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The final combination of tracks onto one composite sound track synchronous with the picture is variously known as mixing, rerecording, or dubbing. Mixing takes place at a special console equipped with separate controls for each track to adjust loudness and various aspects of sound quality. Although some of the new digital processes employ the record-industry technique of overdubbing, or building sound track-by-track onto a single tape, most mixing in films is still performed by the traditional practice of threading multiple dubbing units (sprocketed magnetic film containing separate music, dialogue, and sound effects elements) on banks of interlocked dubbers. The playback dubbers are connected by selsyn motors to one another, as well as to the rerecorders that produce the master, or parallel music/dialogue/effects (M/D/E), track on full-coat magnetic stock. Also in interlock are a projector that allows the mixer to work from the actual image and a footage counter that allows the mixer to follow cue sheets, or logs, which indicate by footage number when each track should be brought in and out.

The mixer strives to strike the right dramatic balance between dialogue, music, and effects and to avoid monotony. Mixing procedures vary widely. Some studios use one mixer for each of the three main tracks, in which case the effects tracks have probably been mixed down earlier onto one combined track. In the early days of magnetic recording, stopping the rerecording equipment produced an audible click on the track; if a mistake were made, mixing would have to be redone from the beginning of the tape reel. The advent of back-up recording in the 1960s eliminated the click, making it possible for mixers to work on smaller segments and to correct mistakes without starting over. This enables the mix to be controlled by one person, who may be combining as many as 24 tracks. An even greater advance is the computerized console that enables the mixer to go back and correct any one track without having to remix the others.

For monaural release, a composite music/dialogue/effects master on full-coat 35-mm magnetic film is converted to an optical sound negative. For stereo, four-track submasters for M/D/E are mixed down to a two-track magnetic matrix encoded to contain four channels of sound information. Optical sound negatives are copied from the magnetic master, and they are then composited with the picture internegative so that they are in projection sync (on 35-mm prints the sound is placed 21 frames in advance of its corresponding image; on 16-mm prints the sound is 26 frames in advance of the picture).

Because of narrow track width, optical stereo sound tracks require a system of noise reduction such as Dolby Type A. The Dolby system works by responding to changing amplitudes in various regions of the frequency spectrum of an audio signal. The quieter passages are boosted to increase the spread between the signal (desired sound) and the unwanted ground noise. When played back, normal levels are restored, and the ground noise drops below the threshold of audibility.

Projection technology and theater design

Projectors. The projector is the piece of motion-picture equipment that has changed the least. Manufacturers produce models virtually identical to those of the 1950s, and even the 1930 model Super Simplex is still in wide use. The essential mechanism is still the four-slot Maltese cross introduced in the 1890s. The Maltese cross provides the intermittent Geneva movement that stops each frame of the continuously moving film in front of the picture aperture, where it can be projected (or, in a camera, exposed). The movement starts with a continuously rotating gear and cam (see Figure 5, left). Each 360-degree rotation of the gear and cam causes a pin to engage one of the slots of the Maltese cross. The pin rotates the cross, which in turn rotates a shaft, one quarter turn. As the shaft rotates, four of the 16 teeth on the intermittent sprocket advance and engage the perforations (sprocket holes) on one frame of the film. The sprocket moves only when the pin is fully engaged in the Maltese cross slot (see Figure 5, right). This is the “pull-down” phase; in the other phases the curved surfaces of the cam and the cross are in contact and the movement is in the “dwell” position. The Geneva movement is also called a 3:1 movement because there are three quarter-cycles of dwell for every one quarter-cycle of pull-down.

Sound, unlike images, cannot be reproduced intermittently; sound must be continuous to be realistic. The optical-sound-reading equipment on a projector is therefore located below the picture aperture (see Figure 6), and the sound on an optical 35-mm print is located 21 frames ahead of its corresponding image. A light beam (supplied by a direct current for stability) is shone through a rectangular slit and focused by a lens to dimensions of .001 by .084 inch onto the sound track. The sound track’s varying bands of light and dark then modulate the amount of light from the beam that is allowed to pass to the optical pickup. In older equipment this pickup was a photoelectric cell that changed electrical resistance under exposure to light. Newer designs employ a solar cell of photovoltaic material to convert light energy to electric energy.

