Method and apparatus for reproducing magnetically recorded signals with a D.C. biasing magnetic field produced by a D.C. biasing coil

A magnetically recorded signal on a magnetic recording medium is reproduced in response to a magnetic field produced thereby by applying a DC biasing magnetic field along the magnetic path of the core of a coil-supporting magnetic head, and adding a high-frequency magnetic field of a small amplitude to the DC biasing magnetic field. The DC biasing magnetic field is produced by passing a DC biasing current through a DC biasing coil on the core, and the high-frequency magnetic field is produced by passing a high-frequency current through another coil on the core.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The present invention relates to a method of and an apparatus for 
reproducing signals recorded on a magnetic recording medium, and more 
particularly to a method of and an apparatus for reproducing magnetically 
recorded signals by employing a ring-shaped head made of a metallic 
magnetic material such as sendust, permalloy, amorphous material, or the 
like as a field-responsive playback head in longitudinal magnetization 
recording, or by employing such a ring-shaped head, a secondary magnetic 
excitation pole of the metallic magnetic material, or a primary magnetic 
excitation pole of the metallic magnetic material as a field-responsive 
playback head in perpendicular magnetization recording. 
2. Description of the Prior Art 
Various efforts have been made to accomplish higher packing densities on 
magnetic recording mediums. In magnetically recording signals with higher 
packing densities, it is preferable to lower the relative speed of 
movement between the recording medium and the magnetic head. Where 
magnetically recorded signals, particularly digital signals, are 
reproduced by a magnetic-induction-type playback head, the level of the 
reproduced signals is directly proportional to the speed of travel of the 
recording medium with respect to the magnetic head. Therefore, so long as 
the magnetic-induction-type playback head is employed, the relative speed 
of movement between the recording medium and the magnetic head cannot be 
reduced beyond a certain speed, resulting in a limitation on packing 
densities. In view of the difficulty of the magnetic-induction-type 
playback head, the tendency in high packing density magnetic recording is 
toward the use of a field-responsive magnetic head capable of generating 
reproduced signals at a level which is in direct proportion to the level 
of the magnetic field induced by the recorded signals irrespectively of 
the relative speed between the recording medium and the head and the track 
width. 
One typical field-responsive head is a magneto-resistive-effect head 
(hereinafter referred to as an "MR head"). Since the MR head has been 
designed solely as a playback head, it has been manufactured independently 
of bulk heads or thin-film heads for magnetic recording. The fabrication 
of the MR head has required a complex process such as photolithography or 
other microscopic pattern formation techniques, and hence has resulted in 
increased cost. Another problem is that the signals reproduced by the MR 
head are subject to variations in voltage due to temperature changes. 
SUMMARY OF THE INVENTION 
In view of the problems with the conventional field-responsive signal 
reproduction, it is an object of the present invention to provide a method 
of and an apparatus for reproducing magnetically recorded signals with a 
field-responsive head of a metallic magnetic material supporting a coil or 
coils, which head can also be used for recording signals. 
According to the present invention, a magnetically recorded signal on a 
magnetic recording medium is reproduced in response to a magnetic field 
produced thereby by applying a DC biasing magnetic field along the 
magnetic path of the core of a coil-supporting magnetic head, and adding a 
high-frequency magnetic field of a small amplitude to the DC biasing 
magnetic field. 
An apparatus for carrying out such a signal reproducing method includes a 
magnetic head composed of a core having a magnetic path, a DC biasing coil 
mounted on the core for applying a DC biasing magnetic field along the 
magnetic path to the magnetic recording medium, a signal detecting coil 
mounted on the core for detecting the magnetically recorded signal on the 
magnetic recording medium, an output resistor having a resistance greater 
than the absolute value of the impedance of the head, a carrier oscillator 
connected through the output resistor to the signal detecting coil for 
enabling the signal detecting coil to add a high-frequency magnetic field 
to the DC biasing coil, and a detector for detecting an output signal 
produced from the signal detecting coil. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description when 
taken in conjunction with the accompanying drawings in which a preferred 
embodiment of the present invention is shown by way of illustrative 
example.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
When a DC biasing magnetic field (H bias) is applied along the direction of 
a magnetic path in the core of a magnetic head, and a small-amplitude 
high-frequency magnetic field (H carr) (hereinafter referred to as a 
"carrier") is added to the DC biasing magnetic field, then the impedance Z 
head of the magnetic head is expressed as follows: 
EQU Z head(.omega.c, H bias)=R head(.omega.c, H bias)+j.omega.c L(.omega.c, H 
bias) (1) 
where 
.omega.c: carrier angular frequency (hereinafter referred to as "carrier 
frequency"); and 
H bias: magnitude of the DC biasing magnetic field. 
