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THE EXCITER LAMP 475
Most manufacturers today think in terms of a slit of roughly % mil. If the tone control of a machine to be tested is set to provide unequalized reproduction, it is not unusual to find the slit-loss characteristic as measured with a standard frequency film such as ASA-Z22.44 poorer than that expected for a %-mil slit. The variation that one is apt to find from the product of one manufacturer to that of another is quite large, and, although the variation from one machine to another of a particular manufacturer is smaller, it is still larger than should be expected with first-grade quality control and inspection in manufacture. One source of difficulty is that there is no standard method of defining slit thickness despite the fact that it is tacitly assumed that slit loss will be expressed in terms of the equivalent theoretical slit. In practice, the slit thickness is always larger by a considerable percentage than the value given by the filament diameter of the lamp multiplied by the lens reduction ratio. The diffusion, the lack of color correction, and numerous other factors vary over such a wide range that any simple geometric estimate of this simple nature is almost meaningless.
Accurate focus and azimuth adjustments are almost as important in sound projectors as in sound recording machines. This is especially true of present optical systems, which are usually little better than marginal in performance. There have been too few improvements in the mechanical design of many machines to make possible more accurate adjustment of azimuth and focus. The combination of marginal optical systems and poor adjustment arrangements in so many machines gives rise to entirely too many cases of submarginal performance in machines in day-to-day use. The situation is especially acute when Kodachrome combination duplicates are projected that have the non-standard emulsion position ; the sound track is out of focus by the thickness of the film and in most of the poorer machines there is no provision for altering the focus to correct for the serious losses that are present. Color duplicates are growing daily in commercial importance, and, although steps are being taken in a few laboratories to eliminate non-standard emulsion position color films, it appears that they will still be with us for some time to come.
The Exciter Lamp. The most common form of power supply for the exciter lamp is a super-audible oscillator operating at about 20-25 kcs; its electron tube is fed from the power supply of the amplifier. Alternating current of mains frequency is not used in commercial equipment, since the hum produced would be excessive.