Projection engineering (Sept 1929-Nov 1930)

Page 38 Projection Engineering, September, 1929 Rotating the "Wax" for Sound Pictures Dealing With the Design and Construction of the "Drive" for Disc Recording By L. A. Elmer* THE machine used in recordingsounds on phonograph discs synchronously with associated pictures consists essentially of a turntable, bearing the "wax" and rotated by a synchronous motor1 of constant speed, and an electrically driven stylus2 cutting the record. In the design of this machine the primary aim is to insure that the record is both faithful to the original sounds and synchronous with the pictures. Fidelity in the performance of the stylus would be vitiated by departures from uniformity in the speed of the turntable while sounds were being recorded or reproduced. Although a constant speed motor is used, its value would be destroyed if the machinery transmitting the drive from the motor to turntable were not equally free of velocity variations. Thus the problem of fidelity involves not only the motor and the stylus but all the moving parts of the machine.3 Fig. 2. The mechanical filter for the turntable drive, as it appears in a commercial disc -recording installation. Even were it possible to connect the motor directly to the turntable, casual variations in the speed would arise, from varying frictional loads on the turntable and bearings. But direct connection is unsatisfacory. Because the turntable must operate at a lower speed than the motor (one thirtysixth of that speed), reducing gears must intervene. In the actual apparatus the motor drives (through a horizontal coupling) a worm engaging 2 -006 V) < Fig. 1. Above: x < The difference ^ between actual ~ t 00* and correct po JJ sitions of teeth >> o on a t y p i c a I 5 gear, relative < a to arbitrary ref erence position. . Below: The"o averaging effect J| 5 of dividing the o 5 same gear into 5 z four layers, cal 5 o culated from *_ data (above) % for one layer. fWV -\A A M /A, . ,r " ¥ ^^\ \r\ .007 006 005 2ND C CLE 3RD. CYCLE 4TH.CYCLE a^/vs aAv\ /^w aaA-n vJAa aa/^ -^vIa AAA/> ^V\ NUMBER OF TOOTH 33 41 4«_ 120 160 200 240 260 DECREES ROTATION OF GEAR ON HEAD * Apparatus Development Dept., Bell Telephone Laboratories. a worm wheel which drives (through a vertical coupling) the shaft to which the turntable is attached. It is cyclic speed-change that must be guarded against in this mechanism ; all such changes with frequencies from about one-half cycle per second up to the higher limit of audibility are to be avoided. Speed changes at audible frequencies introduce extraneous sounds into the records, and speed changes at frequencies below the audible range produce changes in pitch. There are in general two points of origin for these variations : the turntable and its bearings, and the gears. Speed-changing variations in load on turntable and bearings are most likely to have the frequency of the rotation of the turntable (a little more than one-half cycle per second). From the gears three sorts of variation arise. — those accountable to inaccuracies in the spacing of the teeth (Fig. 1). to errors in the shape of the teeth, and to the successive shifts of driving load from tooth to tooth. Together these may occasion variations with quite a range of frequencies. Determination of Permissible Variations The extent to which these variations are permissible is determined, for lowfrequency changes, by the smallest change in pitch the ear will notice when pitch-variation is continuous. It appears that, when a pure tone is projected by a loudspeaker of high quality, the ear can detect variations in its pitch which exceed one-tenth per cent of its frequency. This sets a severe requirement for constancy of rotation. It is, furthermore, an overall requirement, for it applies to differences between the original and reproduced sound ; and both a recording and a reproducing machine intervene be tween these sounds. Since both operate at the same speed, and since there is a high probability that the ultimate record will be lined up on the reproducer correspondingly to the "wax" on the recorder, variations in the speeds of the two are likely to be additive in their effects upon sound pitch. The sum of the variations permitted in the two, therefore, must not be greater than the total permissible variation for the system as a whole. Since the Fig. 5. The oil-damping connection between the gear and turntable shaft. Above, the oil cup; below, the vane-bearing end of turntable shaft.