Patent GB-1141864-A (GB)
Title: Improvements in and relating to the manufacture and use of data records
IPC: G11B5/55
Filed: 1966-04-28
Abstract: 1,141,864. Magnetic tape recorders. ANELEX CORP. 28 April, 1966 [28 April, 1965], No. 18803/66. Heading G5R. In order to centre a pick-up over a track (T2), Fig. 4 (not shown), a first alternating signal (R1) is recorded on one side of the track and a second signal (R2) differing slightly in frequency from the first on the opposite side so that a beat frequency is produced in the output of a pick-up when it is centred over both recordings. The signals may be of 400 and 410 kc./s. respectively to produce a beat frequency, Fig. 6 (not shown). It is found that in practice, after limiting of the beat frequency, a series of irregular spikes rather than a null occurs between the regular pulses and this is employed to centre the head accurately over the desired track. In order to position the head 9, Fig. 1, above a desired track, appropriate binary positioning units B1 . . . B4 are energized to displace by means of a linkage, the arm 10 which carries the arm 35 which in turn is looked to the arm 8 on which the head 9 is mounted. Each unit comprises a motor (23), Fig. 3 (not shown), driving through a ratchet (22) and pawl (25), a disc (21) on a shaft to rotate a further disc (14) carrying an arm 13 to one of two positions, determined by the energization of appropriate solenoids (24, 26). The disc (14) is provided with holes (18) and photo-cells (19) so that the position of the shaft is fed out over lead 61 and when all the binary positioning units are in the correct position, a signal is fed out from the gates 60. The head 9 feeds an amplifier limiter (62), the output of which is applied to a logic circuit, Fig. 5 (not shown), together with outputs from photo-cells 57, 59 arranged behind holes in the sector code disc 55 and the output from gates 60 which occurs when the arm 8 is over the desired track. If the output from the amplifier limiter is a series of irregular pulses, indicating exact positioning of the head above the desired track the logic circuit is arranged to feed an output to operate an indicator light. If the head and its associated arm are not in the correct position, the system is arranged to drive the arm 8 to one extreme position after which its direction of travel is reversed until the position where irregular pulses occurs, is reached. Negative pulses from the amplifier limiter 62 and pulses from the photo-cells 57, 59 co-operate in the logic circuit to cause the motor 53 to run backwards. The motor drives through universal joints a lead screw 46 and co-operating nut 45 to drive a wedge 37, thereby displacing arm 35 and hence arm 8 and the head 9. The drive continues until limit switch S3 closes, whereupon the motor reverses and runs forward. As the centre of the track is approached the output of the limiter amplifier (62) changes from a series of rectangular pulses to irregular spikes and one of the latter is effective to cause the logic circuit to stop motor 53 and light the indicator lamp. In a modification, Fig. 8 (not shown), a chain of timing pulses is derived from the sector zero pulses SZP by means of monostable multivibrators, thereby eliminating the need for the sector pulses.