The impedance due to coil stray capacitance is omitted since it is 
negligibly small. 
The equation (1) indicates that the head impedance is composed of a 
resistance, expressed by a real number, and an inductance, expressed by an 
imaginary number, and the magnitude of the inductance depends on the 
carrier frequency .omega.c and the magnitude of the DC biasing magnetic 
field H bias. The resistance R head (.omega.c, H bias) in the equation (1) 
is given by the following equation: 
EQU R head(.omega.c, H bias)=.mu."(.omega.c, H 
bias).multidot..omega.c.multidot.N.sup.2 (S/l)+Rdc (2) 
where 
.mu." (.omega.c, H bias): imaginary part of the AC complex magnetic 
permeability of the head core, the imaginary part being greater as the 
eddy-current loss of the core becomes larger; 
N: number of turns of a signal detecting and recording coil 7 (FIG. 1); 
S: cross-sectional area of the head core; 
l: length of the magnetic path of the head core; and 
Rdc: DC resistance of the coil 7. 
The equation (2) shows that the resistance or real part of the head 
impedance is increased as the carrier frequency .omega.c goes higher, and 
when .omega.c=0 [Hz], R head (.omega.c, H bias)=Rdc. 
Therefore, the real part of the head impedance is increased to an extent 
commensurate with the high-frequency loss of the head core. Where the head 
core is made of a metallic magnetic material, the high-frequency loss is 
mostly in the form of an eddy-current loss. 
When the DC biasing magnetic field H bias is increased at a given carrier 
frequency .omega.c in the equation (2), the differentiated magnetic 
permeability of the core .mu.diff=dB/dH is reduced toward zero as is 
evidenced by the gradient of the B - H curve in FIG. 3 which approaches 
zero. Since the eddy-current loss is reduced as .mu.diff decreases, .mu." 
(.omega.c) is also reduced as H bias increases. As a result, the 
resistance or real part R head (.omega.c, H bias) of the head impedance is 
reduced toward the DC coil resistance Rdc as the DC biasing magnetic field 
H bias increases at the given carrier frequency .omega.c, as shown in FIG. 
4. 
FIG. 4 shows curves of the resistance R of the head impedance as plotted 
against the DC biasing magnetic field H bias. Based on the graph of FIG. 
4, the following field-responsive signal reproducing method may be 
constructed: 
First, the DC biasing magnetic field H bias is selected to be positioned on 
relatively steep and straight portions of the curves of FIG. 4, and the 
selected DC biasing magnetic field H bias is referred to as an optimum 
biasing field Hbo. At the optimum biasing field Hbo, the flux density of 
the core is B (H bias=Hbo)=Bbo. This flux density Bbo has to be of such a 
magnitude as to produce a magnetic field smaller than the minimum magnetic 
field capable of recording signals on the magnetic recording medium being 
used. This is because if the flux density Bbo should be greater than such 
a magnitude, the DC magnetic field would erase the signals recorded on the 
magnetic recording medium by way of DC erase. In actual heads of a 
metallic magnetic material, it is possible to select the optimum biasing 
field Hbo to be sufficiently small so as not to erase the recorded signals 
on the magnetic recording medium. 
Where the DC biasing magnetic field H bias is selected to be the optimum DC 
biasing magnetic field Hbo in FIG. 4, and the carrier frequency .omega.c 
is fixed so as to be sufficiently higher than the frequencies of the 
signals recorded on the magnetic recording medium, the real part or 
resistance of the head impedance is R head (H bias=Hbo)=Rbo. When the 
magnetic field generated by the signals on the magnetic recording medium 
is added to the optimum DC biasing magnetic field, the head resistance R 
head varies above and below the level Rbo dependent on the signal magnetic 
field (ideally proportional thereto). By keeping constant the current of 
the carrier having the frequency .omega.c and flowing through the head 
coil 7 (FIG. 1) irrespectively of the absolute value of the head 
impedance, the variations of the head resistance R head due to the 
magnetic field of the signals can be detected as variations in the 
amplitude of a carrier voltage appearing across the head coil 7. Stated 
otherwise, the carrier of the frequency .omega.c is amplitude-modulated by 
the recorded signals on the magnetic recording medium, and the recording 
signals on the magnetic recording medium can be reproduced by detecting 
the amplitude-modulated carrier. The output voltage of the reproduced 
signals is consequently proportional to the magnitude of magnetization of 
the signals recorded on the magnetic recording medium, without depending 
on the speed of movement between the head and the magnetic recording 
medium and the track width on the magnetic recording medium. As a result, 
the recorded signals can be reproduced in a field-responsive manner. 