An important element of picture quality on the screen is brightness. For decades the standard light source was the carbon-arc lamphouse, which used disposable electrodes (positive and negative carbon-clad rods) that would be moved together as they burned; the rods needed to be replaced every hour or so. Xenon lamps were introduced in West Germany in the 1950s, and carbon-arc projection is now found only in older theaters. Both carbon-arc and xenon lamps are run off a direct-current power supply in order to minimize brightness variations due to fluctuations in voltage. The xenon bulb replaces the positive and negative carbons with a tungsten anode and cathode in a quartz envelope filled with xenon gas under pressure. Light from xenon bulbs has a color temperature closer to that of daylight than carbon-arc light does; that is, it is bluer and is therefore particularly well suited to color films.

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Projection techniques

A 35-mm exhibition print is furnished to the theater mounted on 2,000-foot (22-minute) reels. Thus, a typical feature film consists of five or six reels. For decades, the 2,000-foot reel was the basic unit of projection, and each screening required four or five changes of projector. Circular cue marks printed in the upper right corner of the picture indicated when each changeover should take place. Today the 2,000-foot reel is used primarily in single-screen theaters and in archival and repertory theaters that may present only a single screening of a film. Theatrical exhibition increasingly requires the film to be “made up”—that is, reels must be spliced together to enable the projectionist to make a single changeover between large reels or to use external transports that contain an entire feature without changeovers. For the former, a feature film of six 2,000-foot reels would be reassembled onto two 6,000-foot reels with a running time of about an hour each. The changeover is made by the traditional switching method using the cues at the end of the reel or by attaching a strip of foil sensor tape to the edge of the film, where it activates the appropriate switching relays. Coming attractions (“trailers”) and announcements (“snipes”—e.g., “No Smoking” or “Starts Friday”) are spliced in sequence at the head of the first reel or may be on a separate reel. Up to three auditoriums may be served from a common booth when large reels are used.

The advent of xenon lamps made it possible to reduce or eliminate changeovers to the point where a single projectionist could operate the equipment for several auditoriums. Although there was an occasional theater with more than one screen in the days of carbon-arc projection, it is xenon projection that truly began the age of multiplex cinemas. With more than three screens, equipment popularly known as the flatbed, or platter, system is mandatory. The entire film is shown without changeovers and does not need to be rewound. The most advanced version of the platter eliminates the need for rethreading. The last frame of film is spliced to the first, as in the Edison Kinetoscope.

Sound reproduction

theater sound systems are divided into the “A” chain and “B” chain. The “B” chain components are the power amplifiers and speakers that, although specially made, are not essentially different from those in other audio systems. The “A” chain components are the optical pickup and preamplifier and employ some principles unique to motion pictures.

The simplest and most common sound system employs a single amplifier channel and one speaker behind the screen. Stereo variable area (SVA), popularly known as Dolby, though in fact made by several manufacturers, employs a split optical pickup for two sets of wires for the left and right channels. Three stage speakers (left, right, and center) are mounted behind the screen, and an array of speakers is spread along the side and rear of the auditorium for “surround” sound. Most feature films are prepared so that dialogue issues from the center speaker, music and on-screen sound effects from the left and right, and off-screen sounds from the surrounds. A processor decodes the four channels from dual variable area tracks; information appearing on the left track is sent to the left speaker, on the right track to the right speaker, while information on both tracks is combined in the center channel. The surround channel is derived from inversion phase relationships between the left and right tracks.

In monaural systems, a treble cut is employed in accordance with the Standard Electrical Characteristic of 1938, or Academy Curve, so that frequencies above 8,000 hertz (Hz) are “rolled off.” This practice dates from an era when sound tracks had a large degree of ground noise and vacuum tube amplifiers produced an audible hiss concentrated in the upper frequencies. A treble boost is added during rerecording so that monaural sound tracks sound shrill and sibilant when played without the Academy filter. The introduction of Dolby noise reduction in conjunction with optical tracks made it possible for frequencies to range up to about 12,000 Hz. With the replacement of tube power amplifiers by solid state ones, large wattages are easily obtainable, and theater sound is generally louder than it was formerly. The normal level for dialogue in a monaural film is 80 decibels (dB) in the center of the auditorium; the normal Dolby level is 85 dB, or nearly double that.

