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1953-54 Theatre Catalog, 11th Edition, Page 270 (234)

1953-54 Theatre Catalog, 11th Edition
1953-54 Theatre Catalog
1953-54 Theatre Catalog, 11th Edition, Page 270
Page 270

1953-54 Theatre Catalog, 11th Edition, Page 270

drive systems or mechanical couplings. It must be checked with the two machines in interlock while in operation. Mechanical coupling systems, unless they have no slack at all, may appear to be exactly synchronized when at rest but when the two projectors are in motion, the slack can be taken up by the projector having the greatest load and if the amount of slack taken up is large then the two shutters may drift slightly out of sync. Mechanical couplings that are springy may tend to tthunt," that is, to drift slowly back and forth, making the shutters slightly out of sync in one direction and then slightly out of sync in the other. The service company representative maintaining theatre equipment can check this condition without difficulty. There are reports of homemade ttstifim coupling systems that work perfectly well and there is no reason why they should not. Some of the coupling problems call for good mechanical design.

The Motion Picture Research Council in its Informational Bulletin Number 4 recommends the use of Selsyn interlocks. Their report is quoted in full:

"Interlocking is required between two projectors when showing 3-D pictures. Where stereophonic sound equipment is installed for either 2-D or 3-D, the separate sound reproducer must be interlocked to the projectors.

The preferred system is an electrical interlock, using 2-pole, 1440 rpm, Selsyn motors, having a peak torque rating of at least 1 lb-ft. The Selsyn motor is to be coupled to the projector motor, the preferred coupling being a timing belt and sprockets. The sprocket ratio must be exactly the same ratio as exists between the motor and shutter. For most machines this ratio is 49 to 40, which is the ratio of the motor speed (1764 rpm on 60 cycle power) to the shutter speed (1440 rpm). For some machines with 2-pole motors, this ratio is 3528 to 1440, which equals 49 to 20. The coupling ratio is correct when one revolution of the Selsyn produces exactly one revolution of the shutter.

The 2-pole Selsyn, coupled 40/49 to the projector motor to result in a one to one relationship between shutter and Selsyn, is a no-error interlock system since it precludes all errors in lineup, provided the Selsyn and shutter are

THE BALLANTYNE MECHANICAL INTERLOCK is shown below. The success of 3-D projection begins at this point because it is necessary In have exact synchronization belween the two interlocking projectors.

coupled in line on the initial installation. The projectionist simply turns the hand wheel on each Selsyn to an index mark which automatically indexes the shutters.

It is recommended that a timing belt having a short pitch (.242) be used to avoid large sprockets. An acceptable alternate to the timing belt is the 3./16inch pitch silent chain, but because of lubrication requirements this is a second choice.

An acceptable alternate to the 2-pole 1440 Selsyn is a 4-pole 720 rpm Selsyn, coupled to the motor on a 20/49 ratio, to produce two revolutions of the shutter for one revolution of the Selsyn. The 720 rpm 4-pole Selsyn should have a minimum peak torque of 2 lb-ft. The 4-pole Selsyn will line up and interlock at either of two points 180 degrees apart, but in both positions shutters are properly indexed.

Selsyn speeds other than 1440 and 720 are not recommended]'

(Since the time this report was issued a number of improved mechanical interlocks have been developed. ED.)


Recommendations for procedures to be followed in film laboratories and exchanges must tie in with studio production methods and with the conversion systems used in all 3-D theatres. This tie-in is provided by the Society and by the Research Council whose Informational Bulletin Number 3 contains a number of details essential to proper preparation of release prints. The following portions quoted from that Bulletin have been selected because they will be of general trade interest.

The following recommendations pertain to stereoscopic 3-D pictures; that is, where two prints are simultaneously projected in synchronism, the one a right-eye print, the other a lefteeye print. These recommendations are based on the use of a projection reel 24 inches in diameter which will hold approximately 5000 feet of color positive film or approximately 5500 feet of black-and-white. Since both projectors run simultaneously, change-overs are no longer possible (with less than four projectors in the booth), and intermissions are necessary for changing reels and retrimming carbons.

Where the booth cannot possibly accommodate magazines for the 24-inch reel, a projection reel of 20 inches in diameter, which will hold approximately 3500 feet of color positive, is recommended. This size reel is definitely not preferred, but where 24-inch reels cannot be used the reel diameter should not be less than 20 inches.

Previous recommendations on the 2000 feet release print make-up specified that the picture action should start and finish on fades at the beginning and end of each reel for smooth changeovers. This recommendation still holds, but in addition, particular attention should be paid to the intermission break which should finish and start on a fade.

The length of the picture section in part one and part two of the 3-D feature is not specified, but the first reel should be as full as possible so that regular Z-D short subject material can be put on the last part of the second reel in order that an intermission be THEATRE CATALOG 1953-54
1953-54 Theatre Catalog, 11th Edition, Page 270