Actually, as is apparent from the equation (1), the imaginary part or 
inductance of the head impedance is also varied by the magnetization by 
the recorded signals on the magnetic recording medium, and the carrier is 
amplitude-modulated by the inductance variation. If such an 
inductance-induced amplitude modulation should be detected and mixed in 
the proper reproduced signal detected only by the resistance or real part 
of the head impedance, then noise would be produced. Therefore, any 
amplitude modulation caused by the inductance variations should be 
removed. The removal of such inductance-induced amplitude modulation can 
be achieved by a phase-sensitive detector (PSD), for example. More 
specifically, the carrier amplitude-modulated by the resistance of the 
head impedance has the same carrier frequency as that of the carrier 
amplitude-modulated by the inductance of the head impedance, but leads the 
carrier amplitude-modulated by the inductance of the head impedance by a 
phase angle of 90.degree.. Therefore, by employing a PSD reference signal 
which is equal in frequency and phase to the carrier amplitude-modulated 
by the resistance or real part of the head impedance, only the signal 
amplitude-modulated by the resistance of the head impedance can be 
detected and produced as the output signal of the phase-sensitive 
detector. 
The high-frequency carrier current flows through the coil 7 (FIG. 1) at all 
times during signal reproduction. The magnetization by the recorded 
signals on the magnetic recording medium should not be erased by the 
high-frequency magnetic field generated from the head by the 
high-frequency carrier current. In reality, the magnetization on the 
magnetic recording medium is not erased for the following two reasons: 
(1) As shown in FIG. 3, the amplitude .vertline.H carr.vertline. of the 
high-frequency magnetic field generated by the carrier current flowing 
through the coil 7 is selected to be sufficiently smaller than the optimum 
DC biasing magnetic field Hbo which is established by a coil 3 (FIG. 1). 
As the DC biasing magnetic field Hbo is so weak as not to erase the signal 
magnetization on the magnetic recording medium, the amplitude of the 
high-frequency magnetic field produced by the carrier current is much 
smaller than the minimum magnetic field necessary to erase the signal 
magnetization on the magnetic recording medium (.vertline.H carr 
.vertline.&lt;&lt;Hbo). Therefore, there is no danger of signal erasure. 
(2) The carrier frequency is selected to be sufficiently higher than the 
frequencies of the signals recorded on the magnetic recording medium (for 
example, .omega.c/2 in the range of from 20 MHz to 50 MHz). The high 
carrier frequency causes the head of metallic magnetic material to suffer 
an eddy-current loss which lowers the AC magnetic permeability .mu.' of 
the head core (or lowers the head efficiency). Therefore, the 
high-frequency magnetic field produced from the head by the high-frequency 
carrier current flowing through the coil 7 is sufficiently small in 
intensity and does not erase the signals on the magnetic recording medium. 
During signal reproduction, the signals recorded on the magnetic recording 
medium are also reproduced in the ordinary magnetic induction process to 
produce a signal voltage across the coil 7. Although the signal voltage 
generated by the magnetic induction process is small on account of the low 
speed of relative movement between the head and the magnetic recording 
medium in the field-responsive signal reproducing system, any such voltage 
signal is responsible for creating noise with respect to the normal 
reproduced signal on the carrier. 
The noise induced by the ordinary magnetic induction process is eliminated 
as follows: While the recorded signals are reproduced, the signal voltage 
across the coil 7 is applied to the phase-sensitive detector. The 
phase-sensitive detector produces only a detected output signal having the 
same frequency and phase as the frequency .omega.c and the phase of the 
carrier used as the reference signal of the phase-sensitive detector. 
Because the frequencies of the signals reproduced by the magnetic 
induction process are equal to the frequencies of the recorded signals on 
the magnetic recording medium and sufficiently lower than the carrier 
frequency, i.e., the reference signal frequency, the phase-sensitive 
detector does not produce as its output signal any information of the 
signals reproduced by the magnetic induction process. 
The carrier frequency .omega.c should be as high as possible for the 
following two reasons: 
(1) The sensitivity of the field-responsive signal reproducing system 
according to the present invention is greater as the carrier frequency 
.omega.c is higher, as can be understood from the equation (2) and FIG. 4. 
