Optical magnetic memory device utilizing non-recorded portions of recording medium to represent data

An optical magnetic memory device includes an optical magnetic recording medium such as a disc having recording magnetic film at least on one side thereof and initialized in a certain direction in advance, a recording section for recording data in such a manner that data-recorded portions and non-recorded portions are alternately formed, and a reproducing section for reproducing data from the recording medium by detecting the interval between the recorded or non-recorded portions. The reproducing section comprises a detector for detecting positive and negative peaks of signals reproduced from the recorded or non-recorded portion and a selector for selecting either of the detection outputs from the detector according to a control signal.

BACKGROUND OF THE INVENTION 
The present invention relates to an optical magnetic memory device having 
magnetic film as a recording medium and which records., reproduces and/or 
erases data by irradiating the recording medium with an optical beam such 
as a laser beam. 
Conventionally, two types of optical magnetic disc are known as typical 
optical memory devices; one having magnetic film only on one side thereof 
and the other having magnetic film on both sides as a recording medium. 
When data in a disc with magnetic film on both sides is to be reproduced 
by the conventional optical magnetic memory device capable of reproducing 
data from the disc with magnetic film only on one side, data in one of the 
magnetic films may be reproduced but it is unlikely that data in the other 
magnetic film will be reproduced satisfactorily. 
Generally, in recording data with an optical magnetic memory device, 
recorded bits and non-recorded bits are formed in the recording medium 
(disc) with magnetic film thereon. The memory device reproduces the data 
by detecting the interval between the recorded bits (non-recorded 
portions) as reference marks. However, a data signal obtained by this 
reproduction process may not be reliable. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide ah optical 
magnetic memory device capable of reproducing satisfactorily the data 
recorded in the recording medium having magnetic film at least on one side 
of the disc. 
Another object of the present invention is to provide an optical magnetic 
memory device capable of reproducing data satisfactorily regardless of 
whether the recording medium has magnetic film on one side or on both 
sides thereof. 
Another object of the present invention is to provide a data bit recording 
and reproducing system which increases recording density and enhances the 
reliability of reproduced data signals. 
Other objects and further scope of applicability of the present invention 
will become apparent from the detailed description given hereinafter. It 
should be understood, however, that the detailed description and specific 
examples, while indicating preferred embodiments of the invention, are 
given by way of illustration only, since various changes and modifications 
within the spirit and scope of the invention will become apparent to those 
skilled in the art from this detailed description. 
To achieve the above objects, according to an embodiment of the present 
invention, an optical magnetic memory device comprises an optical magnetic 
recording medium (disc) with magnetic film at least on one side and 
initialized in a certain direction in advance, data recording means which 
forms data recorded portions with non-recorded portions occurring 
alternately therebetween in the magnetic film of the optical magnetic 
recording medium, and means for reproducing the data from the recording 
medium by detecting the interval between the recorded or non-recorded 
portions, the reproducing means containing means for detecting the 
positive and negative peak positions of the signal reproduced from the 
recorded or non-recorded portion and means for selecting either of the 
outputs from the detecting means according to a control signal. 
To achieve the above objects, according to another embodiment of the 
present invention, an optical magnetic memory device, which contains an 
optical recording medium with magnetic film on both sides thereof and 
initialized in one direction in advance and which records data so as to 
allow detection of the interval between recorded marks, comprises means 
for detecting the positive and negative peaks of a reproduced signal and 
means for reproducing data by selecting either of the outputs from the 
detecting means according to a control signal such as a recorded side 
identification signal. 
To achieve the above objects, according to a further embodiment of the 
present invention, an optical memory device, capable of recording, 
reproducing or erasing data by irradiate the optical data recording medium 
with a laser beam spot, records data in such a manner as to allow 
detection of the interval between non-recorded portions to be used as 
reference marks for reproduction of the data.

DETAILED DESCRIPTION OF THE INVENTION 
An optical magnetic memory device of the present invention is described 
using an optical magnetic disc memory device as an example. 
As shown in FIG. 4, the optical magnetic disc memory device records, 
reproduces and/or erases information by radiating a laser beam 40 
condensed by an objective lens 41 onto a recording medium with magnetic 
film 42 whose easy magnetization axis is perpendicular to the film surface 
formed on a substrate. 
Recording and reproducing operation of the optical magnetic disc memory 
device are described in detail with reference to FIG. 4. 
