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56
BRITISH KINEMATOGRAPHY
Vol. 16, NO. 2
mechanism of any projector, and derives its motive power from the projector drive through a flexible coupling.
The component parts of the instrument (Fig. 1) are a take-up spool, a take-off spool, a recording-reproducing magnetic head containing the electromagnet, a permanent magnet for erasing, and a driving capstan with its associated flywheel and mechanical filtering devices.
These parts are assembled on a mechanism plate which is mounted on top of an amplifier, the circuit of which is so designed that it serves the dual function of recording and reproducing (Fig. 2). A 40 Kc/s. oscillator provides the necessary supersonic bias current, which is mixed with the audio current in the process of recording.
Tape Characteristics
The tape used has a \ in. wide plastic support with a coating of gamma iron oxide. It is driven through the mechanism at three times film speed, i.e. approximately 15 in. per sec. with 16 mm. silent film, or 21.6 ins. per sec. if the film is run at sound speed.
o
P.u
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OutPut
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Amplifier
Record
i
Playback
HEAD
it
Amplifier
Fig. 2. By changing one switch position the amplifier may be used for recording or for reproducing.
As the tape is a third of the thickness of film stock a reel of 1200 ft. of tape is accommodated on a spool of equivalent diameter to a 400 ft. 16 mm. spool.
H.F. pre-emphasis is carried out in the amplifier circuit, and while no claim is made to high-fidelity response, satisfactory speech and musical background recordings are obtainable. The overall response characteristic of the tape and amplifier is flat within 2.5 db. between 100 c/s and 5.5 Kc/s.
The recording-reproducing head is of low impedance, and has a polepiece gap of .001 in. It is transformer coupled to the amplifier. A crystal microphone is provided, and the amplifier has an additional input which enables a gramophone pick-up to be fed in, and its signals mixed with the microphone input.
Maintaining Synchronism with Projector
Since the device is generally driven by a variable speed-projector there is no fixed tape speed, but the flexible driving shaft is always attached to a shaft on the projector which produces a speed of 1 turn per frame. This is usually available at the inching knob shaft, but in some cases modifications to the projector are necessary to obtain a suitable drive source. Satisfactory adaptations have already been made in connection with more than 30 types of 8 mm., 9.5 mm. and 16 mm. silent projectors and 16 mm. sound projectors.