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r ' — ~~V
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M2
Fig 2. Exposure sequence for the B.B.C. continuous-motion recording system.
(a) Television-frame waveform (Fl, F2, etc. — frame period. PI, P2, etc. — complete picture period).
(b) Exposure produced on consecutive film frames by consecutive mirrors in the original B.B.G. equipment (Ml, M2, etc. — exposure period).
(c) Exposure sequence when the cone diameter of the rays is less than the gaps between the mirrors.
is now a period when no light falls on the film, and it is therefore necessary to so phase the mirror drum that the mirror change-over period occurs during the television frame-suppression period. A special motor-drive unit provides a very accurate phase lock to the television frame-synchronizing signals. The phase position is indicated by stroboscopic means.
The film-traction mechanism of the original Mechau equipment employed a four-picture sprocket to pull the film through the curved gate. The new film path is shown in Fig. 3. The sprocket SI withdraws film from the magazine, and, after a free loop, the film passes to a smooth drum Dl, which is mounted on the shaft of an eddy-current brake. After travelling through the curved gate G, the film passes round a second smooth drum D2, which has a flywheel of large inertia coupled to it. A small roller R serves to wrap the film round this drum. The film then passes via a sprocket S2 to the sound-recording drum D3 and thence back to SI and to the magazine. S2 provides the motive force to pull the
film through the picture gate. The framing can be varied by rotating R about the centre of D2.
When static, there is no tension in the picture gate. The running tension is supplied entirely by the braking action of Dl. The inertia of D2 is so considerable that it would take a long time for the film to accelerate it from rest, and advantage is taken of the lack of gate tension under static conditions by arranging that this flywheel is kept rotating at full speed by means of a separate electric motor. Initially D2 withdraws a small amount of film from the gate, but, as the film is then slack round the drum, no further film can be withdrawn until the main motor of the camera, which drives SI and S2, is started. Additional contacts on the starting relay then actuate a magnetic clutch which disconnects the electric motor from D2 so that D2 is driven by the film alone.
The film is held against the gate surface by two curved runners independently spring-loaded. The spring-loading is kept to an absolute minimum to avoid gate friction, which is liable to vary and
370
April 1953 Journal of the SMPTE Vol, 60