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BACKSTAGE AT CINERAMA

Our good friend and colleague, Jack Gieck, with the cooperation of his publishers, Movie Makers (April 1954)
has made available to us as an industry service his recent article on Cinerama. Until recently the Cinerama
process has been cloaked in secrecy. Thus I was elated a few weeks ago when Ed Miller, Cinerama's resident
engineer in Detroit, invited me to have a look into the inner sanctum. I could even bring my camera, he said.
On the appointed night I entered Detroit's Music Hall and found Ed at his desk in the cutting room, talking
simultaneously to the telephone and to the intercom which connects him with the console operator and the four
projection booths. Lining the walls of the room were rewind benches, splicing equipment and enormous square
film cans storing Cinerama's 8000-foot reels.

"How's Charlie?" Ed was asking the intercom. Charlie, I figured, must be a member of the crew; but I soon
learned that this was the phonetic designation for the left projection booth (the others being "Able" and "Baker",
reading from right to left).

Seven Sound Tracks. To review briefly, Cinerama employs three projectors, each in its own projection booth,
to cover a screen 66 feet wide by 24 feet high, curved horizontally on a 25 foot radius. The projectors, like the
lenses on the camera which made the picture, are set 48 degrees apart to cover a field of 146 degrees on the
screen.

Seven tracks of stereophonic sound (representing six microphone locations and one composite track) of ex-
tremely high fidelity are recorded magnetically on 35mm film base. The magnetic sound reproducer is located
in the fourth projection booth, situated above the balcony at the rear of the theatre. This boot also houses the
standard 35mm optical sound projector which projects the conventional movie prolog.

The Sound Room. The magnetic sound reproducer is about 7 feet tall with a 31 inch reel housing at top and
bottom. The centre section contains the soundhead and there are several sprockets to lead the film off the
feedreel and onto the takeup reel. I found that all seven magnetic pickups are side by side in a single head
narrower than the film width. (This is in contrast to the Warner-phonic system- a Ia House of Wax, for example
-which uses 35mm magnetic film, but has only three tracks with the pickups staggered along the film.) What
is even more surprising, under these conditions, is the absence of interrnodulation (cross-talk) between the
seven tracks.

Also mounted on the magnetic reproducer is the hear of Cinerama's synchronization system. Cinerama does
not use conventional selsyn motors to keep the three projectors and sound in "locked-in" sync. Instead, they
"fall in off the line." Each projector is equipped with a small"slave" synchronising device similar in appearance
to the "master" on the sound machine. In Fig. 2 a black disc resembling a strobe card with 32 white graduations
can be seen. Each time this disc revolves, 32 frames, or approximately 3 feet of the film, pass through the
machine.

Maintaining Sync. At the conclusion of each revolution the device sends out an electrical pulse to the slave
units on the projectors. If the latter are in sync, a green light mounted on each machine flashes momentarily.
If one of the projectors is ahead or behind by as little as one frame, the light fails to flash. Instead, one of the
two small (6-volt) synchro timing motors - visible in the illustration as the round black objects to the right of the
disc - takes over and accelerates or retards the projector by the required amount.

This gadgetry is especially busy at the start of the show. When the projectors are loaded, the films are
synchronised on a common start-mark, perforation for perforation. The sound is started slightly ahead of the
picture, and the projectors accelerate as required while 15 feet of leader run through them, so that all four
machines are in step by the time the picture appears and the dousers (arc-house shutters) swing open.
Since this sync mechanism is not completely foolproof, the green pulse-lights also appear on the console
board in the centre of the theatre, and the console operator can correct any projector from his control panel.
We'll have more to say about the console later on.

To the left of the reproducer the preamplifier panel may be seen. The plug cords hanging down are used for
"patching" in case one of the magnetic pickups should fail. That is, in case one sound track is not being
reproduced, an adjacent track can be spread over two of the auditorium speakers to avoid a dead spot behind
the screen. This also can be done from the console, as we shall see.

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