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BACKGROUND OF THE INVENTION
The present invention relates to methods and apparatus for the modification
of recorded data and, more particularly, to a novel technique for the
direct over-write modification of digital data stored in domains of a
magnetic-optic recording media.
The concept of storing binary bits of information from a digital data
stream in a sequential multiplicity of domains formed in magnetic storage
media is well known. While many different types of magnetic media have
been hitherto used, including plated wires, toroidal cores, tapes and the
like, the particular high-information-density media of interest here is a
thin film layer of a magneto-optic recording material. Magneto-optic
recording materials are amorphous ferrimagnetic alloys usually including a
rare earth in combination with a transition metal such as amorphous alloys
of terbium cobalt (TbCo), gadolinium terbium cobalt (GdTbCo), and the like
materials.
With magneto-optic materials, the binary value of a stored bit of
information can be determined by analyzing the effect upon a polarized
light beam reflected from the surface. Ferrimagnetic materials chosen to
have a high coercivity at room temperatures and low coercivity at higher
temperatures, can be "written" by heating a small region to have a net
magnetization which is not only substantially perpendicular to the surface
of the film but is also established in that direction parallel to the
direction in which an external (bias) magnetic field was directed at the
time when that particular region was heated and allowed to subsequently
cool. It is also well known that the external field direction can be
changed to encode the data to be stored. Previously stored information can
be changed by re-heating the film region while an external bias magnetic
field is presented in the desired (opposite) direction through the region.
In such magneto-optic systems in which an external bias magnetic field is
used to change the previously-stored information, the preferred recording
material is a ferrimagnetic material with a compensation temperature (Tc)
at about room temperature. However, the speed at which an external
magnetic field can be made to reverse cannot presently be made as fast as
is desired. Thus, although magneto-optic storage media, have demonstrated
both (1) sufficient data density for the storage of gigabits of
information on a disk and (2) short time for access to the previously
stored data, they have not hitherto allowed randomly stored data to be
modified at any speed even close to the speed at which stored data can be
read from the storage disk.
For general use, data storage equipment should be capable of writing,
reading and/or over-writing data at the same high rate. Accordingly, a
method and apparatus by which to rapidly modify the data stored in
microscopic recording regions of a magneto-optical recording medium is
highly desirable.
BRIEF SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, the need for
applying an external bias magnetic field to modify previously stored
information in a selected region, surrounded by a magnetic domain wall, of
a thin film magneto-optic medium, is eliminated by employing as the medium
a magnetic material which produces a self-demagnetizing field within said
region when reheated. Ferrimagnetic materials with a compensation
temperature only tens of degrees C. above room temperature have been found
capable of changing the magnetic state of a domain when heated with a beam
of energy without using an external magnetic field. In particular, with
such materials, previously recorded domains can be erased applying a laser
energy in the absence of any significant external magnetic bias field. The
medium preferably is a single layer and has substantially homogenous
magnetic properties. The self-demagnetizing field reverses the net
magnetization of the region and therefore alters the stored information.
In presently preferred embodiments, the magneto-optic recording material is
an amorphous alloy of at least one rare-earth element and at least one
transition-metal element, and has a compensation temperature only tens of
degrees C. above room temperature; alloys of terbium cobalt (TbCo),
gadolinium terbium cobalt (GdTbCo) and terbium iron cobalt (TbFeCo) are
especially favored. The required heating of a microscopic region, e.g., a
region with a diameter on the order of 1 micron, of the thin film of this
material is carried out by a pulse of light energy from a laser diode and
the like source. Pulse durations of 10-1000 nanoseconds, at power levels
of 1-20 milliwatts, are satisfactory for causing self-inversion of the net
magnetic moment of the heated region, and therefore, of the related stored
binary logic value.
In accordance with a further aspect of the invention, a novel method for
over-writing information on a magneto-optic recording material layer can
be referred to as "read-before-write" and includes the steps of: reading
the binary value of the bit of digital data presently stored in a selected
region of the recording layer; determining if that binary value differs
from the binary value of a received new bit of digital data to be stored
in that region; and, only if the binary value of the new data bit is
different from the presently-stored value, irradiating that region of the
recording layer with a beam of energy selected to temporarily raise the
temperature of substantially only that region to beyond the compensation
temperature of the recording layer material, in the substantial absence of
any externally-provided magnetic bias field, to cause self-inversion of
the direction of net magnetization in that region.