(2) The higher the carrier frequency .omega.c compared with the frequencies 
of the signals recorded on the magnetic recording medium, the better the 
accuracy of the detection employing the phase-sensitive detector. 
However, a suitable carrier frequency should be determined since the higher 
the carrier frequency .omega.c, the greater difficulty the electronic 
circuit (described later) of FIG. 1 experiences in achieving its normal 
operation. 
The sensitivity and dynamic range in the signal reproducing method of the 
present invention will now be described especially for their relationship 
to the requirements to be met by a head employed in the method of the 
present invention. 
Sensitivity: 
The relationship between the head characteristics and sensitivity is 
expressed by the equation (2). The head characteristics are roughly 
divided into geometric characteristics such as the cross-sectional area of 
the head core, the length of the magnetic path, and the number of coil 
turns, and magnetic and electric properties of the head core. The 
sensitivity is proportional to the square of the number of coil turns, 
proportional to the cross-sectional area of the head core (and hence the 
thickness thereof), and inversely proportional to the length of the 
magnetic path. The sensitivity is also proportional to the imaginary part 
.mu." of the AC complex magnetic permeability of the head core. If the 
magnetic core is made of a metallic magnetic material, then the imaginary 
part .mu." almost entirely depends on the eddy-current loss, and increases 
as the eddy-current loss increases. As is well known, the eddy-current 
loss is proportional to the DC magnetic permeability .mu.dc of the 
magnetic material of the head core, and inversely proportional to its 
resistivity .rho.. Because of the skin effect, the thinner the magnetic 
head core, the greater the eddy-current loss thereof. As the carrier 
frequency .omega.c in the method of the invention becomes higher, the 
eddy-current loss increases substantially in proportion thereto. The 
eddy-current loss coefficient e.sub.1 of the head core should range from 
0.1.times.10.sup.-6 to 0.3.times.10.sup.-6 [Hz.sup.-1 ]. 
Dynamic range: 
As illustrated in FIGS. 3 and 4, the dynamic range is wider as the optimum 
DC biasing magnetic field Hbo is larger, provided that the flux density 
Bbo of the head core at the time the optimum DC biasing magnetic field is 
applied to the head core is sufficiently small so as not to erase the 
recorded signals on the magnetic recording medium by way of DC erase, a 
feature which depends on the static magnetic characteristics of the head 
core (FIG. 3). 
As described above, the various characteristics (such as the number of coil 
turns, the thickness of the head core, the magnetic and electric 
properties of the core head, for example) of the magnetic head used in the 
method of the present invention should appropriately be selected in order 
to provide a practical sensitivity and dynamic range. 
If the head of the present invention is used for reproducing signals in a 
field-responsive mode and also for recording signals, as shown in FIG. 1 
(described later on), then the head should have a good recording 
sensitivity throughout the frequency range of signals to be recorded. The 
recording sensitivity becomes lower in a higher-frequency range as the 
eddy-current loss is larger, which is incompatible with the fact that the 
reproducing sensitivity according to the method of the invention is higher 
as the eddy-current loss is larger. As a result, the thickness of the head 
core and the magnetic and electric properties of the head core should be 
selected so that both recording and reproducing sensitivities of the head 
will be of practical values. 
The preferable AC magnetic permeability .mu.' of the head used in the 
method of the invention should be as high as possible in a low-frequency 
range and should become lowered as sharply as possible as the frequency 
goes higher. 
The head which meets the foregoing requirements and has a practical 
sensitivity and dynamic range in both reproducing and recording modes may 
be a recently available head of metallic magnetic material such as a bulk 
video head of sendust, amorphous material, or the like. The method of the 
present invention is not suitable for use with a head which has had its 
eddy-current loss highly reduced by reducing the core thickness, such as a 
thin-film magnetic head, a laminated head, or the like. 
An electronic circuit arrangement for carrying out the recording and 
reproducing process of the present invention will be described with 
reference to FIGS. 1 and 2A through 2D. 
For recording signals, mode changeover switches 6, 8 which are ganged to 
each other are operated to select the recording mode. Then, no current 
flows through a DC biasing coil 3 of a ring-shaped and coil-supporting 
magnetic head 2 having a core of a metallic magnetic material disposed 
over a magnetic recording medium 1, and a coil 7 of the magnetic head 2 is 
connected to the output terminal of a recording amplifier 16. The circuit 
of FIG. 1 now operates as an ordinary recording system. 
