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Cinema Digital Sound - System Overview
|Read more at|
The 70mm Newsletter
|Written by: Ronald E. Uhlig, Eastman Kodak Co. &
Howard J. Flemming, Optical Radiation Corporation.
Presented by Jack Johnson, Optical Radiation Corporation.
Text prepared from a vintage papers by
Anders M. Olsson,
Controller. Click to see enlargement. Picture by Thomas Hauerslev
With the showing of last summer's hit film, Dick Tracy, the public had its first
opportunity to hear an exciting new sound format for motion picture film:
CINEMA DIGITAL SOUND.
Since last summer a number of other major studio motion pictures have been
released in CDS with more following.
Cinema Digital Sound, or CDS for short, is a format for obtaining six channels
of digital sound on motion picture release prints. Ever since the compact disc
was launched as an alternative to the vinyl phonograph record, the motion
picture industry has dreamed of having that level of quality available on every
motion picture print. CDS offers that possibility in a system that is practical
and cost effective, advantages that will allow it to become the universal
release sound track format.
CDS was born out of work begun at Eastman Kodak to develop a sound format that
would permit compact-disc-quality sound, suitable for high end special venue
formats such as Showscan, but at the same time capable of being recorded in the
normal sound track area of 35mm prints.
It was obvious that the new format would have to be digital as only a digital
system could offer all the desired benefits: six discrete channels, consisting
of five full bandwidth channels and one subwoofer channel; full audio bandwidth;
and the reduction of noise, flutter, and distortion to the vanishing point. No
envisioned optical analog system could offer all of these.
However, to be a suitable candidate for a universal motion picture audio format,
the system had to be practical. We ruled out expensive and exotic systems such
as interlocked optical discs. It is clear that current technologies, such as the
use of SMPTE time code, provide ways of synchronizing just about anything to
motion picture film. However, no system exists that is both reasonable in cost
and convenient for recording a motion picture sound track. Consider the compact
disc. It has a playing time of about 70 minutes and a capacity of two channels.
To record a motion picture that is 90 to 120 minutes long and requires six
channels for an effective presentation would require a total of six compact
discs and perhaps six compact disc players in the theater. Obviously, this is
not the ideal medium.
Even if a practical recording ideal medium did exist, industry experts say that
the problems of getting the right sound with the right reel of picture through
the production and distribution system to the exhibitor preclude any sort of
double system from ever being used on a widespread basis. There will always be
limited run and special venue situations where a double system is best, but
never as the general motion picture sound release format.
Striped magnetic prints overcome the double system problems, but they introduce
a new set of problems. First of all, putting the magnetic stripes on the prints
is costly and time consuming. Secondly, the stripes wear and the magnetic heads
that contact them wear. The initial high quality that magnetic sound offers is
all too often not presented in the real world theater.
Thus we centered out attention on the straightforward approach of putting an
optical digital sound track right on the print, using the dyes of the current
"Eastman" color print film to image clear and dark bits in the normal soundtrack
At Eastman Kodak a program was started to determine whether the films and
equipment have the necessary resolving power to image the small bit size that
would be needed to record the millions of bits per second that were required to
record the desired number of tracks of CD quality digital sound. Subsequent
tests at Eastman Kodak showed that it was indeed possible and that the error
rates were low enough to be easily correctable by today's error detection and
|More in 70mm reading:|
Films presented in 70mm
and 35mm Cinema Digital Sound
List of cinemas with CDS equipment
Cinema Digital Sound - Sounds
Like The Real World
c/o Britannia House
1/11 Glenthorne Road
Penthouse Reader. Click to see enlargement. Picture by Thomas Hauerslev
At this point we at Optical Radiation Corporation entered the picture and
completed the development of CDS. Perhaps ORC needs a little introduction to the
motion picture audio community, although I doubt that it needs introduction to
those in exhibition. ORC has a distinguished history as the supplier of Century
projectors and Orcon high-intensity projector lamphouses. ORC had reached
similar conclusions about the value of a digital sound track to the motion
picture industry, and after hearing about the positive results of their
feasibility study, we contacted Kodak.
