Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, July, 1930 Page 7 Sound Recording Tubes By Joseph B. Zetka Fig. I. New recording tube. THE DEVELOPMENT OF A NEW GLOW LAMP FOR VARIABLE DENSITY RECORDING OF SOUND ON FILM THE two outstanding methods for producing a variable-density sound track on film consist of the mechanical vibrating system and the glow-recording system. Up to the present time the difficulty of producing glow lamps which are sufficiently uniform and which meet the other requirements imposed have resulted in some of the leading producers adopting the mechanical vibrating system. In this system it has been possible mechanically to peak the response of the device in order to compensate for loss of the higher frequencies which are caused by the finite width of the slit used. In the glow recording system there is one outstanding advantage over the mechanical vibrating system, in that there is absolutely no inertia to be overcome. On the other hand any losses of the higher frequencies must be compensated for, either in the amplifier itself or else the lamp must radiate a sufficiently fine beam of actinic rays so that the important range of audio-frequencies is transmitted without any loss up to a frequency of about 5,000 cycles. If some loss begins to manifest itself above this frequency, the result will still be perfectly satisfactory for all practical purposes. Where the recording of sound is made exclusively in a laboratory, the extremely delicate nature of the apparatus used in the mechanical vibrating system is under the constant, watchful eye of laboratory technicians and delicate adjustments can of course be made instantly. When, however, such recordings are made "on location" or under conditions such as confront news reel recordings in surroundings far removed from the laboratory, reliable operation of the system requires the service of an experienced mechanician. The glow recording system, because of its ruggedness, offers a decided advantage for such work. Up to the present time the advantages of the glow recording system have been counter-balanced in practical usage, by a number of shortcomings in the glow lamps themselves, which may be listed as follows : 1. Low light-intensity : necessitating that the lamp shall be used with the mechanical slit or its equivalent as close to the film as possible with the attendant troubles of slit-clogging, etc. 2. Too large wattage consumption in the lamp itself : causing excessive heating of the bulb and element, which in turn causes vaporization of the elements within the bulb and consequent blackening and obstruction of light. 3. Very short and indefinite life : due to blackening of the bulb or change in the gas pressure or surfaces of the electrodes. 4. Lack of uniformity : requiring individual calibrations and adjustments for each lamp. 5. Erratic electrical characteristics during life of the lamp : requiring constant watching and readjustments. 6. Low sensitivity : thus requiring a comparatively large output from the recording amplifier. Glow Lamp Design The design and development of a glow lamp which meets and overcomes these objections has proven an extremely difficult and tedious problem of intensive laboratory experimentation. In order to make lamps, having repro ducibly uniform characteristics, extreme care in each manufacturing process is, of course, absolutely essential. Naturally, it is also vital that the gases used must be absolutely free from any trace of impurity. Furthermore, the bulb and the elements themselves must be thoroughly degassified and evacuated so that the completed glow lamp will contain only the various elements themselves, together with the desired gases, without possibility of other gases being liberated from the elements under operating conditions. As the primary object of the glow lamp is to provide a source of actinic light which is to be effective only through an exceedingly small aperture, the design of the lamp must be such that the light intensity present in the glowing spot within the bulb must be concentrated and located as closely as possible to the end of the bulb through which the light is to pass. In exposing the film, the violet and ultra-violet rays of the spectrum are very important, and consequently in addition to the other factors entering into the development of such a lamp, great care must be taken to insure the conservation of this range of frequencies in every way possible. New Lamp Ready The new glow lamp has been developed as a laboratory product to meet the required conditions and is now available for sound recording on film by the variable density system. The light intensity produced by this lamp makes it suitable for either the slit method or the optical system of producing the desired sound track. Most of the experimental work has been done on standard 35 mm. film with the 125 mm. sound track. A few recordings have been made