A variation of the novel method for over-writing information on a
magnetic-optic recording layer without using magnetic bias can be referred
to as "erase-before-write". Instead of reading the recording medium to
determine the binary value of the recorded bit in advance of the writing
operation, all nucleated domains can be erased in advance of the writing
operation to place the medium in a known binary state. A new bit of
digital data to be stored is compared with the known state and the region
on the recording layer is irradiated only if the binary value of the new
data bit is different from the known state. A simple erase-before-write
operation is established by the ability to repeatedly erase and
selectively write nucleated domains on the recording medium. The erase
operation can be achieved by detecting domains in the region to be
recorded and by selectively erasing any detected domains. Alternatively,
the erase operation can be achieved dynamically by a succession of closely
spaced erase pulses used to erase all domains encountered in the region to
be recorded.
Accordingly, an object of the present invention is to provide a novel
method and apparatus for the modification of the logic value of a bit of
digital dated stored in a region of magneto-optic recording medium.
Another object is to provide a recording medium, recording disk and
recording system for advantageously implementing the novel method of this
invention.
It is another object of the invention to provide a magneto-optic recording
material on which previously recorded domains can be erased using a beam
of energy without any significant bias magnetic field.
This and other objects will be more clearly appreciated from the following
detailed description of our presently preferred embodiments, especially
when considered in conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a magneto-optic recording disk and the
apparatus to read and over-write digital data at a multiplicity of storage
regions;
FIG. 1a is a section view through a recording disk;
FIG. 1b is a schematic block diagram of an electronic circuit for use, with
the apparatus of FIG. 1, in over-writing the digital values stored in a
particular storage region;
FIGS. 2a-2h are graphical representations of net magnetic moments of
several adjacent data bit storage regions, at times before, during and
after two successive changes in stored value.
FIG. 3 is a perspective view of a magneto-optic recording disk and
apparatus to read, erase and write digital data at a multiplicity of
storage regions;
FIG. 4 is a schematic block diagram of an electronic circuit for use, with
apparatus in FIG. 3, for selective erase-before-write operation; and
FIG. 5 is a schematic block diagram of an electronic circuit for use with
apparatus in FIG. 3, for pulsed erase-before-write operation.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
Referring initially to FIGS. 1 and 1a, apparatus 10, for the magneto-optic
storage of digital information, includes a storage disk 11, which may be
of cylindrical shape. The disk has a central aperture 11.degree., through
which a central spindle member 12 protrudes during operation; the disk is
caused to rotate about spindle 12, in the direction of arrow A, by known
mechanisms (not shown). The disk 11 is itself comprised of a discoidal
substrate member 11-1, formed of a substantially non-magnetic material,
such as glass and the like. The substrate has a pair of circular,
substantially parallel and spaced apart surfaces 11a and 11a', upon at
least one (and preferably both) of which surfaces a thin film 11-2 or
11-2' of the magneto-optic recording material is fabricated, as by
sputtering and the like processes. The outwardly-directed circular
surface(s) 11b (and 11b', if second layer 11-2' is present) of the
recording thin film layer(s) is preferably overcoated with an
optically-transparent protective layer 11-3 (or 11-3') of glass and the
like. Each recording layer 11-2 or 11-2' is characterized by a
multiplicity of microscopic data storage regions 11d, e.g., the
consecutive storage regions 11d-1 through 11d-4 defined through the upper
recording layer 11-2, and the consecutive storage regions 11d'-1 through
11d'-4 defined through the lower recording layer 11-2'. Each recording
region has an average diameter D, on the order of 1 micron. Each recording
layer is fabricated of a magneto-optic material, such as an alloy of
gadolinium terbium cobalt (GdTbCo), terbium cobalt (TbCo), gadolinium iron
cobalt (GdFeCo) and the like, which has a compensation temperature T.sub.c
higher than the highest expected ambient temperature T.sub.a, but much
less than the crystallization temperature of the alloy. Compensation
temperatures only a few tens of degrees C. above the normal room
temperature are preferred.