Led by Howard Flemming, the team at ORC developed the recording and playback
equipment necessary to make CDS a reality. Kodak has developed a special film
for recording the CDS sound negative. Both companies continue to work together
to introduce CDS to the motion picture industry.
With that introduction, let me tell you a little bit about the CDS system. I
will be giving you just an overview of Cinema Digital sound. Tomorrow morning,
however, at the Laboratory and Sound sessions Ron Uhlig of Eastman Kodak Company
giving two papers that go into much more detail about the technical wizardry
that has been put into CDS. An interesting feature of modern digital electronic
technology is that it is possible to have a product that is both complex and
Ron will tell you tomorrow about some of the complexity, but I'm here to tell
you now that the CDS decoder in the projection room of a theater is simpler to
operate than the VCR in your family room at home.
Originally CDS was offered in only 70mm format but it is now available in both
35 and 70mm. So just keep in mind that everything I say about CDS applies to
both the 35 or 70mm format. There is no difference in how we put CDS on 35 and
70mm film. The 70mm film just runs a little faster.
This slide shows clips of both 35 and 70mm film with CDS sound tracks. The CDS
track is the magenta colored strip just to the left of the picture. In both
cases, the CDS tracks occupy the area that was formally occupied by the analog
sound track. In 35mm that analog track would have been a variable area optical
sound track and in 70mm, one of the four magnetic stripes.
If we zoom in on the CDS track, we can see that the track is composed of black
and white squares. Actually, they are magenta and white. Each square is one bit
of data, either a one or a zero. For the benefit of those of you sitting further
back, we'll zoom in even further.
Each individual bit is 14 by 14 microns. On one hand, this might seem pretty
small, but on the other hand, it doesn't really push the limits of the system so
much that it requires exotic equipment to print the data onto the film. This
print was made from a CDS negative on a standard Bell & Howell model C printer.
Going back to the overall view of the track, there are 180 columns of bits
across the sound track, which is just slightly over one-tenth of an inch wide.
Since the bits are square, we can easily calculate that there are approximately
32,500 row of bits in each second of film and, if you are interested, that adds
up to a total of almost 6 million (5,850.00) bits per second.
2: Judgement Day" at the Imperial Bio, Copenhagen (Denmark) in full Cinema
Digital Sound, 1992.
Picture by Thomas Hauerslev
However, as you can see, there is virtually no space left between the track and
the picture and the track and the perforation. What about weave in the
projector? It turns out that there is enough redundancy in the data so that only
152 of the 180 bits need to be read. Thus it doesn't matter if the scanner in
the projector reads 152 in the middle or near the perforation or near the
picture. The scanner will find the 152 that it needs, in the process giving
plenty of allowance for weave.
With that brief look at the film, let's take a look at the recording and
reproduction processes, beginning with recording.
Recording and printing a CDS track is little different from recording and
printing a standard variable area sound track, and intentionally so. One goal
was not to require any exotic recording apparatus, such as a laser on the
The recorder for exposing the sound track negative looks similar to a
conventional sound track recorder, although its electronic guts are much
different. A light emitting diode array is used to expose the film.
A special sound negative film, 'EASTMAN' Digital Sound Recording film type 2374,
has been developed by Kodak to work with the spectral output of LED array but
yet be process compatible with the D-97 process used for conventional sound
A useful feature of the recorder is its ability to record a human readable
message in the sound track area. In both the leader and trailer of the film,
sound track identification is printed, along with patches for measuring the
density of the negative and the print. The arrow always points towards the head
of the reel.
