DIGITAL SOUND FOR 70mm

A digital sound system for the 70mm format was introduced in May, 1990. It is called Cinema Digital Sound, or CDS. CDS is the audio system co-developed by the Motion Picture and Television Products Division of Eastman Kodak Company in Rochester, New York, and Optical Radiation Corporation, in Azusa, California.

The CDS system provides six tracks of digital sound, five of which are full-bandwidth channels, and one is a sub-woofer channel (..... in today's terminology, written as a "5.1" surround sound system). The sub-woofer channel reproduces low-frequency bass tones enabling the audience to "feel" the sound, a now widely-used, dynamic effect in cinemas. Like all the new digital cinema-sound configurations, it is a powerful, multi-speaker system of pristine, CD-quality sound. By contrast, traditional 35mm sound tracks provide two discreet "stereo" tracks matrixed to provide four channels of sound, including a surround channel.

In addition to CDS's 6-discreet audio channels, there is a MIDI (Music Industry Digital Interface) control channel which enables the system to be interfaced with a computer. It's a synchronisation track with SMPTE time-code, providing a machine-readable "address" on each frame, plus various identification fields, such as the film's title, dates, and similar information. This control channel can be used for projection booth and auditorium automation, and for synchronising in-theatre special effects, such as moving the seats, triggering strobes and lazers, or other devises to amplify the movie-going experience. ("Did the seat move for you, dear?").

The CDS digital track is slightly wider than a single magnetic track on standard 70mm. The question arises as to where to put a digital track on a 70mm print. There is more than ample room across the hugh 70mm width for a digital track, but unfortunately, there is not a neat slot for it. CDS considered a number of possibilities, such as putting the track outside the perforations where it is more prone to mechanical damage, and changing or moving the picture area. They came to the conclusion that the most reliable, and the least disruptive to existing 70mm systems, was to put the digital track in the position of the number 4, magnetic track. This places it inside the perforations, and next to the image on one side of the film. (The other 5 magnetic-track positions are left unused) The slightly larger digital track robs the image of about one percent of it normal area on each side of the frame. Ronald E. Uhlig, a group leader in the electronic development section of Kodak's Motion Picture and Television Production division, says, "That should have no impact on the way films are composed. Cinematographers very rarely use the extreme edges of the frame in their composition, since most exhibitors use a mask with 70mm projectors in order to create a black frame around the picture area."

The reason that CDS was first introduced in the 70mm format was that 70mm equipped cinemas already use a 6 track system with high-quality front and surround speakers. It is a relatively cheap and simple conversion. They need to retrofit the 70mm projector with a digital-sound, pickup head, and install a digital processor in their equipment rack. The digital pickup will fit any projector, and is easily installed. The processor decodes the digital information and translates it back into 6-channel sound. Another benefit of the digital system, is that it eliminates the head wear associated with magnetic tracks.

Howard Flemming, the programme director for Optical Radiation Corporation, explains further, "One big advantage of digital sound is the mathematical techniques that can be applied for an error detection and error correction system in the processor. Utilizing proven digital technology, such as concealment algorithms, the processor automatically provides an accurate audio signal despite splices, dirt or scratches on the film." Fast audio fade-outs and fade-ins are used if large gaps of data are damaged or missing, making the signal loss virtually undetectable to the human ear. Approximately 6 million bits of data are read and recovered per second, and the process owes a lot to the high resolution characteristics of modern, motion-picture, emulsion technology. Kodak's Federal Systems Division also developed proprietary customized integrated circuits (IC's) for the error-detection and error-correction system, allowing for the imaging characteristics of the film, the number of channels, and so on.

In terms of print making, the digital CDS track is reproduced like a traditional optical track in the lab. This allows existing contact printers to be used to make release prints complete with sound, thus avoiding the additional step of recording the six-magnetic (analogue) tracks used in standard 70mm prints. In the United States of America, there are three labs located in Hollywood making CDS prints, namely, DeLux, CFI, and Technicolor.

While it can be argued that CDS modernizes and expands the 70mm format, it tends to undermine it at the same time. This is because CDS works equally well on 35mm film. This means the larger 70mm format no longer gives superior sound, as it has in the past. One of the reasons for producing in 70mm, or making a "blow up" to 70mm, was for the heightened cinema experience provided by its more sophisticated, high-quality sound system compared to 35mm.

