US2539876A - Electromagnetic transducer head and energizing circuit therefor - Google Patents

Description

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----VJ ----VN Patentcd Jan. 30, 1951 ELECTROMAGNETIC TBANSDUCER HEAD AND ENERGIZING CIRCUIT THEBEFOB Bobex-t A. von Behren, Valparaiso, Ind., assignor to The Indiana Steel Producti Company, Chicago, lll., a corporation of Indiana Application September 10, 1948, Serial No. 48,716

1 This invention relates to an electromagnetic transducer head and energizing circuit, and more particularly, to a bridge-type head and its associated energizing circuit.

In one method of magnetic recording, a lengthy magnetizable medium is drawn across an electromagnetic transducer head assembly at substantially uniform linear velocity. The head assembly includes a magnetic core having a non-magnetic gap over which the medium passes and is provided with suitable current-conducting exciting elements to produce a magnetic field across the gap.

During the recording operation, current is caused to flow in the exciting elements in accordance with the time variations of an intelligence to produce a time-varying magnetic eld in the core in accordance with the value thereof. The lengthy magnetizable medium is subjected to the influence of this ileld as it is drawn therethrough, and magnetization is imparted to incremental lengths of the medium in accordance with the time variations of the intelligence, thus causing variations in the magnetization of the medium along its length in accordance with the time variations of the intelligence.

During reproduction, the lengthy magnetizable medium is drawn across the same or similar head assembly to set up a, flux in the core portion thereof in accordance with the magnetization of the medium along successive incremental lengths thereof as it passes across the gap of the magnetic core member. The resultant time-varying flux induces voltage in the coil with which the flux is linked in accordance with the time rate of change thereof. This voltage may be amplifled and suitably reproduced by a loud speaker or similar device to reproduce the intelligence recorded.

This magnetic recording and reproducing inherently involved the conversion of an intelligence into a time-varying magnetic field during the recording operation, and the conversion of a time-varying magnetic flux to anA intelligence in the reproducing operation. Great improvements have been made in the magnetic recording field during recent years, and to some extent, this has been brought about by the use of a superimposed high frequency current on the audio current during the recording process. When the amplitude of this high frequency current is within a predetermined range so as to produce a eld which bears a predetermined relationship with respect to the coercive force of the magnetic medium 7 Claims. (Cl. 179-1002) 2 improved results are obtained. Many problems, however, arise when efforts are made to mix this high frequency of supersonic bias with the audio signal.

It has been suggested in the past that the audio and bias signals be mixed in an amplier-mixer stage, and the composite signal applied to a. single winding on the recording head. Most recording heads, however, have a large inductive reactance at the bias frequency, and therefore require a high bias voltage to develop the necessary signal. For that reason, the stage driving the recording `head must be capable of delivering a bias voltage many times that of the maximum audio signal needed. A separate winding on the head has also been used for supplying the high frequency bias.

One of the novel features and objects of the present invention is to provide a novel electromagnetic transducer head having a novel core structure and a novel arrangement of audio and high frequency windings thereon.

A further object of the present invention is to provide a novel bridge-type recording head.

A further object of the present invention is to provide a novel electromagnetic transducer head and energization circuit therefor in which the oscillator may be left connected in the circuit of the head during playback operation.

The novel features which I believe to .be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization, manner of construction, and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which Figure 1 is a diagrammatic illustration of an electromagnetic transducer head embodying the novel teachings and principles of the present invention, together with its associated electrical circuit, and in which Figure 2 is a schematic representation of the core of the head of Figure l.

In the embodiment of the invention illustrated in the drawing, the electromagnetic transducer head I0 includes a core member II which is generally in the shape of a gure 8. This core II includes a central cross leg I2, a pair of upstanding legs vI3 and I4, having a pair of converging pole tips I5 and I6, respectively, and a pair of downwardly extending legs I'I and I8 which also have a pair of converging pole portions I9 and 20, respectively. It will be observed that the upon which the signal is to be recorded. greatly lower portion of the core structure Il is symmemessage trical 'with the upper portion. For convenience of designation, this core structure might be said to be arranged in bridge form with V`four outer legs I3, i4, i1 and I8 and a fifth center leg i2, as shown schematically in Figure 2. The gaps are thus in two opposite corners of the "bridgef A relatively high impedance voice coil 2| is wound on the central leg 2 and is connected through conductors 22 and 23 to an audio ampli'- fler 24. When the electromagnetic transducer head I is acting as a recording head, fluctuating electric energy is supplied frfoni the audio amplifier 24 through the conductors 22 and 23 to the voice coil 2|. When the electromagnetic transducer head I0 is acting as a playback head, fiux threading the voice coil 2| induces a voltage therein which is transmitted through the conductors 22 and 23 to the audio amplifier 24. It will, of course, be understood that the connection of the voice coil 2| to the audio amplifier 24 forms no part of the present invention. It should, however, be understood that when the head I0 is acting as a recording head, the output of the amplifier is connected to the voice coil 2|, but when the head I0 is acting as a playback head, the voice coil 2| is connected to the input of the audio amplifier 24.