When reproducing recorded signals on the magnetic recording medium 1, the 
mode changeover switches 6, 8 are shifted to a playback mode to connect 
the DC biasing coil 3 to a DC biasing power supply 5. The optimum DC 
biasing magnetic field Hbo is selected by adjusting a rheostat 4. The coil 
7 is connected to the output terminal of a signal-detecting carrier 
oscillator 9 through a resistor 10 the resistance of which is sufficiently 
larger than the absolute value of the impedance of the magnetic head 2. 
The carrier current flowing through the coil 7 is now rendered independent 
of variations in the head impedance. Therefore, the amplitude .vertline.H 
carr.vertline. of the carrier field added to the optimum DC biasing 
magnetic field is also independent of variations in the head impedance, 
with the result that the signal detecting accuracy is not lowered. The 
oscillator 9 is adjusted to select an optimum amplitude and frequency of 
the carrier field. 
A carrier signal voltage which is amplitude-modulated by the recorded 
signal (FIG. 2A) on the magnetic recording medium 2 is applied from the 
coil 7 to a band amplifier 11 which amplifies the voltage as shown in FIG. 
2B. The amplified voltage is then applied to a phase-sensitive detector 
(PSD) 13. 
The frequency band of the band amplifier 11 is required to have a central 
frequency equal to the carrier frequency .omega.c and a band width 
covering the frequency band of the signals recorded on the magnetic 
recording medium 1. A reference signal of the phase-sensitive detector 13 
is produced by applying the carrier voltage from the oscillator 9 via a 
phase shifter 12 to the reference signal terminal of the phase-sensitive 
detector 13. The reference signal is shifted in phase by the phase shifter 
12 to an extent corresponding to a phase shift caused by the band 
amplifier 11. The band amplifier 11 and the phase shifter 12 may be 
dispensed with if the amplitude of the amplitude-modulated carrier 
appearing across the terminals of the coil 7 is large enough to permit 
detection by the phase-sensitive detector. 
The output terminal of the phase-sensitive detector 13 produces a 
reproduced signal voltage (FIG. 2C) detected by the phase-sensitive 
detector 13. The DC component is removed from the reproduced signal 
voltage by a DC blocking capacitor 14, and the signal passed through the 
DC blocking capacitor 14 is amplified by a playback amplifier 15 from 
which a reproduced signal (FIG. 2D) is output. 
The signal magnetization (FIG. 2A) recorded on the magnetic recording 
medium 1 is therefore reproduced as the reproduced signal as illustrated 
in FIG. 2d. 
According to the present invention, the field-responsive reproduction of 
magnetically recorded signals can be accomplished by using an ordinary 
bulk head made of a metallic magnetic material. Therefore, it is not 
necessary to use, as a field-responsive playback head, a 
magnetoresistive-effect head or the like which requires a complex 
fabrication process and operates less stably. As shown in FIG. 1, the 
field-responsive playback bulk head of metallic magnetic material can also 
be used for recording signals on the magnetic recording medium. 
Consequently, signals can be recorded and reproduced by the signal head, 
and the overall cost of the circuit arrangement can be lowered. 
The method of the present invention can be utilized for recording and 
reproducing digital or analog signals, although it is of greater advantage 
when used for recording and reproducing digital signals. 
The recording and reproducing system shown in FIG. 1 is illustrated by way 
of example only. Two heads designed respectively for recording and 
reproducing signals may be employed. In such a case, the playback head can 
be designed for better signal reproduction without being subject to 
various limitations imposed by the requirements for the recording head. 
The coil 3 for applying the DC biasing magnetic field may be dispensed 
with, and a DC biasing current may electrically be added to a carrier 
current for being passed through the coil 7. 
While a ring-shaped bulk head having a metallic magnetic core has been 
shown by way of illustrative example, other heads can be employed in the 
method of the present invention. Such other heads include, for example, a 
thin-film magnetic head having a metallic magnetic film of an increased 
thickness and a conductive coil of a reduced DC resistance, a 
perpendicular magnetic recording head having a primary magnetic excitation 
pole in the form of a metallic magnetic film having an increased thickness 
remotely from its surface for contact with the magnetic recording medium, 
and a perpendicular magnetic recording head having a secondary magnetic 
excitation pole in the form of a metallic magnetic bulk. The metallic 
magnetic material may be sendust, permalloy, amorphous material, or the 
like. 
Although a certain preferred embodiment has been shown and described, it 
should be understood that many changes and modifications may be made 
therein without departing from the scope of the appended claims.