US2416279A - Magnetic signal reproducing means - Google Patents

Description

Feb. 25, 1947. 5, BEGU'N 2,416,279

MAGNETIC SIGNAL REPRODUCING MEANS Filed Jan. 22, 1943 AMPLIFIER FREQUENCY 'FlliB FIBB AMPLIFIIR HIGH PAH FILTER INVENTOR. .ffN/ JOJTPH BfGU/Y Patented Feb. 25, 1947 MAGNETIC SIGNAL REPRODUCING MEANS Semi Joseph Begun, Cleveland Heights, Ohio, as-

signor to The Brush Development Company, Cleveland, Ohio. a corporation of Ohio Application January 22, 1943, Serial No. 473,165

17 Claims. 1

My invention pertains to magnetic transducers and more particularly to magnetic reproducing heads adapted to operate with a moving magnetized material and to electrical circuits connected to the reproducing heads.

An object of my invention is to extend the range of the frequency response of magnetic systems.

Another object of my invention is to provide magnetic signal reproducing means having an improved low frequency response.

A further object of my invention is to provide a circuit for use with a combination or double reproducing head.

It is also an object of my invention to provide a circuit for use with a combination reproducing head which provides a relatively flat overall signal response.

Another object of my invention is to provide a combination reproducing head wherein the phase relation and the signal level of the signal response of the individual heads are designed to prevent dips and peaks in the overall signal response from the combination head.

Other objects and a fuller understanding of my invention may be had by referring to the following description taken in conjunction with the drawings in which:

Figure 1 shows a combination reproducing head.

Figure 2 is a plan .view of one of thetwo heads of the combination head.

Figure 3 is a sectional view taken along line 3-3 of Figure 2, and looking in the direction of the arrows.

Figure 4 is a graphical representation of the frequency response of each of the two heads which form the combination head.

Figure 5 is a graphical representation of the overall response of the combination head.

Figure 6 shows my improved combination reproducing head.

Figure 7 is a graphical representation of the response of my improved combination reproducing head.

Figure 8 shows another form of my invention.

It has been found in magnetically recording and reproducing signals with a magnetic head (i. e. a pole piece surrounded by a coil)- that a frequency range of about 1 to 100 can be easily handled. That is, if the lowest frequency is about 100 cycles per second the highest frequency would be about 10,000 cycles per second. This ratio holds to a more or less marked degree over a fairly wide range of speeds of the magnetizable material. A magnetic reproducing head "reproduces" a recorded signal by establishing a voltage in accordance with the recorded magnetic signal. The voltage established is a function of the rate of change of flux through the magnetic head. For a given tape speed the flux through the reproducing head is a function of the frequency which was recorded on the tape. Below a certain frequency the rate of change of flux in the magnetic head drops sharply, thereby resulting in a drop in the generated voltage. This is the low frequency limitation of a magnetic reproducing head. The high frequency limitation arises when the length of the wave being reproduced approaches the finite thickness of the pole piece, thus generating voltages which tend to cancel each other and drop the output of the head.

It is desirable in very many cases to extend the frequency range in reproduction beyond the above mentioned ratio of 1 to 100. This I do by providing two reproducing heads in association with the moving magnetized material, and by specially designing one of the heads to permit it to reproduce frequencies which are lower than the lowest which the other head can reproduce. Each of the two heads reproduces the signal which is on the magnetized material, and the outputs from thetwo heads are mixed to establish a single combination or overall output having a frequency range which is extended at the lower end compared to a standard reproducing head.

Figure 1 shows the two reproducing heads l0 and II electrically connected in series and positioned close together and in association with a magnetized tape or wire l2. The outputs from the two heads are connected to an amplifier [3. The reproducing head I!) is adapted primarily for reproducing low frequency signals, such for example as those covered by the range indicated by the response curve ll of Figure 4, and the reproducing head II is adapted primarily'for re-v producing higher frequency signals, such for example as those covered by the range indicated by the response curve I5 of Figure 4.

As shown in Figure 1 the reproducing head It is a head comprised of two pole pieces l8, l1, one of which is surrounded by a coil l8. The coil it which surrounds the pole piece 16 is connected in series with the reproducing head l0 and the amplifier IS.

