Patent GB-1191845-A (GB)
Title: Improvements in Mountings for Transducers of the kind having Stylus Means for Engaging a Spiral Trace Containing a Signal and Formed on a Rotatable Disc
Inventor: BIRCH RICHARD WYKEHAM BEAUFOY
IPC: G11B3/38
Filed: 1966-08-17
Abstract: 1,191,845. Transducer mounting and guiding for disc record players. R. W. B. BIRCH. 1 Aug., 1967 [17 Aug., 1966], No. 36840/66. Heading G5R. A mounting for a transducer (including stylus) comprises (a) a transducer arm, (b) a slave arm rotatable in a plane parallel to that of the disc, the transducer arm being supported on or from the slave arm for rotation in relation to it about an axis normal to the disc, (c) means between the stylus and the transducer arm axis for sensing departures, of the line joining the stylus point to the transducer arm axis, from tangential relationship to the spiral record trace at the stylus point, and (d) drive means for driving the slave arm rotatably as the stylus follows the trace, the dimensions of the slave arm and its path of rotation being such that during the traverse of the stylus the line abovementioned coincides with or it close to the tangent at the stylus point, whilst the transducer arm is so located that the line is normal or substantially normal to the plane of vibration of the stylus. Fig. 1 illustrates the geometry on which the invention is based. Stylus S at the end of a stylus arm SP engages a spiral trace whose centre is O. During transducing, P is the pivot point of stylus arm SP, but P itself is caused to move so that (a) S traces out part of a semicircle on OX as diameter (the "ideal path" for S) (X being a fixed point) and (b) points X, P and S are always in a straight line. The theoretical path of P can be closely imitated by a portion of a circle, whose centre lies to the right of the line OX and is spaced from X by a distance approximately one quarter of OX. Point X is arbitrarily chosen, but the lengths OX and PS (e.g. 12 inches and 7 inches respectively) are selected to be convenient for sizes of commonly used discs. In four embodiments P follows the part-circular path above referred to while in the fifth, P follows the theoretical path. In Fig. 3 the apparatus consists of three main parts: a slave arm C,NP (see also 136, Fig. 7 (d)) centred at C and constraining point P to move in a curved path, a control arm XHR, centred at X and maintaining the collinear relationship of X, P and S, and the stylus arm SPW extended beyond P for counterweighting. The control arm, Fig. 5b, includes two parallel bars 101a and 101b, Fig. 3, secured to a boss 102 supported and pivoted on pillar 147, the bars being slidable (for variation of distance XP during transducing) in tubes 100a, 100b, respectively. The control arm is further supported by resting on a shoulder on a stern M, Fig. 6, carried by the slave arm 136, Fig. 7 (d); the control arm is rotatable with respect to stem M. The stylus arm, Fig. 4b, hangs by two wires K1, K2, Fig. 6, from the control arm, and is rotatable about a point P constituted by the centre of a sphere, fixed at the lower end of stem M, Fig. 6, and engaging in a bore in the stylus arm. The motivation of the slave arm is described in connection with the description of operation below. Operation.-The stylus arm carries a blade Q, Figs. 3 and 4 (b), which, when the stylus arm and control arm are correctly aligned, is just outside (radially of the disc) the free ends of two wires R1, R2, Fig. 5 (b), extending from a portion of the control arm which is located between the limbs of the stylus arm in the assembled condition. As the stylus arm is rotated clockwise (Fig. 3) by engaging the spiral trace, blade Q comes into contact with wires R1, R2, thus closing the circuit of a relay which, in turn, switches on the motor 114, Fig. 7(a) of the slave arm. The slave arm is thereupon rotated clockwise, such rotation causing the control arm to rotate clockwise about point X. The wires R1, R2 are thus moved away from blade Q, and the slave arm motor stops. In order to reduce to a minimum the number of switching actions during a traverse, a variable speed linkage which is pre-set, is arranged between the motor 114 and the slave arm. Thus motor 114 drives a friction wheel 115, Fig. 7(a), located at a fixed distance from a further friction wheel 116. The friction wheels are normally pressed between two discs, not shown, freely rotatable on a spindle 120, so that wheel 115 