The crystallization temperature for alloys of gadolinium terbium cobalt
(GdTbCo) and terbium cobalt (TbCo) is about 250.degree. C. and, therefore,
the compensation temperature should be below 200.degree. C. and preferably
below 140.degree. C. to avoid any crystallization of the recording medium.
A few tens of degrees C. above normal room temperature sets the minimum
compensation temperature T.sub.c for this invention at 40.degree. C. Since
disk drives are commonly designed for operation up to 55.degree. C.
ambient, a compensation temperature above 60.degree. C. is preferable.
Experimental work has established that magnetic domains in ferrimagnetic
materials can be erased without magnetic bias in materials with a
compensation temperature in the range of 40.degree. C. to 140.degree. C.
Erasure, although possible, does not work reliably outside this range.
A preferred target compensation temperature for the recording medium
according to the invention is 80.degree. C. The preferred range of actual
compensation temperatures about the target temperature is 60.degree. C. to
100.degree. C.
In general it is known that the compensation temperature of a ferrimagnetic
recording medium is a function of the ratio of the two types of magnetic
materials. In rare earth, transition metal, magneto-optic formulations,
the compensation temperature is a function of the rare earth to transition
metal ratio. If more than one rare earth is included in the formulation,
the compensation temperature is approximately independent of the ratio of
the rare earths. In formulations including iron, such as gadolinium
terbium iron cobalt (GdTbFeCo), the compensation temperature is
approximately independent of the iron/cobalt (Fe/Co) ratio. In
formulations in which cobalt is the transition metal, about 77% cobalt
yields a compensation temperature of 50.degree. C., whereas about 75.8%
cobalt yields a compensation temperature of 100.degree. C.
The binary value of the data bit stored in any one region lid is initially
random, immediately following fabrication of disk 11. Each region is
heated to a temperature greater than the compensation temperature T.sub.c
while an external bias magnetic field 14, established in a direction
(e.g., upwardly, as shown by arrow B) substantially perpendicular to the
plane of the disk surface, is caused to pass through the heated region
upon cooling, under the influence of the external field 14, each region
has a net magnetic moment directed in the same direction, e.g., upward, as
the direction of the initializing bias field 14. This initial magnetic
moment direction can be assigned to either binary value, so long as that
direction:value assignment is consistently utilized. It should be
understood that run-length-limited codings could be used and that each
domain could contain more than a single bit of data.
While large amounts of data may be substantially simultaneously written
into the initialized memory regions, for the purpose of illustration, a
single beam 15 of heating radiation is considered radially movable along a
line 16 so as to be directed to fall at a presently-selected one of a
plurality of points presently each defining a selected one of concentric
circular tracks of sequentially-located regions 11d-w (although a spiral
track can be used); one bit of digital data is to be initially written
into each of regions 11d-w. The beam can be formed of optical radiation,
as produced by a light source means 17, such as a laser diode 18 and a
focusing lens means 19, and will be directed toward the disk, as shown by
arrow C. The laser diode produces its optical radiation output responsive
to a current I caused to flow therethrough, from an associated laser power
supply 18', responsive to reception of a write-enable signal at a control
input 18a; advantageously, the current is of a pulsed nature, to produce a
light pulse signal having a duration from a minimum time interval on the
order of 10 nanoseconds to a maximum time interval on the order of 1
microsecond, with power levels on the order of 1-20 milliwatts.
Preferred values for nucleating micrometer size domains in a gadolinium
terbium cobalt (GdTbCo) film is by locally heating the film with seven
milliwatts of laser power for a 300 nanosecond pulse duration without an
externally applied magnetic field. A domain written with a pulse of 300
nanoseconds duration can be completely erased by a succeeding pulse of
100-200 nanoseconds duration at the same power level. With 12 milliwatts
of laser power, a domain written with a 100 nanosecond pulse can be erased
by a 30-80 nanosecond pulse; a domain written with a 150 nanosecond pulse
can be erased with a 35-130 nanosecond pulse; and a domain written with a
200 nanosecond pulse can be erased with a 45-175 nanosecond pulse.