The low frequency reproducing head is shown in detail in Figures 2 andv 3, and'comprises a. spool 25 having a shank portion 24, and a flange 26 at one end and a flange 21 at the other end.

of turns of light 'wire 30 is wound on the spool.

The shaft 24 of the spool and the plug 29 prevent the moving tape I2 from cutting into the wire 30. For best results the wire 30 should be Just as close to the magnetizable tape l2 as possible. For this reason the slot 28 through the spool should be just slightly wider than the tape l2, and the thickness of the spool shank 24 should be as small as possible. Because the spool extends for some distance in the direction of movement of the tape l2 it is relatively non-responsive 1 f to signals of short wave length, and is relatively more responsive to signals of longer wave length than reproducing heads which utilize pole pieces. Its excellence in picking up low frequency signals arises from the fact that it is not focused on a small portion of the tape like a pole piece is, and the reason it is relatively non-responsive to the higher frequency signals is that when the wave length of the signal to be reproduced approaches the length of the head l there will be cancelling voltages induced in the head, and consequently the output will be small. Thus, by combining two reproducing heads, one of which permalloy pole piece reproducing head ii, there is a rounded peak at about 1500 cycles per second and a smooth fall of! to around 4000 cycles per second. At the lower end ther is a sharp drop in response from around 100 cycles per second to 30 cycles per second. Theresponse curve .of the low frequency head l0 compared to the response curve of the head H would show that at about 30 cycles per second the response from the head l0 would be about 15 d. b. higher than the response from the head II, and that the curve falls rather sharply to 200 cycles per second at which point the response is quite small. At around 80 cycles per second the responses from the twoheads are about the same.

The overall response curve 3'! taken, using the combined output from both heads "I and II,

' which were placed .46 of an inch apart along the is particularly responsive to low frequencies and the other of which is particularly responsive to higher frequencies, a composite or combination reproducing head is formed which has a frewill, however, always be a certain finite distance between the two heads, and it will occur for a certain speed of the magnetized material and for 3 a certain recorded frequency, that the voltage generated in one pickup device is 180 degrees out of 3 phase with the voltage generated in the other 3 pickup device. In such a case the voltages buck each other, and if they are of the same magnil tude they will cancel out leaving no resultant 1 output. For other frequencies, if the speed of a the magnetized material is assumed to be constant, the voltages will be in phase and will add 1 to each other giving an increase in the resultant .1 output. For frequencies between those which i establish bucking voltages and those which establish'aiding voltages there will be a wide variation in the relative phase shift and therefore the resultant voltages will change depending upon the comparative amplitude of the two reproduced signals. This cancelling and adding of voltages 3 is shown in the graphical representation in Figure 5, where the curve I4 represents the response from the head I0 and the curve I5 represents the 3 response from the head H. 1 for a certain frequency range the responses over- 5 lap. It is in this range that the voltages will 8 add and subtract to produce an undesirable overj all response curve such as is shown by curve 31.

It will be seen that An example of the responses may be as follows:

tape i2, shows a sharp minimum at about 94 cycles per second due to the spacial separation and interference between the two reproducing heads. There is also aslight reinforcement at 4'7 cycles per second and another at 141 cycles per second. There is also a cancellation at 188 cycles. Further reinforcements and cancellations do not show because the level of one or the other of the heads is down too low.

Several things may be done to prevent or lessen the dips and peaks in the overall response curve. One way of greatly improving the overall response is to cause the voltage generated by the'head ID to fall down for frequencies which overlap with the frequency range of the head i l, and to cause the voltage generated by the head II to fall down for frequencies which overlap with the frequency range of the head I0. Thiswould mean that the upper end of the response curve of the head [0 would drop sharply, and that the lower end of the response curve of the head ll would drop sharply. Such a drop in the frequency responses may be accelerated by suitable networks, such for example as, resistance-condenser networks.

Figure 6 shows the use of these net-works. The output from the head [0 passes through the low pass filter 43, then to the amplifier 44 where the level of the signal can be adjusted, and then to the mixer 45. The output from the head I l passes through the highpass filter 46, then to the amplifier 41 where the level of the signal canbe adjusted, and then to the mixer 45.