To record information, the laser beam 40, condensed to about 1.mu.m 
diameter by the objective lens 41 and modulated in intensity according to 
the recording signal, is radiated onto the magnetic film 42. Then, the 
irradiated part of the magnetic film 42 rises in temperature and decreases 
in coercive force. With simultaneous application of an external bias 
magnetic field H.sub.B, the magnetization direction is reversed to be 
oriented in the same direction as the bias magnetic field H.sub.B so that 
information is recorded. 
To reproduce the information, a linear polarized laser beam of lower 
intensity than the beam for recording is radiated onto the magnetic film 
42. Then, the magneto-optic effect (Kerr effect) causes the plane of 
polarization of the reflected light to be inclined. This inclination is 
detected by an analyzer and converted into luminous intensity which is 
detected by an optical detector. A reproduced signal 46 with positive 
peaks corresponding to the recorded marks is thus obtained as shown in 
FIG. 4 (b). Information can be reproduced by detecting the peak positions 
of the reproduced signals 46. 
The procedure for reproducing information data from the reproduced signal 
is described with reference to FIG. 5. 
The signal reproduced by a reproducing head 1 is amplified by the amplifier 
2 before being input to a differential circuit 4 and to an amplitude 
detector circuit 11 in a peak detector circuit 3 (enclosed by broken 
line). Signal output 13 from the differential circuit 4 is converted to a 
digital signal by a zero-cross detector circuit 5. The peak position is 
then detected in the following manner. The circuitry comprising an 
inverter 6, a resistance 7, a capacitor 8 and a NOR gate 9 detects 
positive peaks of the reproduced signal 12 or the falling edge of the 
signal output from the zero-cross detector circuit 5 and outputs a peak 
position identifying signal 14. An AND gate 10 takes the logical product 
between the signal 14 and a signal output 15 from the amplitude detector 
signal 11, and outputs a peak detection signal 16 which corresponds to 
positive peaks alone. The peak detection signal 16 is transmitted to a 
data demodulator circuit where it is converted to information data. 
The above procedure is now described referring to FIG. 6 which shows the 
signal waveforms. The same signals are described by the same numbers in 
FIGS. 5 and 6. The reproduced signal 12 has positive peaks at positions 
corresponding to information data. The reproduced signal 12 is converted 
to a differential signal 13 in the differential circuit 4. The 
differential signal 13 crosses the zero level from the positive to the 
negative side at the positive peak position of the reproduced signal 12. 
Accordingly, the signal output from the zero-cross detector circuit 5 
starts falling at the positive peak position. Therefore, the signal 14 has 
detected the positive peak position of the reproduced signal 12 by 
detecting the falling edge of the signal 13. The signal 14 and the 
amplitude detection signal 15 are input to the AND gate 10 which takes 
their logical product and outputs the peak detection signal 16 which 
corresponds to the positive peak alone. 
As mentioned above, the optical magnetic disc, whose recorded information 
is erasable, has magnetic film on one or both sides. For the optical 
magnetic disc with magnetic film 42 on both sides as shown in FIG. 7 and 
8, it is natural that the optical memory device is so constructed as to 
record, reproduce and/or erase information on each side of the disc 
independently. 
Such construction is described below. The two magnetic films (which are 
separated by a separation layer 43) are referred to as the side A and the 
side B, respectively. It is assumed that both magnetic films have been 
initialized or magnetized in the same direction. 
First, the mechanism of recording and reproduction on the side A is 
described referring to FIG. 7 (a). For recording, laser beam 40 condensed 
by the objective lens 41 is radiated on the side A of the magnetic film 
42. Then, the magnetization orientation is reversed to coincide with the 
direction of the bias magnetic field H.sub.B whereby recording is 
conducted. When reproduced, the reproduced signal 44 contains positive 
peaks at positions corresponding to the recorded marks as shown in FIG. 7 
(b). 
For recording on the side B, the direction of the bias magnetic field 
H.sub.B needs to be reversed as the side B has been initialized in the 
same direction as the side A as shown in FIG. 8 (a). In other words, 
recording is conducted from the opposite side for the side B, compared 
with that for the side A. As a result, the magnetization orientation of 
the recorded mark on the side B is reversed from that on the side A. 
Accordingly, the reproduced signal 45 of the side B has opposite polarity 
from that of the reproduced signal 44 of the side A. Namely, the signal 45 
has negative peaks at positions corresponding to the recorded marks, as 
shown in FIG. 8 (b). Therefore, it is necessary to detect negative peak 
positions for reproduction from the side B. 
As understood from the above, when the recording medium has magnetic films 
on both sides, namely the side A and the side B, and when the films have 
been initialized in the same direction, reproduced signals from the side A 
and the side B have opposite polarities from each other. Supposing 
reproduction is based on detection of the intervals between recorded 
marks, if information is to be reproduced by a data reproducer capable of 
detecting positive peaks alone as shown in FIG. 5, information data may 
not be reproduced for side B due to the incapability of detecting negative 
peaks of the side B. 