Printing the sound track is done on a conventional contact printer. Successful
prints have been made on all the types of contact printers commonly used for
release printing. Of course, since there is no current optical sound track on
70mm prints, 70mm printers don't have separate sound heads. However, labs can
either print in two passes, the practice that has been followed to date for
obtaining 70mm CDS prints, or a sound head can be added to existing 70mm
printers. We have arranged for such a sound head to be designed and tested.
Another interesting feature of 70mm CDS tracks is that they are printed from a
35mm width negative. The advantage of this is that the same recorder and film
stock can be used as for 35mm tracks and it is not necessary to develop 70mm
D-97 processing capability. Of course, the negative film runs 25% faster when
making 70mm negatives and the bits are stretched out by the same proportion.
Experience has shown that, while the printer does need to be fairly well
adjusted and maintained for CDS tracks, no special modifications are necessary.
While we are talking about CDS in the laboratory, its a good time to note that
CDS tracks are printed with the sound and pictures in editorial synchronisation.
There is no sound track offset. The playback equipment can be easily adjusted
for varying distances between the sound head and the picture gate. Lack of an
offset makes it easy to splice prints without sound track sync. problems.
The final printing feature I will mention is that sound track application is
optional. The scanner is designed to read dye sound tracks but will also work
perfectly well with applicated tracks. The processing industry has long been
looking for a way to get rid of sound track application, and this is it.
However, until a lab is substantially converted, it may actually be simpler to
just applicate everything, and not have to worry about giving CDS prints special
at the Imperial Bio, Copenhagen (Denmark) with CDS reader on top of the
Picture by Thomas Hauerslev
All in all, it is a significant advantage of CDS tracks that they can be printed
with the same techniques and equipment used for conventional sound track prints.
High volume, low cost prints will continue to be a way of life.
Playback of CDS sound takes place with the addition of these two pieces of
equipment to the projection booth. The CDS scanner mounts on top of the
projector. If the projector has an existing penthouse, the CDS scanner mounts on
top of it. There is no interference with existing sound scanners, either
magnetic or optical. As I mentioned before, no particular sound head to picture
gate displacement in the projector is required. The equipment has an
electronically adjustable sound delay to obtain synchronization. The scanner
uses a tungsten light source and a 512 element CCD array to scan the sound
track. Each row is scanned once, and only once, and the information is fed to
the decoding package.
The scanner we see here can be used to play either 35 or 70mm CDS tracks.
Changing from one format to the other takes only a few minutes.
The decoder is where all the wizardry takes place. It must control both the
horizontal and vertical scanning so as to synchronize it with the data on the
film, and then undo all the encoding that took place when the negative was
recorded. In the process, it corrects for any damage in the form of dirt and
scratches that may exist on either the negative or the print. All in all, the
process is very complex, and once again I invite you to hear more about it in
tomorrow morning's papers. However, the vast majority of the electronics is
digital, and once it is implemented in integrated circuits as the CDS system is,
the entire system is very reliable and stable and will seldom, if ever, need
adjustments or maintenance.
A front panel liquid crystal display allows control of the few functions the
projectionist needs control over, such as a master gain control. A password can
be keyed in which allows authorized service personnel to access the setup
functions, such as synchronization delay and individual channel volumes.
The unit is fully compatible with existing sound equipment the theater and
allows feed through of the analog audio signals from, for example, a Dolby
CP-200. The CDS decoder automatically switches to digital if valid CDS data is
on the film and back to analog when it is not. Thus digital and analog material
can be intermixed on the same platter, if necessary, and the switch over will be
Also automatic is projector changeover. Two scanners can be connected to one
decoder package and the decoder can be interfaced with the booth's existing
changeover circuitry to make the process completely automatic.
Now let me talk a little about performance of the system. First, let's consider
digital data performance. The most fundamental measure of performance of any
digital recording system is what is known as the bit error rate. This is simply
the fraction of bits that are misinterpreted on playback. For example, a small
piece of dirt may cause a clear bit, a zero, to be read back as a one.