The output from the mixer 45 is across the terminals 48, which may be connected to a loud speaker, transient analyzer, oscilloscope,- or any other such device for utilizing the signal. By using a low pass filter 43, the response curve of the head l0 will appear somewhat likecurve 52 of Figure 7, and by using a high pass filter 46 the response curve of the head II will appear somewhat like curve 53. These two response curves fall sharply where they overlap. An overall response curve will appear somewhat like line 54, which has only several very small peaks and dips, and those extend over a very small frequency range. The reasons why filters improve the overall response is that they narrow the range of interference and they also may shift the phase of the signals. The position of the filters and the amplifiers is immaterial as the amplifiers may just as well be placed before the filters as after. It is possible to develop the net-works to shift the phase in a direction which will in effect reduce the spacial distance between the two heads.

Figure 8 illustrates a circuit for shifting the phase of the signal from head III to in effect reduce the spacial distance between head HI and head II. The circuit includes a network consisting of resistor 55 and condenser 56, and the resistor and condenser are so proportioned that the resistance of 55 is large compared to the reactance of condenser 56 throughout the frequency range in which the responses of the two heads overlap. With the elements so proportioned the signal delivered -to amplifier M is delayed an amount of time corresponding to one-quarter of a wave-length, thus, in eifect reducing the spacial distance between the effective center of coil l stood that the present disclosure has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

I claim as my invention: v

1. A magnetic reproducing device for use with a magnetizable medium comprising, in combina-. tion, a first magnetic reproducing head associated with said magnetizable medium for reproducing the long wave-length signals therefrom, a second magnetic reproducing head positioned closely adjacent to said first magnetic reproducing head and associated with said magnetizable medium for reproducing shorter wavelength signals, the length of the spacing between the said first and second reproducing heads corresponding to the length of one of the ,shorter waves which is reproduced by said second head, amplifier means, and circuit means connecting said first and second reproducing heads to said amplifier means and including signal time delay means in the output circuit of the reproducing head which first reproduces a given recorded magnetic signal.

2. A magnetic reproducing device for use with a magnetizable medium comprising, in combination, a first magnetic reproducing head associated with said magnetizable medium for reproducing the long wave-length signals therefrom, a second magnetizable reproducing head associated with said magnetic medium for reproducing the short wave-length signals therefrom, amplifier means, circuit means connecting the output from said first reproducing head and the output from said second reproducing head to said amplifier means, said circuit means including low pass filter means in the output circuit of said first reproducing head and high pass filter means in the output circuit of said second reproducing head.

3. A device as set forth in claim 2 in which the upper limit of said low pass filter and the lower limit of said high pass filter are set at substantially the same wave-length in the range covered by said reproducing heads.

4. A magnetic reproducing device for use with a magnetizable medium comprising, in combination, a first magnetic reproducing head associated with said magnetizable medium for reproducing the long wave-length signals therefrom, a second magnetizable reproducing head associ- 6 ated with said magnetic medium for reproducing the short wave-length signals therefrom, signal mixer means connected to the output signal circuit from each of said reproducing heads, and phase shifting'means positioned in the said output signal circuit from one of said reproducing means to said mixer means. I

5. A device as set forth in claim 4 in which said phase shifting means is in the output circuit between the head which reproduces the low wave-length signals and the mixer means.

6. A device as set forth in claim 4 in which said phase shifting means is a signal integrating circuit.

'7. A magnetic reproducing device for use with a relatively moving magnetizable medium comprising, in combination, reproducing head means including a first magnetic reproducing'head associated with said magnetizable medium for reproducing the long wave-length signals therefrom and surrounding a portion of the said magnetizable medium and a second magnetic reproducing head associated with said magnetizable medium and spaced from said first head for reproducing shorter wave-length, signals, the length of the spacing between the said first and second reproducing heads corresponding to the length of one of the said shorter length waves, amplifier means, and circuit means connecting said first and second reproducing heads to said amplifier means, said circuit means including means for delaying the signal reproduced by the one of said reproducing heads which first reproduces a given signal with respect to the signal reproduced by the other reproducing head.

8. A device as setforth in claim '7, further characterized in that said circuit connected to said reproducing head means includes signal time delay means which integrate the signal.