An optical magnetic memory device is now described having magnetic films on 
the side A and the side B which have been initialized in the same 
direction as mentioned above. 
FIG. 1 shows the construction of the data reproducer in an embodiment of 
the optical magnetic memory device of the present invention. FIG. 2 shows 
signal waveforms generated by reproduction from the side A of the memory 
device of FIG. 1. FIG. 3 shows signal waveforms by reproduction from the 
side B of the memory device of FIG. 1. In FIGS. 1, 2 and 3, the same 
components are described by the same numbers. 
Referring to FIG. 1, a signal 12 reproduced by a reproducing head 1 and 
amplified by an amplifier 2 is led to a positive peak detector circuit 3 
(enclosed by a broken line) and to a negative peak detector circuit 17 
(enclosed by a broken line). 
The positive peak detector circuit 3 detects positive peaks in the 
following manner. A signal output 13 from a differential circuit 4 is 
converted to a digital signal in a zero-cross detector circuit 5 and sent 
into the subsequent circuit for positive peak detection. Specifically, the 
circuit made up of an inverter 6, a resistor 7, a capacitor 8 and a NOR 
gate 9 detects the falling edge of the signal output from the zero-cross 
detector circuit 5 or namely the positive peak of the reproduced signal 
12, and outputs a signal 14. The signal 14 and a signal output 15 from an 
amplitude detector circuit 11 are input to an AND gate 10 which takes 
their logical product and outputs a positive peak, detection signal 16. 
The signal 16 which has detected the positive peak position alone is led 
to one of the two inputs of a switching circuit 30. 
The negative peak detector circuit 17, detects negative peaks in the 
following manner. A signal output 26 from a differential circuit 18 is 
converted to a digital signal in a zero-cross detector circuit 19. The 
differential circuit 4 or 18 and the zero-cross detector circuit 5 or 19 
may be shared between the positive and negative peak detector circuits 3 
and 17. Negative peak position is detected by the subsequent circuit in 
the following manner. The circuit made up of an inverter 20, a resistor 
21, a capacitor 22 and an AND gate 23 detects the rising edge the signal 
output from the zero cross detector circuit 19 or namely the negative peak 
of the reproduced signal 12, and outputs a signal 27. The signal 27 and a 
signal output 28 from an amplitude detector circuit 25 are input to an AND 
gate 24 which takes their logical product of them and outputs a negative 
peak detection signal 29. The signal 29 which has detected negative peak 
position alone is led to the other input of the switching circuit 30. 
The switching circuit 30 selects either the positive peak detection signal 
16 or the negative peak detection signal 29 according to a control signal 
32 such as a recorded side identification signal, and outputs a signal 31 
which is converted to information data by a data demodulator circuit. 
FIGS. 2 and 3 are signal waveforms related to positive peak detection on 
the side A and negative peak detection on the side B, respectively. As 
clear from FIGS. 2 and 3, the reproduced signals 12 on the side A and on 
the side B have reversed polarities. For the side A, the signal output 13 
from the differential circuit 4 crosses the zero level from the positive 
to the negative side at the position corresponding to the positive peak. 
For the side B, in contrast, the signal output 26 from the differential 
circuit 18 crosses the zero level from the negative to the positive side 
at the position corresponding to the negative peak. For peak detection, 
therefore, it is only necessary to detect the falling edge of the signal 
output from the zero-cross detector circuit 5 on the side A, and to detect 
the rising edge of the signal output from the zero-cross detector circuit 
19 on the side B. The positive peak detection signal 16 which corresponds 
to the positive peak position alone is obtained based on the logical 
product taken from the falling edge detection signal 14 together with the 
amplitude detection signal 15, on the side A. The negative peak detection 
signal 29 which corresponds to the negative peak position alone is 
obtained based on the logical product taken from the rising edge detection 
signal 27 together with the amplitude detection signal 28, on the side B. 
The peak detection signal 16 for the side A or the peak detection signal 
29 for the side B is selected by the switching circuit 30 shown in FIG. 1 
according to the control signal 32, and transmitted to the data 
demodulator circuit. 
When information is recorded in the recording medium in such a manner as to 
allow reproduction to be achieved by detecting the intervals between 
recorded marks as described above, data reproduction is possible from both 
A and B sides simply by selecting the positive or negative peak detection 
signal according to the control signal. In the above description, it is 
assumed that a signal reproduced from the side A has positive peaks and 
that reproduced from the side B has negative peaks. Alternatively, a 
signal reproduced from the side A may have negative peaks and that from 
the side B may have positive peaks. 
