British Kinematography (1951)

May, 1951 BOWEN, MOIR AND f UfcNER : TELEKINEMA SOUND EQUIPMENT 149 on the outgoing machine turns his gain control to zero, gives an aural or visual signal to the operator of the incoming machine, who has only to depress his " sound " push button to select the appropriate speakers. These " sound " buttons are located on each machine in close proximity to the gain controls. A loudspeaker selector unit containing the change-over control circuits and the deaf-aid amplifiers is located adjacent to the right hand SUP A machine. It carries indicator lights of different colours which show which speaker system is in operation. Monitor Speakers Four monitor speakers of conventional design reproduce sound from the four output channels; those corresponding to the three sets of screen speakers being mounted on the front wall of the box, while the fourth, monitoring the auditorium sound effects, is mounted on the rear wall of the box. 5. CONTROL SYSTEM When projecting stereoscopic films or using the magnetic sound heads it is essential to operate two or more machines in exact synchronism from standstill. Normal synchronous drives are insufficiently accurate to maintain such exact correspondence even though the films are correctly framed when threaded, but the application of a Selsyn interlock system makes it possible to obtain the synchronisation necessary in a relatively simple manner. Selsyns are similar to A.C. induction motors, with wound rotors and stators, the stator winding being energised from the mains while the rotors of the two Selsyns to be interlocked are connected in parallel. The stator windings induce mains-frequency voltages into the rotor windings and the two rotors move relatively until the voltages induced in the two rotors balance each other with no current flowing in the rotor circuit. If one rotor is turned mechanically, the rotor of the second machine will move to keep the same relative position, and this occurs even with the rotor turning at 1,440 r.p.m. The Telekinema requirements are met bv mounting a Selsyn motor adjacent to each driving motor, the two motors being coupled by a roller chain drive'. Correct phasing at standstill is achieved by supplying singlephase excitation to the stators before film threading commences, and on depressing the " start " push button all interlocked units will run up in exact synchronism. As the number of machines in operation depends on the programme being presented all the control switches are grouped together in a Selsyn selector panel, thus permitting the operator to verify at a glance the combination selected for operation. Change-over Devices The light and sound change-over devices on the standard SUPA equipment are electrically operated, depression of the " changeover " button on the incoming machine opening the " incoming " shutter and closing the " outgoing " shutter. This facility is retained in the Telekinema equipment, but the arrangement has to be modified when projecting stereoscopic films, as both machine change-over shutters must open or close together. Appropriate arrangements ensure that switching of the equipment to the " stereoscopic " position automatically closes both machine shutters. During stereoscopic operation the normal change-over push-button operates both shutters in phase, i.e. they open together. This completes a rather brief description of the Telekinema equipment, but it is probably true to say that it represents an advanced view of the projection room of the future, embodying some of the equipment that will undoubtedly prove essential if the kinema is to retain its present pre-eminent position. Acknowledgments The authors would acknowledge the efforts and ideas of several other engineers in the B.T.H. organization who have contributed to the design of the equipment, and would also mention the co-operation of the factory personnel in producing the equipment in the relatively short time allowed. Our thanks are also due to Mr. H. Jack, Director and Chief Electrical Engineer of the British Thomson-Houston Co., Ltd., for permission to publish this paper,