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116
MAGNETIC SOUND for 8mm. PROJECTION
MARVIN CAMRAS, Armour Research Foundation
Because of its very great interest and importance to all amateur filmers, Movie Makers is pleased to present, in the field of amateur motion pictures, this first authentic report on the possibilities of magnetized sound track recording directly on existing 8mm. films.
The discussion which follows will be, we realize, somewhat technical in nature for many of our readers. Despite this aspect of it — which is inescapable — we helieve that the basic facts of this report are of such present interest and future importance to all amateur filmers as to merit their presentation in our magazine.
Magnetic Sound for 8mm. Projection was first presented by Marvin Camras as a paper at the 1947 convention of the Society of Motion Picture Engineers. Later published in the Journal of the Society of Motion Picture Engineers, it is reprinted by Movie Makers with the gracious permission of the author and of the Society. — The Editors.
ALTHOUGH sound on 8mm. film has been considered in the past, results were discouraging, and up to the present time no 8mm. sound (on film) projector has appeared on the market. One difficulty with 8mm. films is the limited space available for a sound track. Fig 1 shows the relative dimensions of standard films. The 35mm. film has a track about 100 mils wide; on 16mm. film it is about 80 mils wide. With 8mm. film the maximum track width is only about 30 mils. This track can be located at the film edge, either on the sprocketed side A or on the picture side B.
h m
a — □ — o — a — □ —
a
a
□
a
o —
JL
.030
FIG. 1. Relative sound track dimensions.
FIG. 2. Region of uneven development.
L
MICROPHONE
AMPLIFIER
AND EQUALIZER
-tr]
I5KC OSCILLATOR
-o4
MAGNETIC HEAD
FIG. 3. Magnetic sound on film recording system.
1
10
^s^>
08 0
\
1 \
-10
16 18 24 FRAMES
1
50 tOO 1000
FIG. 5. Overall response of 8mm. system.
10,000
FIG. 4. Eight-mm. magnetic sound projector.
SENDS IN FILM (COMPLIANCE)
MAGNETIC HEAD
FRICTION BLOCKS (RESISTANCE)
R -VW
OUTPUT
EQUIVALENT ELECTRIC CIRCUIT
POSTS FOR DAMPENING INTERMITTENT FLUTTER
FIG. 7. Adapter for 8mm. projectors.
FIG. 8. Adapter plate mounted on 8mm. projector.
FIG. 6. Converted 8mm. silent projector.
Problems and possibilities of sound on 8mm. film, recorded magnetically, discussed by the research engineer responsible for this development.