British Kinematography (1951)

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146 BRITISH KINEMATOGfc^PHY Vol. 1ft, No. 6 gate is parallel to the plane of the screen. The lantern gives normal illumination, a= its centre line passes through the centre of the picture aperture to the centre of the screen. The projection lenses are mounted in the standard focusing holder with their centre lines normal to the plane of the screen. The projection lenses are mounted offset in the standard focusing holder. The Screen The screen has a front surface of fine grain particles, each of which gives a specular reflection and maintains the plane of polarisation. The polar diagram of such a screen stretched flat when set up in a vertical plane has a rather narrow lobe. The screen is therefore curved in the horizontal plane, and coverage is obtained over the whole of the seating capacity with good uniformity of brightness over the whole surface of the screen. Vibration Insulation Normally the projectors stand directly on the floor of the operating room, but in the Telekinema the room is located directly above part of the audience. It is necessary, therefore, to mount the projectors on antivibration mountings. These mountings are designed to give a periodicity of about 16 cycles per second in the vertical plane in order to be effective in reducing noise transmission to the floor. The machines, being tall in proportion to their width, have a slow torsional period about the fore-and-aft line of the base, and this would have an adverse effect on the picture but for a bridge piece which braces the two machines and holds them together. With the projectors mounted non-rigidly, it would be reasonable to assume that the picture as projected would be less steady than that from a similar projector rigidlv mounted. It is however found, first that the steadiness of each picture is still within normal limits, and secondly that in the stereoscopic picture the remaining movement disappears, giving a remarkably steady picture. 4. STEREOPHONIC SOUND Present techniques of reproducing sound in the major kinemas have reached a fairly high degree of perfection, and there appears to be little doubt that further refinements of these techniques are unlikely to arouse any intense public interest, or, to put it more directly, they are unlikely to be good " box office." In spite of this, it must be agreed that no audience could be deceived into thinking that the present electro-mechanical reproduction is the real thing, and as this must be the ultimate target of the sound reproducer designer it was decided to investigate alternatives to present techniques rather than mere refinements, although such refinements are incorporated wherever possible. Three-Channel Reproducer Of the various possibilities, the installation of a three-channel stereophonic reproducer, with additional loud-speakers to reproduce special sound effects in the auditorium, seemed to be the most attractive, and after this had been confirmed by an experimental installation in one of the B.T.H. works theatres it was agreed to proceed with an installation in the Telekinema. Three channels rather than two were chosen because side-by-side comparison indicated the superiority of three, particularly where dramatic action is to be reproduced. The majority of acoustic close-ups always occur in centre stage and the third (centre channel) is invaluable in securing the necessary intimacy. It is noteworthy that Bell Laboratories came to the same conclusion after their tests in America.5 The dramatic appeal of sound effects produced in the auditorium, rather than from the loudspeakers behind the screen, was confirmed in first experiments, and at a very early stage in the discussions it was agreed to install additional loud-speakers in the auditorium back walls and in the main and underbalcony ceilings. Independent Sound Tracks Four independent sound tracks were therefore required, three for the stereophonic sound effects and one for the auditorium sound effects. Although experiment has confirmed that a wide frequency range is less necessary in a stereophonic system than in a monaural system, it was decided to make the system