drives wheel 116. The speed ratio is governed by the lateral position of spindle 120, which is mounted in a frame 124 slidable between guides 125a, 125b. The frame is moved by rotation of a cam 126 mounted on a spindle 127, which has a knob 128, Fig. 3, click stops (not shown), and a pointer 129, above the casing, the pointer moving over a fixed scale 149. The click stops provide fixed ratios suited to commonly-used discs. Friction wheel 116 is connected to a worm 117, Figs. 7(a) and 7(d), which drives a worm-wheel 118, Figs 7(a) and 7(d), fixed on the axle 130 of a disc 131. Disc 131 is supported by a central boss on a fixed arcuate disc 140 and its outer edge rests on rollers 148 mounted in openings in disc 140. Disc 131 drives, by frictional contact, a pad 139 at the end of a bracket 138 mounted on an axle 135. Bracket 136, fixed near the top of axle 135, constitutes the slave arm. For any position of knob 128, arm 136 is thus rotated at a uniform speed about C: but if axle 135 (not attached to axle 130) is displaced laterally from axle 130, the speed of rotation of the slave arm varies during one cycle: such variation is an improvement on the uniform motion. In a second embodiment the stylus arm carries a small horseshoe permanent magnet instead of the blade Q, and the control arm carries a reed switch instead of the wires R1, R2 (Figs 8(a) to 8(e), not shown). In a third embodiment the control arm incorporates a photo-electric cell illuminated through a slit by a light source, and the stylus arm incorporates a shutter freely located between the slit and the photo-electric cell, the shutter having a vertical edge which controls the amount of light falling on the cell, and thus the motor circuit in which the cell is connected, (Figs. 9(a) to 9(#), not shown). In a fourth embodiment automatic control of the slave arm motor is dispensed with. For this purpose the control arm carries a pointer cooperating with a scale carried on the stylus arm, to indicate relative positions during a cycle, and the slave arm motor runs continuously during the cycle. For any particular disc the speed of the slave arm (controlled by knob 128) which gives the best performance (as indicated by the arm pointer and scale) is chosen by trial and error. The above-mentioned variant where the speed of the slave arm is not uniform may be used with this embodiment, (Fig. 10, not shown). In a fifth embodiment the path of point P is the ideal one. This is achieved by mounting the stem M, Fig. 7(d), in a block slidable on two rods forming part of the slave arm 136; the block is urged towards the axis of rotation by springs, and incorporates a horizontal roller-follower which traverses the edge of a fixed horizontal cam. The cam profile is selected to ensure that point P traces out the ideal path (Figs. 11(a) and 11(b), not shown). Miscellaneous variants and optional features.- In the third embodiment the slave arm motor may be variable-speed, its speed being governed by the output of an amplifier to the input of which the photo-cell is connected. This superimposes fine control on the coarse control provided by the variable friction drive of Fig. 7(a). In all embodiments, however, the slave arm motor may be variable-speed, and have its speed controlled; the variable friction drive is in that case replaced by a transmission train of invariable ratio. One of the counterweights, W1, on the stylus arm, Fig. 4(b), has a stack of removable riders 109. The stylus arm may be raised and lowered (being rotatable about the ends of wires K1 and K2) by screwing down a screw L, Fig. 5(b), provided in the control arm. The stylus arm has limited freedom of movement relative to the control arm by the provision of a stop J on the control arm. The control arm is hinged at H so that its outer part may be raised by the stylus arm engaging stop J when screw L is operated. The slave arm may be rotated by hand, using wheel 142, Figs. 3 and 7(d), or wheel 143, Figs. 3 and 7(d), or, for slow motion, wheel 144, Fig. 3, operating through wheel 142. The slave arm may be driven by the motor which rotates the disc, the control means of the invention being used to effect connection and disconnection between the disc motor and slave arm, e.g. by acting on an electromagnetic clutch interposed between motor and arm. The sphere-and-bore arrangement at the lower end of stem M may be replaced by a needle and diaphragm or by a needle and crossed wires.