International projectionist (Jan-Dec 1954)

Ernemann and Ernophon S sound reproducers is an antimony-cesium bluesensitive cell patented by Zeiss Ikon, but similar in response characteristics to the RCA photocell No. IP37. Photocell Types The regular-type photocell, having a cathode composed of a mixture of cesium and silver oxide coated upon a silver plate, is called a red-sensitive cell because it is most sensitive to red and infrared light, and responds only very feebly to blue rays. In fact, the sensitivity-peak of this cell, which , is still the most widely used in all countries, lies in the invisible infrared region of the spectrum. Consequently, the red photocell works well only with silver-image tracks, not with coloreddye tracks which, if they could be used, would lower the cost of all types of multilayer, dye-coupler natural color prints. Silver images absorb red and infrared as well as all other wavelengths emitted by the exciting lamp, but dye tracks, no matter what color they may be, are quite transparent to low red and infrared radiation. The blue cell is practically insensitive to red and infrared light, and thus gives excellent response with red (magenta plus yellow) soundtracks as well as with ordinary silver soundtracks. The Zeiss Ikon blue cell has, in addition to this property, superior electron-emitting characteristics and an exceptionally long life. There are, however, two disadvantages of the blue cell which will soon be mentioned. Strontium Photocell A strontium photocell having its sensitivity-peak in the green region of the spectrum is just now being developed in Germany. This green cell would work with magenta dye tracks, thus further simplifying the work of processing color prints. Another solution of the problem involves producing a cyan dye which completely absorbs both red and infrared. While the common cyan dyes in use absorb "high" and "medium" red wavelengths very efficiently, most of them transmit bands in the low red and, what is more serious, practically all of the infrared emitted by an incandescent exciting lamp. If a perfect cyan dye could be produced, cyan tracks would give good response with the regular red-sensitive photocell. It all amounts to making the soundtrack images "look" as dense as pos FIG 4. The Ikosol II (left) and Magnasol IV (right) convertible arc-lamps for both LI and HI operation at maximum optical efficiency. Note the clean design and centralization of controls. sible to the light-sensitive cathode of the photocell. To an ordinary red cell a dye track looks almost blank because the colored dyes, even when superimposed (cyan, magenta, and yellow) to look black to the eye, pass along waves which such a photocell readily perceives. To return to the blue photocell, we must point out that this type of cell, though very sensitive, is excited only by a very small portion of the rays emitted by the filament of an exciting lamp. Incandescent lamps give out mostly infrared, red, orange, and yellow rays, a fairly large amount of green rays, but relatively few blue and violet rays. And it happens that the intensity of the blue-violet radiation of an incandescent bulb varies much more than that of the red and infrared with fluctuations in the current which lights the lamp. This fact is revealed by a reddening of the light when the voltage is decreased. Blue PC Faults The sound-current output of a blue photocell excited by an incandescent lamp is therefore at the mercy of exciter-voltage fluctuations too small to affect the performance of a red cell noticeably. In theatres where line voltage is none too steady, the blue cell would behave very badly indeed and force the projectionist to stick by the volume control in an effort to maintain a constant level of volume. Then too, we must take into consideration that the greater number of color prints in America are made by the dye-imbibition process (Technicolor), a method which is much less expensive than the use of multilayer dye-coupler color films. But imbibition printing, generally satisfactory for pictures, cannot be used for printing soundtracks because it involves a rather serious loss of image definition. Satisfactory "resolution" is a neces sity in soundtrack printing — a "fuzzy" track gives fuzzy sound, and no amount of electrical high-frequency equalization in re-recording can compensate frequencies which fail to print through in the projection copy. For this reason Technicolor films will always have to be made with silver soundtracks. Small Preamplifier The sound unit of the Ernemann X and IX projectors has a small preamplifier (visible in Fig. 2. See December IP) close to the photocell. This single-stage voltage-gain amplifier makes it possible to set up the main amplifier in any part of the projection room without danger of picking up noise or of attenuating the high frequencies. A preamplifier attached to the projector, it will be recalled, was employed in the very first Western Electric (ERPI) equipments; and most American manufacturers of soundheads now favor photocell coupling amplifiers. Zeiss Ikon, the manufacturer of Ernemann projectors and sound reproducers, also makes "Dominar" theatre amplifiers, "Ikovox" speaker combinations, a separate cell-coupling preamplifier for use with earlier Ernemann models, or when the Ernophon S reproducer is used with other (European) makes of projector. Ernemann exciter rectifiers are part of the projector equipment, and in the Model X the rectifier is located in a metal drawer under the lamp table. Dominor Amplifiers The Dominar amplifiers require no detailed description, since their characteristics are very similar to the highest grade theatre amplifiers made in England and America. They have less than 2 per cent distortion, utilize degenerative feedback, and have frequency-response networks for adjusting to auditorium conditions. Noteworthy, however, are the very heavy 10 INTERNATIONAL PROJECTIONIST • JANUARY 1954