GB816678A - Improvements in or relating to moving coil electro-mechanical transducers - Google Patents

Abstract

816,678. Pick-ups and recording apparatus. ELECTRIC & MUSICAL INDUSTRIES Ltd. Sept. 10, 1956 [Sept. 23, 1955], No. 27136/55. Class 40 (4). In a moving-coil transducer for producing, or reproducing from, records having either or both of two different directions of groove modulation, such as lateral cut and hill-anddale traces, respectively, two moving coils are wound on a single support capable of movement in the two corresponding directions, each of the coils producing, or responding to, vibrations in one of the directions only. The principle of operation is illustrated in Fig. 1 in which the coil support (not shown) is assumed to be capable of angular movement about the axis XY and of translational movement along the axis of the air gap 3 at right angles to XY. The first coil FG is wound in a non-reversing manner so as to encircle the central pole portion 2, and the second coil HK is wound in a figureof-eight pattern with the cross-over along the axis XY in a hole or slot in the pole portion 2. Alternating current fed to FG causes translational motion of the support (hill-and-dale), whereas similar current fed to HK produces a torque causing oscillation of the support about the axis XY (lateral cut). The coils respond in like manner, FG to hill-and-dale and HK to lateral cut traces. A practical embodiment suitable for cutting or reproducing records having either or both types of modulation is shown in Fig. 4. The magnetic system includes a central pole portion 13 of one polarity and two co-operating pole portions 14, 15 of the opposite polarity providing a part-annular air gap. Brackets 16, 17 of non-magnetic material are secured to pole portion 14 and to two pairs of spring strips 18, 19 and 20, 21 of, for example, phosphor-bronze. The lower pair 18, 19 which are flat and in the horizontal plane have their inner ends fixed to the lower end of the coil support 22, whereas the upper pair 20, 21 each have a right angle twist intermediate the ends, the outer ends being vertical and the inner ends horizontal. The inner ends of 20, 21 are secured to the upper end of support 22. The two coils are wound as in Fig. 1 and the support 22 is free to vibrate in the two directions, the stylus 29 moving as indicated by arrows 27 and 31, respectively. When the stylus 29 is vibrated in the direction of the arrow 27 the translational movements imparted to the support 22 result in the induction of a corresponding signal. in the second (non-reversing) coil but substantially no response from the first (figure-of-eight) coil. Vibration in the direction of the arrow 31 produces angular movements of the support 22 and give rise to a corresponding signal in the first coil but substantially no response from the second coil. If it is desired to produce a complex-cut record in which the two recording axes are mutually at right angles and each at an angle of 45 degrees to the surface of the record by means of a cutter embodying the transducer of the invention, the sum of the two signals to be recorded is fed to one of the coils and the difference between them to the other coil. To reproduce signals from such a record, the two signals derived from the respective coils are added to provide one of the reproduced signals, and are substracted to provide the other. Means for summing and differencing signals are well known, for example as described in Specification 394,325.