Harrison's Reports (1928-1928)

132 HARRISON’S REPORTS either case recording is far superior to that of the disc system, and is capable of recording a greater range either of high or low frequency sounds. Recording on Film There are two types of recording of sound on film: the variable density, and the variable width. The variable density system is used by Western Electrict, and the variable width by the R. C. A. Photophone. The Fox-Case Movietone uses the variable density system. (For the sake of clarity, let it be said that the word “Movietone” does not denote a particular type of talking machine; it has been used by Fox for their own instrument. The same instrument may be given a different name by another company. The same is true of the Vitaphone; it is a name applied on their own system by Warner Bros. Another company may secure the rights to the same kind of instrument from Western Electric and call it by another name. But for convenience, let us use the words “Vitaphone,” to denote the disc type, “Movietone” to denote the variable density type, and “Photophone” to denote the variable width type.) The Movietone, or variable density, sound track, consists of different density lines running across the sound track, over the entire width. The density of the lines are the result of the strength and quality of the sound. If the sound is soft, the lines are very light; if the sound is strong and has great volume, the lines are dark. The Photophone (variable width) sound track is black on one side, and transparent on the other, the division line resembling sometimes a saw, with the teeth of different lengths, sometimes a miniature mountain range. The division line is always irregular, and its shape depends on the pitch and volume of sound. In both types a ray of light strikes the sound track and reaches the photocell, reproducing the sound that was recorded on the film. (How the sound is produced is omitted because it is outside the scope of this article.) But in the Movietone type the light that passes through the sound track and reaches the photocell is regulated by the density of the lines on the emulsion, whereas in the Photophone type the amount of light that passes through the sound track is regulated by the width of the transparent part of the sound track. Figuratively speaking, in the Movietone system the amount of light that goes through the film to reach the photocell is regulated by curtains, drawn across its path or removed from such path, just as the requirements of sound dictate, whereas in the Photophone system such light is regulated by a sort of valve. Reproduction The greatest enemy of good reproduction is imperfections in developing or dirty sound track. It causes a ground noise. Of the two film systems, the Movietone is subject to ground noise more than is the Photophone system, for the reason that the sound shadings of it can be affected by oil or by other dirt, whereas the Photophone system, not depending on “shadings," cannot be affected to an equal degree. Grain structure, too, enters into the matter; any defect in it will produce a ground noise, just as will any defect in developing. A defect on the Movietone sound track cannot be painted over. Remember that we are dealing with fine shadings of sound and any imperfections on the sound track of the systems that use the variable density type of track cannot help having a detrimental effect on the quality of the sound. In the Photophone, defect in the grain structure of the emulsion as well as imperfections in developing have no effect on the quality of the sound, for the reason that the defective part can be painted over. Since this type of sound recording and reproducing docs not depend on shading, no matter how black the defective part is made by painting, the effect on the sound is not detrimental. Types of Sound Reproduction The Movietone and Vitaphone systems use the same kind of horn, and the same kind of diaphragm. But it is a different kind of diaphragm from that used in telephone; it is not flat. It is like a cigar ash tray, cupped at the one end, and with a flange at the other: it is made out of duralumin, an aluminum composition. The cupped part of the diaphragm is used for an air cushion effect, by having a ball fit into the cupped part and kept August 18, 1928 at some distance from it. When the diaphragm vibrates, the air between its cupped part and the ball acts as a cushion and prevents it from rattling. But even though it is an ingenious piece of mechanism, it has its limitations. It cannot stand overloading. In sounds of great volume, it is liable to destroy itself. In low-frequency sounds, it must rattle, for the reason that the vibrations being few per given time, fewer than the vibrations in the case of high-frequency sounds, the air between its cupped part and the ball does not act as a cushion in the same manner as it acts in the higher frequency ranges. The Photophone does not use a diaphragm; the paper cone that is used for projecting the sound acts as a diaphragm. The area of the cone is approximately sixteen times greater than the area of the Western Electric diaphragm, and, as the Photophone installation uses sixteen cones, whereas the Vitaphone and Movietone use an average of four horns, the Photophone system has at least sixty times as much vibrating area as has the Vitaphone-Movietone system. This enables the Photophone to produce volumes of sound that cannot be produced by the systems that use the horn, and to reach low-frequency ranges that cannot be reached by systems that use the Western Electric diaphragm. It is my opinion, in fact, that no diaphragm-using system can reach the volume or the frequency ranges that are reached by the instruments that employ the cone system of sound reproduction and projection. The cone system produces a better quality of sound, too. There is no instrument in the market at present that can give out as melodious a sound as can the voice or the instrument that produces the music; but the cone approximates the natural sound far more than does the horn. Sound Projecting The next thing of importance is the projection of sound: There are two types of sound projection: The horn tjrpe, and, as said, the cone type. The Vitaphone and the Movietone use the horn; the Photophone, the cone, made out of paper. The horn has several disadvantages: (1) It cuts off the low-frequency sounds (bass notes). As you know, the cornet produces high notes because the blasts are sent through narrow tubes, and short. On the other hand, the bass horn produces low notes because the blasts are sent through wide tubes, and long. Thus it is seen that low notes require length and width in the tubes. There is a limit in the talking picture field as to how long the tube part of the horn can be made, and how wide the horn itself. The horns used by the Vitaphone and the Movietone are twelve and fourteen feet long, (coiled), and can reproduce successfully low sounds only up to a certain range. To reproduce the entire range of low sounds it will require a horn so long and so cumbersome that it will be necessary for the theatre owner to install a winch to lift it up whenever he needs to use the stage for some other purpose. The paper cone system presents no such defects and difficulties; it is in two sizes, either twelve or sixteen inches in diameter, about six inches deep, and tapers to a hole of about the size of a silver dollar. At the outer circumference it is mounted on soft, flexible kid leather, and on the inside on three fine silk threads. It is naturally placed close to the electro-magnets. This mounting arrangement allows it to vibrate freely in and out, like a piston, producing as good a quality of sound as could be produced by a mechanism when one takes into consideration present mechanical limitations. (2) The horn distorts some sounds and suppresses others. This occurs when the low-frequency waves strike the sides of the horn. The result is unnatural. This defect is noticeable in the human voice more than it is in the musical or other sounds; the voice sounds hollow, or as if “it has come out of a barrel,” as some have put it. The horn possesses some other defects. But as these are not so important as the ones already mentioned, they are left undiscussed for the present. Next week: Interchangeability; prices for the different types and for theatres of different seating capacity; added cost of theatre operation because of the installation of a talking picture device, and other matters of importance to exhibitors seeking light.