The beam focusing means 19 should be configured to focus the beam, at the
half power diameter thereof, to an area having a diameter less than the
diameter D of the region to be heated. Thus localized heating will raise
the temperature, and cause a localized decrease in the coercivity, of
substantially only one region (e.g., region N, see FIG. 2) into which data
is to be written.
The datum contained in any written region 11d-r can be non-destructively
read therefrom by causing a reading light beam of plane-polarized light 21
(of amplitude insufficient to heat to a temperature high enough that the
magnetization changes in any region upon with the reading beam impinges)
to be projected in the direction of arrow D, toward the disk surface 11c.
A portion of the impingent light is reflected from the region surface 11b;
the reflected beam polarization is rotated in a direction dependent upon
the direction in which the net magnetic moment of the region extends.
Therefore, if a preselected polarization is imparted to the impingent
reading beam, the polarization of the reflected beam can be analyzed to
determine the binary state of the datum stored in the region being read.
For example, a separate laser diode 23, of lower output power than the
output power of writing laser diode 18, may provide a beam 24 of light
which is polarized by passage through a polarizing means 26. It should be
understood that a single, variable-power laser diode can be used with
known optics to provide both a lower-power polarized reading beam and a
higher-power writing beam to essentially the same region 11d (which region
is the union of the regions 11d-r and 11d-w as the distance along tract 42
is reduced toward zero). The polarized beam 28 is focused by lens means 30
to a diameter less than the diameter of region 11d-r, after passage
through a beam-splitting means 32. The reflected beam 34, traveling in the
direction of arrow E away from disk 11, is redirected by means 32. The
redirected beam 36 is focused, by lens means 38 and the like, upon the
active portion of a detector means 40, which provides, at a disk data read
output 40a, a logic output signal having a state commensurate with the
state of the net magnetic moment of the region 11d-r being read. The
present invention facilitates the direct over-writing or erasing of
previously written information in selected regions of the magneto-optic
medium. The method employs the self demagnetizing field created within a
reheated region by the specially formulated thin film magneto-optic
recording medium to reverse the net magnetization of the region and thus
eliminates the requirement of the prior art for an externally applied bias
magnetic field. The physical processes believed to be involved in the new
method will be described hereinafter in conjunction with FIGS. 2a-2h but
first, one particular application (referred to as the "read before write"
approach) of the self-inverting data modifying method of the present
invention will be described.
In accordance with the invention, our method to over-write the binary value
of each bit of a multiplicity N of sequentially storable bits of a new
data sequence over the binary value presently stored in each of the
associated N sequential storage regions, causes each associated region to
be interrogated and the present value of the bit stored in that region to
be read therefrom. The read-out data value is then compared with the new
data value to be stored in that region. If the comparison indicates that
the proper binary data value is already stored in the interrogated region
11d-r, action is not necessary and comparison of the next sequential bit
of new data is made against the value of the next data bit already stored
in the magneto-optic media layer. If the comparison indicates that the
wrong binary value is presently stored in the interrogated region, an
over-write enable signal is provided to request heating of the
interrogated region, to a temperature above the compensation temperature
T.sub.c of the magneto-optic material (and in the absence of any
substantial magnetic field external to the storage media layer and
deliberately applied to reverse the net magnetic moment) to cause the
actual net magnetic moment M in that region to be self-inverted and then
be maintained in a stable magnetic domain. Because the disk is rotating,
preferably at a well-regulated speed, the region 11d-r which was read will
have moved from the location at which regions are read, along an imaginary
circular track 42, to another position; the actual position at which
over-writing of a region (now identified as a region 11d-w to be
over-written) occurs should be somewhat beyond that required for a normal
read-before-write decision sequence to occur, to allow for adjustments and
the like. A time delay may be introduced to cause the over-write-enable
signal, resulting from the read and compare operations, to enable the
over-write energy source only at a time when the proper region has arrived
at the position 11d-2 at the focus of the over-write beam 15.