Obviously, we would like the number of errors to be zero but this will never be
the case. Even a piece of film fresh from the processing machine will have some
dirt on its surface and the amount can increase each time the film is projected.
Thus the system must be built to expect errors and correct for them. The CDS
system has been designed to be able to play sound with no loss in quality even
with a bit error rate as high as 10, of in other words, with one bit in every
Although it depends somewhat how the errors are spread out on the film, this
means that in a typical second, you could have as many as 50,000 errors, and
still not be able to hear any problem with the sound. So, if that is how many we
can stand, how many do we typically have? Well, a fresh print will have in the
range of 1000 to 1500 errors per second, and after a few hundred passes, this
may increase to perhaps 5000. As you can see, this still gives us a generous
margin of protection relative to the 50,000 errors per second handling capacity
of the system.
Even if there are occasional bursts of errors over that rate, such as might
occur at a splice, an efficient concealment mechanism renders them almost always
But what is really important is audio performance, so let's run down the audio
of CDS reader at the Imperial Bio, Copenhagen (Denmark), 1992.
Picture by Thomas Hauerslev
First, CDS is a six channel system. Five are full bandwidth channels, and the
sixth is a dedicated low frequency subwoofer channel. Over the years, this
number of channels has developed as the optimum number even for the most
dramatic audio special effects. The channels are placed around the theater as
follows: three behind the screen, left, centre, and right; separate left and
right surrounds; and the subwoofer, also typically placed anywhere behind the
The next important specification is frequency range. CDS covers the entire
audible frequency range from 20 to 20,000 Hertz with a consistency and flatness
only a digital system can attain. The improvement in high end clarity over what
can be attained with current analog optical tracks will be truly significant.
Distortion and flutter are two defects in audio quality that are reduced to the
vanishing point in the digital CDS system.
CDS will deliver six channels of crystal clear sound, free of distortion, hiss,
pops, and other noises on either 35 or 70mm print film. It is, in effect, the
motion picture equivalent of the compact disc sound we now enjoy at home.
But good sound is not all that CDS will offer. There are three additional
streams of digital data. Altogether, they have a total capacity of about 5000
characters per second.
The first is a control channel that outputs data in the Music Industry Digital
Interface -- or MIDI -- format. Data recorded in this area can be used for a
variety of theater automation purposes or perhaps for subtitles in several
languages. An example of a theater automation function that this channel might
be used for would be for control information to hydraulically controlled seats
that tilt, shake or move.
The second type of data is SMPTE time code, identifying each frame of film with
a complete address. This also can be used for automation purposes in theater but
in addition can be a useful tool when searching for and inserting replacement
sections of damaged prints.
Finally, there is a film ID field which can be used to record the title of the
feature along with, perhaps, a version number, language, and production number.
These bits of data are important keys that will allow the CDS sound track to
interface with ever more sophisticated systems well out into the future.
Now that you know what CDS is, let me give you a little information about its
use so far and about our future plans.
During last summer's release season, two films were released with CDS sound
tracks. These were Dick Tracy and Days of Thunder. A total of eleven theaters
were converted to play these films and hundreds of showings were made. Since
that time a further twenty cinemas have been converted in the USA and twelve in
Europe for such releases as Edward Scissorhands, The Doors, Final Approach and
Flat Liners to mention a few.
I hope this introduction to Cinema Digital Sound has helped you understand some
of the excitement we have regarding the future of motion picture sound. Analog
sound tracks for motion pictures have been around for about 50 years now. We
fully expect that 1991 will mark the turning point and that Cinema Digital Sound
will become the standard sound system for the next 50 years. The reaction of
those who have heard it, from industry experts to the average theater goer,
certainly points in that direction.
Finally if you wish to hear CDS will you please contact the BKSTS for tickets
for this evening's screening at BAFTA of our demonstration film and a pre: UK
premiere of a major studio release encoded in CDS.
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