TERMINATOR 2 was the first major 35mm release with CDS digital sound. Unlike 70mm, 35mm has the advantage that the area currently used for the optical soundtrack is wide enough for the new CDS digital track. In San Francisco, one cinema was running a standard 70mm, "blown up" print and a 35mm CDS print of Terminator 2 in the same complex. The 35mm print outgrossed the 70mm presentation by 8% in the opening week, and increasing to 15% in the second week. It would seem that the audience went for the better sound. Of course, it would be a more interesting comparison if the 35mm and 70mm prints both employed CDS digital tracks, and the 70mm version was a truly large-screen presentation ..... after all, if they are shown on a similar size screen, there will be little difference in image quality. The only real difference will be the sound quality, and digital sound should always outclass analogue presentations. However, many producers are finding there is now less need to strike 70mm "blow ups" to extend the distribution potential of new releases, if the cheaper 35mm digital prints do the same, or better, job. After all, a typical 35mm print costs under $2000, while a 70mm print with magnetic tracks costs $7,000 to $10,000. The comparison of 35mm prints of the same title, with and without digital sound, is also an eye opener. Variety reports that 35mm CDS prints grossed 60% more than standard analogue stereo prints screening in the same multiplexes.

Cinema chains and cinema managers are understandably cautious about investing in one of the many new digital systems for the cinema. Understandably, they are responding with their classic "wait-and-see" approach. This is the traditional stumbling block for all new technical innovations in the cinema, causing most improvements to fail without a chance to catch on. There is, after all, the relative merits of the very-fine Dolby SRD system, and other similar competing systems, to consider. Dolby has the hugh advantage that all the Dolby systems (Dolby A, Dolby SR, and Dolby SRD) have a very high degree of inter-system compatibility. Also, Dolby A and Dolby SR are already standard cinema sound systems in the majority of quality cinemas around the world.

And there is a rather large capital outlay ..... it cost about $20,000 per cinema for the full conversion to CDS, for example. Who wants to risk converting to one digital system, only to find out later that a competing system has captured the greater share of the market, forcing it into prominence and the others into decline? As we've seen before, it is not necessarily the best, or the most versitile technology that eventually catches on. The outcome has more to do with the skill, or perhaps aggressiveness, of marketing stratagies.

It is interesting to look at a more precise comparison of digital and analogue sound. Firstly, in terms of DYNAMIC RANGE. Dynamic Range describes the range of loudness in decibels (dB). The higher the number, the broader the range from very soft to very loud. Conventional 35mm monaural (mono), optical sound typically has dynamic ranges from 44 dB to 52 dB depending on the condition of the print. Stereo optical sound in the 35mm format ranges from 51 dB to 59 dB, again, depending on the condition of the print. 70mm analogue magnetic tracks show a marked improvement with a range from 78 dB to 80 dB. By comparison, CDS digital sound retains a level of around 96 dB for the life of the print. The difference between 70mm magnetic and 70mm digital optical sound is most noticeable in cinemas with low background noise levels. The difference between 35mm monaural / stereo optical sound and 35mm digital sound is described as "dramatic".

Next, lets look at CHANNEL SEPARATION. It is the systems ability to isolate sounds coming from specific channels, or in practical terms, specific speakers around the auditorium. Conventional 35mm stereo optical sound has separations of one channel from the other that range from 12 dB to 49 dB. This can mean that there is a tendency for the sound to "leak" from one channel to the other, which degrades the effect of separation and direction of the sound. 70mm magnetic sound can deliver a 50 dB level of separation. By comparison, CDS can deliver a 100 dB level of separation.

Next, lets compare the more familiar figures for FREQUENCY RANGE. For 35mm mono, the frequency range is about 30 to 6,300 Hz. For 35mm stereo, it is about 40 to 12,500 Hz. There is a slight improvement for 70mm magnetic tracks which can reproduce a range of frequencies from 30 to 14,500 Hz. For Cinema Digital Sound, the frequency range is 20 to 20,000 Hz which is outside people's normal range of hearing. The digital system automatically detects and corrects FLUTTER caused by the slightest variations in projection speed.

Finally, lets look at DISTORTION. For 35mm mono and stereo optical sound tracks, the range of harmonic distortion is from 1% to 7%. It depends primarily on the density at which the print was made and the exact nature of the sound. A 70mm print has a harmonic distortion level frequently reaching 3%. The average harmonic distortion on a CDS print is 0.01%.

There is no doubt, digital cinema sound systems are pretty cool. As to what system we will all end up listening to, it seems, only time will tell. As for the future of 70mm as a cinema system, it will no longer be in the forefront by virtue of its superior sound. It is its phenomenally superior image quality on the big, big screen that will continue to make it so special.

Darwin, Australia, June, 1995

Peter Gray
(near Los Angeles)
P.O. Box 5132
Pine Mountain Club, CA 93222
United States of America
telephone: +1(661) 242-1234

dp@petergray.org