Exhibitors Herald and Moving Picture World (Apr-Jun 1930)

Record Details:

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52 Better Theatres Section May 10, 193 ILEX F:2.5 Projection Lenses Assure Maximum Projection Efficiency Greater Sharpness — Greater Brilliancy Comparative test with any other make will prove their superiority. Now furnished in two Series — The Standard Ilex F :2.5 equipped with adjustable mechanism permitting the exact screen covering desired, within its focal range. No. 1 — In position for The Ilex F :2.5 Dual Focus type with which it is possible to maintain the same size screen covering when running sound-on-film as is obtained with silent film or disk — The superior optical qualities of the Standard Ilex maintained in both positions of the Ilex Dual Focus. Inject into your screen that clean cut brilliancy that Ilex alone can give. ILEX OPTICAL COMPANY Rochester, N. Y. Established 1910 on the record. The pickup is a device which translates slight movements of the needle (or rather the armature through the stylus and needle) into electrical currents. In this the pickup is impartial and just as ready to generate an electrical current from an unwanted mechanical vibration as from a movement of the needle by the record groove. Pickups vary in the amount and kind of vibration they will convert into sound through the amplifier and speakers. Most vibration of the pickup head (with respect to the record) which results from vibration of the turntable unit, is along, or in the direction of, the pickup arm. This vibration modulates or distorts the sound, because it causes rapid changes in the rate at which the record groove passes under the needle. The pickup needle may, for example, start to move to the right in a groove, only to have a sudden jerk of the pickup toward the observer make the needle start back in the groove and move to the left for an instant. A “perfect” pickup would only produce a current when moved in the direction in which the record groove produces motion. Many pickups will also produce a slight current when the needle is moved at right angles to its normal movement, or when moved in the direction of the pickup arm; that is, in the direction in which there is most vibration. ( In the design, this is eliminated by preventing movement of the armature in this plane, or by preventing a change in the flux when it does occur.) This current is combined with that due to the distortion mentioned above, and produces a sort of “rumble.” The solution of the problem of vibration should really rest with the manufacturer of the equipment, but all too often the projectionist is left to his own devices in solving it. In so far as it is possible, the thing to strive for is absolute immobility of the pickup with respect to the turntable. This can best be achieved by using a heavy, solid turntable, a well braced rigid pickup arm support, a good flexible coupling between the drive and the turntable and, in many causes, cushioning between the turntable base and the booth floor to prevent the transmission of any vibration from the floor. If little or no vibration is transmitted to the turntable from the drive or from the floor, and the gears in the turntable reduction unit introduce none, the pickup arm base should be mounted directly on a rigid bracket from the turntable support casting. Where this arm cannot be made rigid enough and the “free” end which supports the pickup arm vibrates with respect to the turntable and turntable support, then a thick, soft felt cushion is frequently used between the pickup arm base and the supporting bracket. Rubber bands are then used to hold the pickup arm base in place. This is a remedy of a sort but should be resorted to only if the other changes cannot be made. Aside from vibration, the most common trouble with disc installation is due to “waver,” or to “wows,” as the insta * lation engineer calls them. These ai due to a slow (slow compared to thos possible in the changes due to vibratio mentioned above) and regular change i the rate at which the record groov moves under the pickup needle. This i usually due to a change in the speed o rotation (angular velocity) of the turr table itself. The action is in many case similar to that used in the type of cloc * in which a heavy weight is suspende from a flat strip spring. The weigh first rotates in one direction until all o its energy is used in twisting its sup port, after which it reverses in directio and repeats the process. In the turn table, the flexible drive usually acts a the spring support, and the turntable a the weight. What play or irregularitie there are in the gear train of the reduc tion unit, are also frequently importan in determining both the frequency o rate at which the “wow” or waver oc curs, and also its intensity. A numbe of turntables use a mechanical filter be tween the 33 1/3 r.p.m. drive shaft ant the turntable. While this is desirabli 1 when properly designed, it may actually aggravate the normal waver, unless properly damped and constructed. In addition to the “wow” due tt changes in the speed of rotation of the turntable, one may be introduced by thi pickup itself in periodic movement wit! respect to the record. In some pickup;; the pivot point is very near, or at thi pickup head. This means that the pivo is within three or four inches of thi needle point, rather than 14 inches or si from it, as is normally the case with : long arm pivoted at the arm support. A moment’s consideration will show tha if the record is raised one-eighth of ai inch, the needle will move along thi groove almost four times as far whei the pivot is near the needle as it wil when the pivot is near the end of the arm. This movement is equivalent to ; change in speed of the turntable and wil produce a waver. A waver which results from a changt in the speed of the drive to the turn table, may be due either to the motor oi to a “ragged” load at the projector end Any binding or trouble in the gear trail in the projector which produces a chang< in the load on the driving motor, maj be transmitted back through the motoi to the turntable. The possibility of this is usually minimized by having a flexibh drive between both the motor drive, the projector and the turntable. If the pro jector is suspected, it may be discon nected in some types of equipment anc the turntable be left on. If the troubk is present in only one machine, anc either the motor or the projector heac is suspected, they may be changed one at a time. Changes in pitch which occui at irregular intervals of several minutes or more, may be due to a change in the motor speed resulting from a change ir line voltage. [The sixth article in this series will appear in ih June 7th issue.]