The motion picture projectionist (Nov 1930-Oct 1931)

26 Motion Picture Projectionist January, 1 93 1 e Laboratory Mechanical Reproduction of Sound Film PUBLICATION in a recent issue of the International Review of Educational Cinematography of a "new" method of reproducing sound-on-film once again invites attention to the various contributions to the art of mechanical reproduction of sound-on film. Many and varied have been the efforts to obviate the necessity for using optical means for this purpose, but although one method offers a possible solution, none of these processes may be said to have provided the answer to this problem. The contribution mentioned above is the work of Gastone Frediani,1 Italian inventor, and is presented here in abstract form: The object of this apparatus is to dispense with the use of photo-electric elements in the reproduction of sound photographed on film, by photographing the sound either directly on the edge of the film or on separate film, which is then turned in perfect synchronization with the picture film. The reproduction of the photographed sounds is obtained by substituting for the photo-electric elements in the amplifier a device consisting of two electric contacts in the form of metal drums placed at a certain distance from each other and electrically insulated. They rest upon the layer of developed sound images which connects them electrically with the grid circuit of the first audion (low-frequency amplifier). The photographic layer of sound images, as the sound film passes over it, acts on the grid circuit as a conductor of variable resistance. In point of fact, the layer constituting the photographed images of sounds is made of very fine particles of silver and possesses more or less electrical conductivity according to the degree of darkness or intensity of the sound images — that is, according to the quantity of silver salts reduced by the light during the process of photographing the sound. Accordingly the passage of the film with its developed layer beneath the two aforementioned metal drums, connecting them like an electric bridge in the grid circuit of the first audion, produces variations of electric potential in the grid of this audion in proportion to the electrical conductivity of the photographed sound images, and these variations of potential in the grid will result in corresponding variations in the electron current, which when amplified by the other tubes will be transmitted to the loudspeaker. This system of reproducing sound photographed on film — which can be specially adapted to sound cinematography— eliminates the photo-electric elements with their optical illuminating device, and therefore, since the film no longer need be transparent, it can be made of paper, oil-cloth, etc., instead of celluloid; these paper copies of the original transparent sound negative can be sensitized not only by silver salts but also by bichromate gelatine with the addition of colloidal metallic solutions or electrically conductive powders. These films, too, will be electrically modulated, according to the same law of modulations which governs the transparent sound negative. Corresponding, in fact, to the more transparent parts of the sound negative, a greater quantity of light will pass through and, when printing the copy on gelatine paper, after development, a greater quantity of electrically conductive powder will adhere to the film, which therefore at these points will possess maximum conductivity (as is the case with sound-film copies on paper sensitized with silver salts) . If the very fine powders added to the bichromate gelatine consist of magnetic elements such as iron, cobalt or nickel, the result will be a sound-film on paper which is not only electrically but also magnetically modulated. •Conducted by SAMUEL WEIN After being developed, it will, according to the transparencies in the original negative, offer the maximum thickness of magnetic powder and therefore the minimum magnetic resistance. If this sound-film — of bichromate gelatine and magnetically modulated — is then passed between the poles of a permanent magnet having at its poles two coils which by means of two other coils act upon the grid circuit of the first audion in the amplifier, these coils will set up electro-motor tension which will alter the potential of the grid in the first electron tubes and reproduce the sounds in the loudspeaker in the usual way. Process Dates Back to 1886 There are approximately 25 patents relating specifically to this purpose. The idea is decidedly not new, the first patent relating to a similar purpose having been granted to L. Bock.2 Bock photographed the sound using an "optical slit" on a "photographic slit." Contacts were applied to the emulsion on the "strip" and then fed into the primary of a transformer, the secondary of which was connected to a telephone receiver. "Film" was not commercially used at the time of Bock's disclosure (1886) ; but while Bock did not specify any particular medium he did claim a "transparent film or any other medium." It was of course, much too early in the art to use an "amplifier" and so he had to use a "head phone." Bock's patent refers to the use of a "special emulsion," but it is obvious that silver emulsions are applicable for the same purpose. More recently, Roy J. Pomeroy of the Paramount Famous Lasky technical staff was granted a patent3 which is of such scope as to include practically every advance made in this particular field of endeavor. Indeed, Pomeroy's patent is the only one of all those granted for this purpose which gives promise of being equally valuable in practical application as it appears to be in theory. Frediani's method employs a resistance change; while Pomeroy utilizes a capacity change. The Pomeroy patent is extremely complete as to detail and presents a complete circuit and hook-up. A few salient facts pertinent to this process are appended hereto and are worthy of consideration: 1. Old film will gives rise to so-called surface noise, as do old phonograph records. 2. It is common practice to coat all film with a certain compound to "harden" its surface (emulsion side), to prevent too easy scratching. This coating would naturally increase the resistance of the emulsion and therefore make it rather difficult to reproduce the sound. 3. A new kind of "sound patch" would have to be devised to meet the requirements of this process. 4. It appears to this writer that it is possible to secure a greater initial "kick" in the first tube of an amplifier by this means than is the case with light sensitive cells. In addition, no pre-amplifier, other than a "C" battery and transformer, will be needed. The output of the latter would be fed into the conventional amplifier. REFERENCES: iThis invention was patented in Italy under No. 275,669 on Feb. 14th, 1930; but a description thereof was deposited with certain Royal Academies of Science in Nov., 1929. 2U. S. Patent No. 364,472, June 7, 1887. "U. S. Patent No. 1,715,863, June 4, 1929.