9. A device as set forth in claim 1, further characterized in that said signal time delay ,means which is in the output circuit of the reproducing head which first reproduces a given recorded magnetic signal is an integrating cirtion, a first magnetic reproducing head associated with said magnetizable medium for reproducing the long wave-length signals therefrom, a second magnetic reproducing head associated with said magnetizable medium for reproducing the short wave-length signals therefrom, signal mixer means connected to the output signal circuit from each of said reproducing heads, and a phase shifting integrating circuit in the output circuit between the head which reproduces the long wave-length signals and the mixer means.

11. A magnetic reproducing device for use with a magnetizable medium comprising, in combination, a first magnetic reproducing head associated with said magnetizable medium for reproducing the long wave-length signals from a given portion thereof, a second magnetic reproducing head associated with said magnetizable mediumfor reproducing the short wave-length signals from said given portion after said first magnetic reproducing head has reproduced the long wavelength signals, signal mixer means connected to the output signal circuit from each of said reproducing heads, and signal time delay means positioned in the output signal circuit from the said first magnetic reproducing head for delaying its reproduced signal with respect to the v said reproducing head.

signal reproduced by the said second magnetic reproducing head.

12. A magnetic reproducing device for use with a magnetizable medium comprising, in combination, a first magnetic reproducing head associated with said magnetizable medium for. repro ducing the long wave-length signals therefrom,

a second magnetic reproducing head independent from said first reproducing headand associated with said magnetizable medium for reproducing the short wave-length signals therefrom, signal-mixer means for combining at least two signals, circuit means connecting each of said reproducing heads' to said signal .mixer means, and means in said circuit for modifying the signal reproduced by at least one of said reproducing heads whereby the combined signal from said mixer means is altered to improve its overall characteristics.

13. A ma netic reproducing device for use with a magnetizable medium moving at a given rate of speed comprising, in combination, first magnetic reproducing head means including an output circuit in flux linkage relationship with said magnetizablemedium for reproducing the long said first reproducing head means has a low pass filter in its output circuit and said second reproducing head means has a high pass filter in its output circuit, the high frequency cut-off frequency of said low pass filter and the low frequency cut-off frequency of said high pass filter being substantially the same, and means for combining the output filters.

17. In a signal reproducing circuit for a magnetic reproducing device having a magnewave-length signals therefrom, second magnetic which first reproduces a given recorded signal,

the output of said phase shifting means being connected to the output circuit of said other of said magnetic reproducing head means.

14. A magnetic reproducing device as set forth in claim 13 further characterized in this: that at least one of said reproducing head means has in its output circuit filter means for sharply cutting the output at one end of the frequency range of 15. A magnetic reproducing device as set forth in claim 13 further characterized in this: that said first reproducing head means has a low pass filter in its output circuit and said second reproducing head means has a high pass filter in its output circuit, the high frequency cut-off frequency of said low pass filter and the low frequency cut-off frequency of said high pass filter being substantially the same.

tizable medium moving at a given rate of'speed, first magnetic reproducing, head means in flux linkage relationship with said magnetizable-n'u'adium for reproducing. signals therefrom which are within a' given frequency range andincludingan output circuit having two wires, second magnetic reproducing head means in flux linkage relationship with said magnetizable medium' for reproducing signals therefrom which are within a second given frequency range and including an output circuit having two wires, a portion only of said second frequency. range overlapping a portion only of said first given frequency range whereby throughout the overlapping frequency range both said head means reproduce signals, said first and second reproducing head means being spaced apart from each other along said magnetizable material in the direction of motion of said material, and phase shifting circuit means in the output circuit of the one of said two reproducing head means which first reproduces a given signal recorded on said magnetizable medium, said phase shifting circuit means comprising a resistor connected to one of the wires from the magnetic reproducing head, and a condenser connected across said two wires from the magnetic reproducing head, the resistance of said resistor being large compared to the reactance of said condenser throughout the frequency range in-which both of said heads reproduce signals.

SEMI JOSEPH BEGUN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,251,300 Star Aug. 5, 1941 2,317,199 Kirschbaum Apr. 20, 1943 I FOREIGN PATENTS Number Country Date 656,831 German Sept. 13, 1932 8 in claim. 13 further characterized in this: that signal from said two

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