The high frequency component for use during the recording operation is supplied through four coils 25, 26, 21 and 28 which are wound, respectively, on legs I3, I4, I1 and I8. These four coils 25, 26, 21 and 28 are connected in series to an oscillator 29 which is arranged to supply a high frequency bias current for the recording operation.

It is a function of the oscillator 29 to generate a high frequency voltage for simultaneous application to the windings 25 to 28, inclusive, when a signal to be recorded is fed from the amplifier 24 to the signal coil 2| on the head I8. This oscillator 29 may be arranged to operate, for example, at a frequency of approximately 18 kilocycles, the frequency being determined principally by the natural resonant frequency of the inductor 30 and the capacitor 3|. It will be observed that the oscillator 29 is a conventional Hartley circuit wherein positive feed-back is obtained by connecting a tap on the inductor 38 to the cathode 33 of the electron discharge device 32 and the opposite ends of the inductor 38 to the control electrode 34 and the anode 35 of the electron discharge device 32, respectively.

Spaced path voltage for the electron discharge device 32 is derived from a supply line 36 which connects to a suitable source of uni-directional electric energy indicated at B+. A resistor 3l and a switch 38 are connected in this plate bias circuit. An output winding 39 is inductively coupled to the inductor 38, and this output winding 38 of the oscillator 29 is directly connected at all times to the windings 25, 26, 21 and 28 of the head I8.

The confronting poles I5 and I6 of the legs I3 and I4 on the head I0 define a small non-magnetic gap across which the magnetic record member 40 passes at substantially constant velocity. While this gap between the pole portions I5 and I6 may vary to a considerable degree depending upon the wave length of the average signal to be recorded and reproduced, it has been found in practice that this gap should preferably be between .0005 inch and .002 inch.

Referring now to the arrangement of the various coils on the head of the electromagnetic transducer head I0, it will be observed that if the @ons es, ai and rs are identicai in mani ber of turns and are wound as shown, magnetic flux from these four coils is in an additive sense around a path which includes legs I3, I1, I8 and I4. The flux produced by coils 25 and 28 also tends to flow in one direction across, central leg I2, but it will be observed that the flux produced by coils 21 and 28 tends to flow across central leg I2 in the opposite direction. Since the magneto-motive forces which tend to set up these fluxes across the central leg I2 are equal and opposite, no flux is established across the central leg I2 as a result of the combined action of the high frequency bias coils 25, 26, 21 and 28.

There are two parallel iiux paths for the flux set up by coil 2|, one path being through the upper half of the core structure II and -the other path being through the lower half of the core structure II.

Since the voltages induced in coils 25 and 26 are equal and opposite to those induced in coils 21 and 28, nonet voltage will appear across the bias windings 25, 26, 21 and 28. For that reason, it will be apparent that the flux in the loop which includes legs I3 and I4 due to the windings 25, 26, 21 and 28 will not be effected by the external impedance connected to the voice winding 2I.

Furthermore, the flux in this same loop due to the voice winding 2| will not be effected by the external impedance connected to the windings 25l 26, 21 and 28. It Will thus be apparent that the voice coil winding and the bias windings are in effect completely isolated from each other. Flux linking the loop which includes legs I3, I4 and I2 alone lwill generate an E. M. F. in coils 25 and 26. Due to the fact that these coils are connected in series with coils 21 and 28, the same current will flow in all four coils. The M. M. F. due to this current has a demagnetizing effect on the ux in the loop including legs I3, I4 and I2, but reinforces the flux in the loop which includes legs I1, I8 and I2 to restore the original flux through the voice coil 2|.

It Will thus be understood that changes in the impedance of the coils 25, 26, 21 and 28 will have no effect on the magnetic coupling between the loop which includes legs I3, I4 `and. I2 and coil 2|. The same relationship exists between the lower magnetic loop and the coil 2|.

When the electromagnetic transducer head I0 is used as a recording head, the bias oscillator 29 is arranged to supply high frequency bias current to the windings 25, 26, 21 and 28 and the signal to be recorded is supplied from the amplifier 24 to the voice coil 2|. Since there is no interaction between these circuits, each may be adjusted to optimum match Without effecting the other. The coil 2I is highly inductive and may be used as a frequency-sensitive element in the audio circuit for purposes of pre-emphasis. During recording, it has been found advantageous to connect a peaking condenser 4I by closure of a switch 42 in parallel with the coil 2| to gain simultaneous pre-emphasis of the high frequencies and a sharp cut-off of frequencies higher than those desired to be recorded.

When the electromagnetic transducer head I0 is being used as a playback head, the coil 2| is connected to the input of the audio amplifier 24. Since the reluctance of the leg I 2 is very low compared to that of the loop which includes legs I1 and I8, loss of signal due to the magnetic shunting actionof this latter loop is negligible.

One of the important features of the present invention which is made possible by a head of andere the type described above lies in the fact that the bias source may be left connected to the coils 25, 28, 21 and 2t (provided that the source is no longer active) since the flux linkages are not effected by this additional load. Thus, with this arrangement, it is simply necessary to disconnect the plate voltage supply from the Oscillator stage when switching from the recording function to the reproducing function. The resultant gradual decay of'bias oscillation will effectively demagnetize the head, thus yielding ra, substantially lower noise level in the reproduced signal. This feature thus provides a very substantial improved result and enables results to be obtained with a single head for both recording `and reproduction which were hitherto possible, if at all, only with two separate heads.

