Patent GB-388819-A (GB)
Title: Improvements in or relating to sound recording or reproducing systems
IPC: G11B3/00
Filed: 1931-09-04
Abstract: 388,819. Recording sound. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A.-(Assignees of Keller, A. C. ; 3, Park Lane, Mount Vernon, New York, U.S.A.) Sept. 4, 1931, No. 24911. Convention date, April 18. [Class 40 (ii).] A stylus for cutting hill-and-dale type sound records, consists of a conical member of small angle of taper which is sliced to produce a flat cutting face of parabolic, hyperbolic or preferably elliptical shape, adapted to be mounted perpendicularly to the record surface, the angle between the front edge and the trailing edge of the middle section of the cone, taken through the axis of the cutting face, being 50‹ or less. The stylus is designed to cut sound-record grooves of large amplitude without distortion and to allow overlapping of adjacent spirals of a groove so that the playing time of the record is increased. Figs. 2 and 4 illustrate a stylus according to the invention in which an elliptical cutting face has a semi-major axis band semiminor axis a, the values of a and b being chosen to give a suitable groove spacing and a desired groove depth and at the same time to allow a suitable radius of curvature at the stylus tip, this radius of curvature being, usually, À0022 inch. The " available groove depth " d is somewhat smaller than b since the curved surface 25 of the cone must not project beyond the cutting edge of the ellipse at the grooveengaging portion and this dimension d is therefore determined by the choice of the angle of taper # of the cone and the angle (#) at which it is sliced to form the cutting face. Moreover the angle (#) which the cutting edge makes with the record surface when the depth of the groove is a maximum must be slightly less than 90‹, usually 85‹, in order that the shavings cut from the record may be removed in a satisfactory manner, and the groove depth determined by consideration of this factor also is called the "Useful Depth." A formula is developed which expresses the " Useful Depth " in terms of a, b and #, A stylus form is usually chosen, by selection of the values of # and #, so that the " Useful Depth " and the " available depth " are the same. The curves 41, 42, 43 of Fig. 11 show variations in groove width with depth of styli having different corresponding values of band a. The curves 47, 48, 49, Fig. 12, show the corresponding variations of depth with the angle of taper (#). Thus by reference to the dotted line curves, Figs. 11 and 12, it is seen that for values of b, of 5, 1.0, 15 mils. respectively having corresponding values of a of 3À3, 4À7, and 5À8 mils. respectively. The values of d are 4À35, 8À15, and 11À6 mils. respectively, and for these depths the styli must be sliced from cones having double angles of taper of 7À5‹, 8À7‹, and 9À0‹ respectively. The curves 44, 45, 46, Fig. 12, show values of the slicing angle plotted against values of # which show that the three stvli should have slicing angles of 43À5‹, 28À5‹, and 23‹ respectively. A further consideration in the design of the stylus is to make provision that the trailing edge shall not contact with the edge of the groove and prevent faithful response to the impulses of the recorder. The stylus is designed, therefore, to clear the portion 27, Fig. 6, of the record groove in recording the large amplitudes and for this purpose the back angle # is made greater than 40‹, and since # + # + # = 90‹, this large value of # is therefore allowed for by a stylus designed according to the curves of Figs. 11 and 12. Fig. 1 shows in section the overlapping grooves of a recording at high intensity levels effected with the stylus of the invention. A suitable form of reproducing stylus for a record sc formed is a cone 54 of which the portion entering the groove has an angle of 36‹ and a tip radius of À002 inch. Specifications 262,839, [Class 40 (ii), Phonographs &c.], and 363,449, [Group XXXIX], are referred to.