An optical magnetic disc memory device has been described as an example of 
the optical magnetic memory device of the present invention. The invention 
is not limited to a disc recording medium but may be applied to an optical 
magnetic memory device of other type such as tape with magnetic film on 
both sides. 
According to the above embodiment of the present invention, when data is 
recorded on the recording medium in such a manner as to permit detection 
of the interval between recorded marks for data reproduction, the optical 
magnetic memory device can reproduce information data from either side of 
the magnetic films of the recording medium which have been initialized in 
the same direction. In this embodiment, magnetic film may be formed on one 
side or on both sides of the recording medium. 
The present invention may adopt a recording method which enables data 
reproduction to be achieved by detecting the interval between reference 
marks. Such recording method is described now. A method of detecting the 
interval between reference marks is also described. 
According to this second embodiment, an optical magnetic disc memory device 
as an example of the optical memory device of the present invention uses 
the RZ method in recording data on a data recording medium (hereinafter 
called disc) in such a manner as to permit detection of the interval 
between reference marks for data reproduction. FIGS. 11 (a) and 11 (b) 
show the recording characteristics of this embodiments. In this 
embodiment, a reference mark is a data-recorded bit. To record a reference 
mark, a recording laser beam P.sub.2 (Refer to FIG. 11 (a)) is radiated on 
the disc after being modulated in intensity by the RZ method as shown by a 
solid line according to an information signal. Then, only the part of the 
magnetic film on the disc exposed to the laser beam of high intensity 
rises in temperature and reduces in coercive force When an external 
auxiliary magnetic field is simultaneously applied to the disc with this 
state, magnetization orientation is reversed so that a recorded bit "a" is 
formed as a reference mark on the disc (see FIG. 11 (b)). 
According to this method, the recorded bit length "l.sub.1 " is shorter 
than the non-recorded bit length "l.sub.2 ". In FIG. 11 (a), the broken 
lines indicate a laser beam of reduced intensity. 
A light spot "b" (see FIG. 11 (b)) which is obtained by condensing laser 
beam of lower intensity than the recording laser beam P.sub.2 is used for 
reproducing information from the recorded bit "a" formed on the disc. 
Reproduced signal S.sub.2 thus obtained is shown in FIG. 11 (c). Data 
signal D.sub.2 shown in FIG. 11 (d) is obtained by detecting peaks of the 
reproduced signal S.sub.2. The data signal D.sub.2 has detected the 
intervals between adjacent recorded bits "a" and "a" (or the intervals 
between recorded marks formed by the RZ method). 
However, as the difference in the area between the recorded portion and 
non-recorded portion within the range of the light spot "b" is very small, 
the reproduced signal S.sub.2 reproduced by the light spot "b" from the 
recorded bit "a" formed by the RZ method has a small amplitude. If the 
intensity of the recording laser beam P.sub.2 reduces as shown by broken 
lines, the area of the recorded bit "a" becomes smaller, further reducing 
the amplitude of the reproduced signal S.sub.2, which hampers satisfactory 
reproduction of information. This phenomenon is more conspicuous as the 
length of the recorded bit "a" is made smaller to raise the linear bit 
density for higher recording density. Therefore, when information is 
recorded and reproduced by the method in which the recorded bit length 
"l.sub.1 " is longer than the non-recorded bit length "l.sub.2 ", 
reduction in the intensity of laser beam or increase in the recording 
density may result in less reliable information signal reproduction. 
Furthermore, formation of recorded bits may possibly be hampered not only 
by the reduction in the laser beam intensity but also by deterioration of 
the recording sensitivity and of the recording capacity of the data 
recording medium. 
For the purpose of solving the above problems, another embodiment described 
below is characterized in that a reference mark is a non-recorded portion. 
FIG. 10 schematically shows the construction of an optical magnetic disc 
memory device, an example of an optical memory device capable of 
recording, reproducing and erasing information. 
102 is a semiconductor laser which emits a laser beam of a predetermined 
intensity. To record information, the recording laser beam P.sub.1 emitted 
from the semiconductor laser 102 is converted by a collimator lens 103 
into a parallel beam which is then converted to an approximately circular 
beam by a shaping prism 104. The beam is passed through a polarization 
splitter 105, a total reflection prism 106 and an objective lens 107 to be 
condensed to about 1 .mu.m diameter beam, and then radiated on the 
magnetic film surface of a disc 101. The disc 101 is a magnetic recording 
medium whose easy magnetization axis is perpendicular to the magnetic film 
surface. 