From the above description, it -will be apparent that the novel electromagnetic transducer head herein described and its associated electrical circuit provides many distinct and novel improvements and advantages. Since the head will work inw and out of high impedance audio sources,

` no audio transformers are necessary in conjunction with the recording head. Because of the isolation between the high frequency bias circuit and the signal circuit, a higher impedance winding may be used without destroying the head emciency, thus reducing the head input requirements and/or delivering higher voltage to the audio amplifier during reproduction. It will also be observed that high frequency pre-emphasis can be obtained with a minimum expense by resorting to a peaking condenser across the head during recording. It will be further observed that since the bias oscillator need not be disconnected from the bias winding during reproduction, switching transients are eliminated, and in addition, an inexpensive means for demagnetizing the recording head is also provided. It will also be observed that while in normal use only one recording surface is available for recording or reproduction, nevertheless the symmetrical arrangement of the head with the nonmagnetic gap at the bottom as well as at the top in effect provides two recording surfaces which are available for either simultaneous recording or simultaneous reproducing. It will also be observed that the head may be turned end-forend when one surface is worn out and only a single recording gap is being used at a time.

From the above description, it will further be observed that the coils may be arranged for circuitsrequiring a low impedance winding on the center leg and a high impedance winding on the outside legs without having each adversely affect the operation of the other. Also, both sets of windings may be of a low impedance type, or both sets may be of a high impedance type. It will be apparent to those skilled in the art that instead of mixing an audio signal and a high frequency signal, two audio signals or pulses may be mixed.

While I have shown a particular embodiment of my invention, it will, of course, be understood that I' do not wish to be limited thereto since many modifications may be made, and I, therefore, contemplate by the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

I claim as my invention:

l. An electromagnetic transducer head comprising a figure 8 shape core with a nonmagnetic gap at the top and a second non-magnetic gap at the bottom thereof, a signal winding on the central cross member of said and a distributed high freue'ncy bias winding disposed around the remaining portion of said core.

2. An electromagnetic transducer head comprising a figure 8 shape core with a non-magnetic gap' at the top and a second non-magnetic gap at the bottom thereof. a high impedance signal winding on the central cross member of said core and a low impedance distributed high frequency bias winding disposed around the remaining portion of said core.

3. An electromagnetic transducer head comprising a ligure 8 shape core with a non-magnetic gap at the top thereof and a second nonmagnetic gap at the bottom thereof, a signal winding on the central cross member of said core, and a distributed high frequency bias winding disposed around the remaining portion of said core, said distributed winding being wound on said core in such a manner as to produce a circular flux path around the outer portion of said core and across said gaps but to be canceled out across said central cross member.

4. An electromagnetic transducer head comprising a figure 8 shape core having a central cross leg, a pair of upwardly extending legs terminating in a pair of spaced confronting pole portions which form a non-magnetic gap therebetween and a pair of downwardly extending legs terminating in a pair of spaced confronting pole portions which form a non-magnetic gap therebetween, a signal winding wound on said central cross leg, and four high frequency bias windings wound on said four remaining legs, respectively.

5. An electromagnetic transducer head comprising a ligure 8 shape core having a central cross leg, a pair of upwardly extending legs terminating in a pair of spaced confronting pole portions which form a non-magnetic gap therebetween and a pair of downwardly extending legs terminating in a pair of spaced confronting pole portions which form a non-magnetic gap therebetween, a signal winding wound on said central cross leg, and four high frequency bias windings wound on said four remaining legs, respectively, said high frequency bias windings having the same number of turns and being wound in a sense to produce a circular high frequency flux path around said upwardly and downwardly extending legs but with the fiux produced across said central leg by said windings on said upper legs canceling out the flux produced in said central leg by the coils around said downwardly extending legs. i

6. A balanced bridge electromagnetic transducer head having a core including four legs arranged in bridge form with non-magnetic gaps in two opposite corners thereof, a fifth leg connecting the other two corners thereof, a high frequency bias winding on each of said first four legs connected to produce flux in an additive relation around said bridge, the total flux produced by the windings on one side of said fifth leg being substantially equal to the total flux produced by the two windings on the other side of said fifth leg, and a signal winding on said fifth leg.

7. A balanced bridge electromagnetic transducer head having a core including four legs arranged in bridge form with non-'magnetic gaps in two opposite corners, thereof, a fifth leg connecting the other two corners thereof, four high frequency bias windings each having the same number of turns'substantially as the other on core u said four legs respectively, said windings being @www wound on their iesgichive lege t0 module@ ux in an additive reiatim around said bridge, ami a signal winding on said mth ieg.

ROBERT A. von IBEHREN.

REFERENCES @MED The fovllowing references me of i'ecord in "whe me @i this patent:

Number Cami-as June 13. 1944 Camras June 13, 1944 C9. June 13, 1944 Camas June 13, 1944 Gamma .m Dec. 21, 1948

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