really a byproduct of feature films, and thus technological advances in the audio and visual qualities of these films will directly benefit the owners of home theater systems.
Contrary to what most home theater enthusiasts believe, the digital sound tracks of these movies' videodiscs are not derived from original digital masters. The majority of Dolby Stereo movie soundtracks are recorded with Nagra quarter-inch analog magnetic tape recorders that provide special features like pilot tones and SMPTE time code. In the same way that many CDs are made from analog tape masters, the digital soundtracks of video discs are commonly derived from analog Nagra soundtracks.
There has been some use of professional R-DAT recorders to provide digital movie soundtracks. But in the very conservative (and expensive) world of film recording, the prime consideration is reliability of equipment. The Nagra is a proven device, while the jury is still out on R-DAT recording for sound tracks. Meanwhile, drawing attention is the increasing use of Dolby Spectral Recording (SR) soundtracks. Dolby SR is an analog process, but with a dynamic range of over 100 dB it actually exceeds the 96 dB of 16-bit digital recording. Given the wide frequency response and dynamic range of 35-mm Dolby SR films, theaters that have been acoustically modified and equipped with high-quality amplifiers and loudspeakers can deliver sound of stunning impact and clarity.
Nonetheless, the movie industry is well aware of the power of "digital" as a buzzword and a marketing advantage. Many consumers, for example, will not buy CDs if they do not have a SPARS code of DDD. So for some time, a number of companies have been trying to develop a film format with a digital optical soundtrack. This is a formidable task-and a particular aspect of it is of paramount importance.
As a foreword to this, let me sound a personal note. In 1932, when I was 12 years old, my Aunt Josephine married a brilliant Danish engineer, Gustav Plahn, whose field was optical systems and photography. Uncle Gus was a fascinating man of ideas and a great teacher. At that time, "talking pictures" were in their infancy, and color movies were rare indeed. One day, Uncle Gus took me to his studio/laboratory and showed me a large movie camera of very unusual design. He then handed me a length of 70-mm sprocketed black-and-white movie film. When I held it up to a light, I could see what appeared to be three identical images across the width of the film. The cam era had been equipped with beam splitters behind the lens to produce three images. A red filter was placed behind one beam, a green filter behind the second, and a blue filter behind the third. A disc about a foot in diameter, which had various geometric cutouts, was mounted behind the filters and was rotated at high speed by an electric motor. Thus, the three filtered images were exposed on the film. Being standard black-and-white film stock, it was developed in ordinary black-and-white film chemicals. In essence, there were red, blue, and green negatives on the film for every frame.
Gus had filmed my red-haired aunt wearing a turquoise dress as she was walking around a blooming flower gar den. Then he ran the black-and-white film through a projector that reversed the order of elements in the camera color wheel, RGB filters, beam splitters, and on through the lens to the screen. The result was astonishing:
There was my pretty aunt in "living" but very natural and realistic-color! By contrast, the color film processes of the time produced rather garish, over saturated hues.
A year later, Path& the French company whose newsreels were quite popular, offered to buy Uncle Gus' color system for a million dollars--a great amount of money in those days. He foolishly refused the offer because he wanted royalties as well. Pathe soon realized it was lucky in this negotiation, because Uncle Gus' system had a fatal flaw which has been the bane of other motion picture devices through the years. The flaw was that the system was not even remotely compatible with the projection technology of the time.
Uncle Gus' technique was conceived in the depths of the Depression, and movie theater owners simply could not afford to install a costly new Plahn projector. To make matters worse, the projector could not be used to show standard 35-mm movies.
About that time, early Technicolor films began to appear. Technicolor, employing a multi-layer film, was a process that required laborious dyeing of each layer with its separate color. Yes, it was cumbersome. Yes, the color was very chromatic and overbright. How ever, the system was acceptable-and most of all, it could be shown through the standard projectors in any theater.
Today, with the movie industry apparently ready for films with digital soundtracks, a number of companies have set out to develop such a recording system. A French company developed what is known as LC Concept, in which a time code is recorded on the film to synchronize with an optical disc that plays back the digital soundtrack.
While the quality may be quite high and while the integrity of the analog soundtrack on the film is maintained this system does raise the question of compatibility, in that two different elements are required, and it is easy to imagine the film and the optical disc being separated or one of them lost, with obvious consequences.
Another contender is a digital soundtrack system jointly developed by Kodak and Optical Radiation Corp.
This is called Cinema Digital Sound, or CDS. Compact Discs have their digital signals in their information pits; the CDS system uses tiny spots that are light and dark in a matrix pattern on the film, positioned in the same area that is normally used for the analog stereo soundtrack. This last point is very significant, because once again, there is a compatibility problem. If a CDS soundtrack is damaged or otherwise becomes unusable when running through the projector, there is no analog counterpart available to maintain sound continuity in the movie. The CDS system includes much clever technology, but I understand ORC has laid off personnel and is attempting to sell its interest in the system, citing the recession and the slow acceptance of CDS. Kodak, however, remains committed.
This basically leaves the playing field to Dolby Labs, which, from its earliest dealings with motion picture sound, has had an ironclad policy of maintaining compatibility with cinema systems in current use. After trying a number of ideas, Dolby engineers came up with the clever idea of employing the unused space between the sprocket holes of 35-mm film to en code a digital optical soundtrack. This SR-D system is wholly compatible. If the digital soundtrack between the sprocket holes should become unusable, the SR Stereo Variable Area (SVA) soundtrack is automatically switched in, thereby maintaining sonic continuity.
Dolby SR-D follows the SMPTE recommendations for digital film sound. Six channels are used: Left, right, center, sub-bass, and left rear and right rear surround. Most important, unlike Dolby's optical analog stereo, which uses a matrix for its four channels, the digital six-channel system is totally discrete-and thus separation and localization can be much more precise. The six channels operate in a 48-kHz sampling rate to code 16-bit words. In order to help get six discrete channels in the spaces between the sprockets, Dolby has used its AC-2 adaptive transform coding algorithm to com press the 48-kHz, 16-bit data to one-sixth the original rate. On the film, the optical digital data-the ones and zeros-are represented by dark and light spots. This data is read by a CCD scanner, which is easily mounted on standard movie projectors. The error-correction and digital data are very robust. In a conversation I had with Ray Dolby, he told me his engineers "looped" film with the digital sound track and played it over 1,000 times and it was apparent the visual quality of the film would be impaired before the digital data would corrupt! Ray also commented that in production form, beginning in mid-1992, the coding will be increased to 18 bits to accommodate the 100-plus dB of the system's dynamic range. The SR-D equipment should be widely available by the fall of this year.
There is much more to SR-D, of course, but for now its significance for the surround-sound home theater owner lies in the prospect of better sound quality in today's videodiscs and in future videotapes. In Japan, S-VHS VCRs with 16-bit PCM audio are available, and 8-mm models are coming. However, availability in the U.S. may await English-language software.
(adapted from Audio magazine, Feb. 1992; Bert Whyte)
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