SVA is a direct replacement for the four-track magnetic sound introduced in 1953 in conjunction with CinemaScope. Today, magnetic sound is used only with 70-mm prints where six tracks are contained in four stripes of magnetic oxide embossed on the film. The magnetic reproducer, called a penthouse, is mounted above the projector. On a magnetic print, the sound displacement is behind the picture (28 frames in 35 mm and 23 frames in 70 mm).

Until recently, theater speakers were not capable of reproducing sounds below 80 Hz. The standard theater speaker was a two-way system with a high-frequency horn mounted atop a cabinet containing a wide, shallow paper cone woofer. The impetus given to 70-mm six-track sound by the great success of Star Wars led to the development of the THX system for exhibition. In the six-track system, five stage speakers are mounted in a flat baffle wall behind the screen; each has double 15-inch woofers for low-frequency reproduction down to 40 Hz. For frequencies down to 30 Hz, sub-woofers are connected to a bass extension module that augments signals below 100 Hz on the tracks. At this level, sound is not heard but felt as vibration in the viewer’s diaphragm. The THX system delivers undistorted sound up to a level of 108 dB per channel.

Auditorium design

The most crucial consideration of theater design is the relationship of picture size to the seating area. In the 1940s the Society of Motion Picture Engineers propounded the “two and six rule,” which stated that the first row of seats should be at a distance from the screen equal to twice the picture width and the last row at six picture widths. This rule was based on the Academy picture ratio of 1.33 to 1, which is no longer used except for revival showings. The rule is still valid, however, because the wide-screen formats derive their impact from extension of the picture into the viewer’s peripheral vision, and proper installation will maintain constant picture height through all formats.

Depending upon the seating capacity of the auditorium, the image may be made larger or smaller by changing the focal length of the lens. The lens size is calculated by multiplying the “throw” (distance from lens to screen) by the width of the aperture and dividing the total by the picture width. Thus, to produce a picture 18.5 feet wide in 1.85 format (aperture width .825 inch) in an auditorium having a 90-foot throw would require a 4-inch lens.

The recommended level of screen brightness is 16 foot lamberts in the center of the screen (with no film in the aperture), but a level of 12 to 14 foot lamberts is more typical for commercial cinemas. It is difficult to illuminate a large picture, because screen brightness decreases in proportion to the square of the increase in screen size; i.e., the light source used to produce a 30-foot-wide picture will have to be not twice but four times as bright as that for a 15-foot image.

Light from the screen is wasted if it comes back over the heads of the audience, is too low down, or is too far to the sides. Light may be conserved, at the expense of even illumination, by the use of various screen surfaces. The ordinary matte-white screen exhibits approximately the same level of brightness at wide angles as from the center axis. It is possible to increase the light reflected to the center axis by using pearlescent screen surfaces that contain a brightness enhancing agent. Such screens conserve light but cannot be used in a theater with a wide audience area. Another screen surface is the aluminized, or silver, screen associated with old-style movie palaces with very long throws. This screen is even brighter than the pearlescent version but loses its brightness markedly if viewed from beyond 20 degrees from the center axis. It is mandatory for 3-D presentation, however, because an ordinary white screen depolarizes the light.

theater screens are perforated to allow transmission of sound from speakers behind the screen. The perforations account for only about 8 percent of the screen surface and do not substantially degrade the picture.

Reverberation times in excess of one second degrade speech intelligibility from the speakers. Very large, old theaters built for vaudeville and live musical accompaniment of silent films have high ceilings and large interior volumes that produce reverberation times of two seconds or more. Well-designed theaters employ curved, often serrated walls and avoid parallel walls and right angles that can produce short-path reflections.

Elisabeth Weis Stephen G. Handzo