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SOUND REPRODUCTION 259
in the speaker system and obviates the need for a D.C. supply to the exciter lamp. The shape of the filament is always a straight line at right angles to the elevation shown in the figure and, as will be seen later, some systems employ this Tine Filament' to obtain the scanning slit. Sound volume will naturally fall when, after some considerable use, the lamp envelope becomes blackened. Because of this, and also to provide an immediate replacement in the event of a lamp breaking during a show, most manufacturers now fit some form of double holder in the exciter lamp house, one side carrying the lamp which is in use and the other carrying a spare lamp already centred up with the optical axis of the sound head. The wiring is so arranged that this second lamp will come into operation immediately it is swung into position the action of rotating the mount, to transpose the lamp positions, automatically transfers the electrical supply from one lamp to the other.
In the arrangement shown, light from the filament is brought to a small highly intense spot on the film surface by means of a condenser lens. An image of the sound track is thrown by the objective lens onto the scanning slit. This slit is of such dimensions that it will isolate one cycle of the highest frequency required in the reproduction, and is situated just in front of the photo-cell.
The Position and Size of Scanning Slits
The size of the overall or effective scanning slit may be easily obtained from two factors. Firstly the film speed and, secondly, the highest frequency which is required to be reproduced. The highest recorded frequency is usually about 9,000 cycles and, although the amplifiers may 'Cut-Off' below this frequency, it is necessary to resolve a 9,000 cycle note at the scanning head since such a frequency will alter the timbre or quality of tone in those lower frequencies which the amplifier will reproduce. The distance occupied by one cycle of a 9,000 cycle frequency, at a film speed of 24 pictures per second is 0-002-inch (0-05-mm). If the effective slit width is too great the high frequency response is lost and distortion is introduced whereas, if the slit width is too narrow, the response over the entire range is lowered. Apart from these factors mechanical considerations place a limit on the size of slit which can be made to any given accuracy. Curves plotted to show the frequency response with slits of various widths indicates that the most efficient slit is one 0-0013-inch (0-033-mm) in width.
It should be noted that the term 'Effective Slit Width' has been used. This has been done to draw attention to the fact that large mechanical slits may be optically reduced to the required