The read-before-write (e.g., read-compare-delay-enable) operational
sequence may be obtained at least in part, for example, by the functions
of a subcircuit 50 such as is illustrated in FIG. 1b. The data read from
the disk is, after suitable buffering and processing after output 40a,
introduced at a first input 50a of the subcircuit. In one possible
embodiment, the incoming data is first delayed, in a data delay means 52,
for the time interval required for the region 11d-r in which the read
magnetic domain resides to advance to a preselected position just prior to
the position at which region 10d-w can be over-written. The delayed read
data bit value is provided to a first input 54a of a comparison means 54,
e.g., an exclusive-OR (XOR) gate. The incoming data bit value is provided
to a second subcircuit input 50b and is coupled to other input 54b of the
XOR gate. If both gate inputs are of the same binary value, then gate
output 54c will be at a first level, e.g., a logic zero state, while the
output 54c will be at the opposite second level, e.g., the logic one
state, only if both input states are different. The gate output is
connected to the data D input of a flip-flop logic element 56, which
receives a clock C input signal from a sychronization SYNC. signal
subcircuit input 50c. This SYNC. signal is prepared, in manner well known
to the arts, from at least those sychronization signals provided by
formatting of the disk storage regions. Thus, the logic level at the
comparison means output 54c is clocked through to the Q output of the
flip-flop only at such time as that output signal should be properly
present at the subcircuit output 50d, for coupling to the enabling output
18a of the laser/power supply, for causing a pulse of laser light to be
focused upon the associated storage region, which has now been moved from
the location at which a region 11d-r is read to the location at which a
region 11d-w is over-written. It should be understood that the comparison
can be carried out first, as by connection of input 50a to gate input 54a,
with placement of the delay means 52 after the comparison means and
immediately prior to the synchronization means 46. It should also be
understood that the delay means 52 can be positioned after the
synchronization means 56. Further, it is preferable that the data delay
means itself receive a clock signal, as at input 50e, recovered from the
actual read data, to cause the desired N region, or domain, delay to occur
without loss of accuracy. Additionally, another reading station (not
shown) may be located after the overwriting station (of elements 18 and
19) to re-read the region and verify that the net magnetic moment of the
domain has in fact been reversed and the value of the data bit stored in
that region has been actually over-written.
Referring now to FIGS. 2a-2h, the process which we believe to be physically
occurring for our self-inverting data over-write method (suing the
apparent self-demagnetizing field of the thin-film magneto-optic recording
media) is illustrated. Prior to the time at which a first over-write
operation is to occur, the media layer 60 is at an ambient temperature Ta
less than the compensation temperature Tc of the magneto-optic material.
The recording regions each contain one bit of a first set of data. FIG. 2a
illustrates that, for the starting data set with the same data value,
e.g., a binary one state, in each of three sequential regions (N-1), N,
and (N+1), the net magnetic moments (symbolized by the broad arrows
62a-62o) are all directed in the same (e.g., upward) direction
substantially perpendicular to the media layer surface 60a and are all of
approximately the same amplitude. The amplitude and direction of the net
magnetic moment M is established by the relative amplitudes and direction
of the magnetic moment of the individual components of the magneto-optic
alloy. Here, the downwardly-directed moment 64a of the transition metal
(TM) component of the alloy is of smaller amplitude than the
upwardly-directed moment 64b of the rare earth (RE) alloy component, in
each subregion.
As a region N receives energy from the overwriting means (laser 18), the
temperature of that region is raised until the compensation temperature Tc
is exceeded. Because the impingent light beam 15 has a substantially
Gaussian energy distribution, the entire region N is not uniformly heated.
Thus, while the individual alloy component magnetic moments (e.g., moments
62a and 62b) and the net magnetic moments (e.g., net moments 62a, 62b,
62n and 62o) all remain substantially unchanged in subregions removed from
the region N receiving energy, those subregions nearer to the heated
region N receive energy from the fringes of the beam. Responsive to the
increased temperature, which is less than the compensation temperature in
these other regions (N-1), (N+1), etc., the magnetic moment 64c of the
transition metal TM component is decreased by some amount, which is not as
great as the decrease in the magnetic moment 64d of the rare earth RE
component; the net moment (e.g., net moments 65c' and 64m') of that
subregion is reduced. As the temperature increases, the reduction in the
magnitude of the net magnetic moment (e.g., net moments 64d' and 64l')
continues, responsive to the faster reduction of the RE moment 64f than
the reduction in the TM moment 64e, with closer location to region N. In
some subregions the compensation temperature is just attained and the
reduced amplitudes of the TM and RE moments 64g and 64h become by
definition, equal; the net magnetic moment 62e', 62k', . . . are of zero
magnitude (and define the periphery of the region N). Inward of the
regions 62 with zero net magnetic moment, the subregion temperature
exceeds the compensation temperature; the reduced amplitude of the TM
moment (e.g., magnetic moments 64i, 64k, 64m . . .) is now larger than the
reduced amplitude of the RE moment (e.g., magnetic moments 64j, 64l, 64m,
62f-62j'. . .) are all now reversed, having increasing magnitude but in
the opposite direction (e.g., into the recording layer).