In signal reproduction, a laser beam of lower intensity than that for 
recording is emitted from the semiconductor laser 102. The laser beam 
travels over the same route as in recording, and irradiates the magnetic 
film surface of the disc 101. Information-bearing light reflected from the 
magnetic film has an inclined plane of polarization due to the 
magneto-optic effect (Kerr effect) of the magnetic film. After passing 
through the objective lens 107, the total reflection prism 106 and the 
polarization beam splitter 105, the reflected information light is 
separated from the incident laser beam and split into two light beams by a 
polarization beam splitter 108. One of the two beams is led through a 
cylindrical lens 110 into an optical detector 111. A tracking reference 
signal and a focusing reference signal are obtained from the signal output 
from the optical detector 111. The other reflected information light beam 
is led to a halfwave plate 112 where the direction of polarization is 
rotated by a specified angle. Then the reflected light beam is split by a 
polarization beam splitter 114 into S-polarized light and P-polarized 
light components. Simultaneously, these information light components are 
detected by optical detectors 115 and 116, respectively, and converted to 
electric signals. Thus, to obtain reproduced signals of better S/N ratio, 
the optical magnetic disc memory device of this embodiment detects 
information light beams by the differential detection system which divides 
the information light into P-polarized light and S-polarized light 
components and obtains the difference between the detection signals of the 
P and S polarized light components. 
In the above embodiment, to permit non-recorded zones to be used as a 
reference mark, the recording laser beam P emitted from the semiconductor 
laser 102 is modulated in intensity by the RZ method so that the recorded 
bit length L.sub.1 is longer than the non-recorded bit length L.sub.2 as 
shown by solid line in FIG. 9 (a). Then, only the part of the magnetic 
film on the disc 101 exposed to the laser beam of high intensity rises in 
temperature and reduces in coercive force. Application of an external 
auxiliary magnetic field on the disc with this state causes the 
magnetization orientation to be reversed, so that a recorded bit 120 
corresponding to the recording laser beam P.sub.1 is formed on the disc 
101 (see FIG. 9 (b)). The broken line of FIG. 9 (a) indicates a recording 
laser beam P.sub.1 of reduced intensity. According to this embodiment, 
even if the intensity of the laser beam P.sub.1 drops, reduction in the 
length of the recorded bit 120 is negligible as indicated by broken line 
in FIG. 9 (b). Recorded bits 120 of satisfactory length can be formed in 
any case. 
For signal reproduction, laser beam spot 122 is radiated on a non-recorded 
bit 121 as a reference mark. Since the difference in the area between the 
recorded zone and the nonrecorded zone within the beam spot 122 larger in 
this embodiment than in the previous embodiment, the reproduced signal 
S.sub.1 obtained has a larger amplitude as shown in FIG. 9 (c). Besides, 
if the intensity of the recording laser beam P.sub.1 drops as shown by the 
broken line, the amplitude of the reproduced signal S.sub.1 is still 
sufficiently large as indicated by broken line in FIG. 9 (c). Accordingly, 
reliability of the data signal D.sub.1 (see FIG. 9 (d)) will increase if 
it is obtained by detecting the interval between the non-recorded bits 
121. 
As mentioned above, recorded bits 120 of sufficient length are formed on 
the disc 101 and a satisfactory reproduced signal S.sub.1 is obtained by 
the RZ method if the recorded bit length L.sub.1 is made longer than the 
non-recorded bit length L.sub.2. As a result, a highly reliable data 
signal D.sub.1 can be reproduced even with a laser beam of reduced 
intensity or with a high density recording medium. In the above 
embodiment, the influence of laser beam intensity reduction on the 
formation of recorded bits 120 is prevented. According to the present 
invention, it is possible to obtain reliable data signals even when the 
recording sensitivity or capacity of the data recording medium is 
deteriorated. In addition to the optical magnetic disc memory device, the 
recording and reproducing method as described above may be applied to 
another optical memory device which records, reproduces or erases 
information by using a laser beam. 
According to the present invention, as understood from the above, since 
information is recorded by the RZ method so that recorded bit length is 
longer than non-recorded bit length, the recording characteristic is 
sufficiently good to reproduce a signal of high quality. Therefore, the 
present invention makes it possible to realize an optical memory device 
which reproduces highly reliable data. 
While only certain embodiments of the present invention have been 
described, it will be apparent to those skilled in the art that various 
changes and modifications may be made therein without departing from the 
spirit and scope of the present invention as claimed.