Patent GB-964973-A (GB)
Title: Improvements in or relating to magnetic transducers
IPC: G11B5/49
Filed: 1960-08-15
Abstract: 964, 973. Magnetic recording processes. ARMOUR RESEARCH FOUNDATION OF ILLINOIS INSTITUTE OF TECHNOLOGY. Aug. 15, 1960 [Aug. 20, 1959], No. 28217/60. Heading G5R. [Also in Division H3] The cores of a stack or series of cores in a transducer head are switched in sequence from saturation of one polarity to saturation of the opposite polarity. As each core switches, it is only momentarily non-saturated and in this short period of time it may read or write information in the adjacent section of a magnetic record, for example, a magnetic tape record of audio and video television signals. A series of cores is switched to scan a tape from side to side. Signal windings on the cores may be replaced by Hall effect elements. Scanning. Cores A to H, Fig. 1, which may be transducer elements with gaps,are wound with two series of graded windings, one series 21 to 28 being regarded as sweep windings on the other series 32 to 38 as bias windings. The two series are fed with sinusoidal currents i1, i2 of the same amplitude but 90 degrees out of phase and the windings are so graded that the magnetising force vectors of the cores have the same moduli but are spaced from the vectors of adjacent cores by equal phase angles as shown in Fig. 5A. Each core is switched as its magnetising force vector passes through a vertical position in Fig. 5A. The cores are switched on the same order twice in each cycle and there is not time required for flyback. As shown in Figs. 2, 3 the output pulses from windings 41 to 48 change polarity in successive halfcycles. The cores may be wound so that the output pulses from successive cores alternate in polarity throughout a whole cycle. In an alternative scanning arrangement,transducer cores 411 to 415, Fig. 7, associated with tracks on a record medium 407 have graded bias windings proportional to 0, 1, 2, 3 and 4 ampere-turns. When fed with current of the wave-form 500 shown in Fig. 9,the ampere-turns in the respective cores are as shown by the time axis and curves 512 to 515. A sweep oscillation of the form shown in Fig. 8, applied to windings with the same number of turns on each core switches the core at equally spaced times marked by dots on the "ramp" portions of the wave but there is a "return time" involved between points 535, 541 during which no magnetic change takes place. The saturable portion of a core may be arranged in shunt with the gap instead of in series therewith. Permanent magnets may be used to provide the graded bias (see later). The sweep wave form is then as shown in Fig. 24. Construction of head with bias windings. A single transducer core,say 411 of Fig. 7, has the form shown in Fig. 12. A major loop with a magnetic gap 430 comprises two complete minor loops with limbs 420,421 and 422,423. The graded bias windings 471 comprise separate windings 459, 452, 454, 456 each limb of the one element having the same number of turns and having a magnetic effect of the polarity shown. Similarly, the sweep windings 481 comprise separate windings 451,453,455,457. A signal winding 460, is connected in series with compensating windings 748, 749. The transducer core of Fig. 12, may be controlled either by sinusoidal sweep and bias currents or by the current wave form described with reference to Figs. 7 to 10. In an alternative construction of transducer core, Figs. 13, 14 a pair of symmetrical core sections 770,771 defines two gaps 772,774 and four limbs 781 to 784. Cores are stacked, Fig. 15, with intervening copper spacers 800 having upper and lower portions 801, 802 for supporting the pole portions 791 to 794. In the stack shown, groups of cores are of different sizes so as to accommodate the windings. Although few cores are shown, a number such as 360 or 525 cores may be used to define a line of video information across the width of a tape. Corresponding limbs of the sixteen cores of Fig. 15, are wound as shown in Fig. 16, to magnetize the cores in accordance with the vector diagram, Fig. 17. Unsymmetrical cores, Fig. 25, may be stacked in groups displaced as shown in Fig. 26, with the gap aligned to fit a straight gap spacer 3345. The windings are described in detail. All the half-sections of the arm on one side of the spacer 3345 may be replaced by a common unwound polepiece. Construction of head with permanent magnet bias. A core comprises polepieces 3021, 3022 Fig. 18, defining a gap, a base 3025 and portions 3026, 3027 of small cross-section which are normally saturated by permanent magnets 3037, 3038. A signal winding 3035 is linked with the base 3025. The core is switched by sweep currents in coils 3040 to 3043. The cores are stacked with copper spacers 3049, Fig. 21, to form the assembly shown in Figs. 22, 23. Magnets 3037,3038,3060,3061 are disposed so as to vary the magnetic bias along the stack. The sweep generator 3044 provides a wave form shown in Fig. 24.