The self-demagnetizing field appears to be applied by those subregions
still magnetized in the original (e.g., upward) direction; the component
moment directions are now re-inverted (FIG. 2c) in those subregions still
at a temperature greater than the compensation temperature. The component
moments in subregions within, but adjacent to, the region N periphery
remain fixed in the former direction (e.g., as shown by moments 64i and
64j) so that the net moment remains fixed in the new (now inverted)
direction. The component moments in the more central subregions, however,
are direction reversed to the original direction (e.g., the upward
direction, as at new magnetic moments 62g", 62h", 62i", . . .). As the
subregion temperature decreases upon cooling, after removal/off-switching
of the light beam, the amplitudes of the alloy component magnetic moments
increase to their ambient temperature values; as each subregion passes
through the compensation temperature, the net magnetic moment 62 thereof
is decreased to zero. As shown in FIG. 2d, in each subregion, at some
temperature less than the compensation temperature, the RE magnetic moment
(e.g., magnetic moment 64n') amplitude is again greater than the amplitude
of the TM magnetic moment (e.g., magnetic moment 64m') and the direction
of the net moment (e.g., net magnetic moment 62h') is again in the same
inverted direction. The rest of the region N subregions experience the
same inversion of their net magnetic moments (e.g., net magnetic moments
62g', 62i'. . .). Thus, the subregions of region N all have net magnetic
moments aligned in a direction opposite to the alignment direction prior
to the heating of the region N to a temperature greater than the
compensation temperature. As the opposed moments nucleate a magnetic wall
(as at the periphery subregions 66a and 66b upon opposite sides of region
N), a stable magnetic domain is created, with diameter D, now storing the
new value of the associated data bit.
Referring now to FIGS. 2e-2h, at some later time, a comparison of the data
value (e.g., a logic zero) stored in the domain and the logic value (e.g.,
a logic one) of a new bit of binary data for storage in domain N indicates
that the region N must be over-written. This decision enables the write
laser diode and causes region N to be again heated, in the absence of any
substantial intentional external bias magnetic field, to a temperature in
excess of the compensation temperature. The domain wall, as exemplified by
wall portions 66a and 66b of FIG. 2e is not abruptly destroyed; there is
an inversion of the net magnetic moments 62 of the subregions within
region N, due to the reversal of the alloy component magnetic moment
dominance. Thus, the net magnetic moments 62f'-62j', of those subregions
within the heated region N, are not only modified in amplitude by the
Gaussian energy distribution of the impinging light beam, but are also
inverted in direction, to point upwardly and away from the magneto-optic
material layer. The self-demagnetizing field of the immediately-adjacent
subregions (i.e., the upwardly directed net moments of the subregions 62f'
and 62j") cause a reversal in the local magnetic field in the center
subregion(s), as here represented by subregion 62h", of the heated region
N, as shown in FIG. 2f, so that at least one of the interior subregions
now has the net magnetic moment thereof directed in an again-inverted
direction (e.g., the downwardly directed net magnetic moment 62h" of a
smaller, region of diameter D.sup.1, less than region/domain diameter D,
within the larger domain/region N). A second, inner domain wall, as shown
by opposed wall portions 68a and 68b , is now present about the subregion
periphery. Local wall motion causes the portions of the inner wall to
expand to the locations of the associated portions of the outer wall; the
two walls meet and mutually annihilate one another, so that the diameter
D", of the region in which the net magnetic moments 62g"-62i" (see FIG.
2g) are still inverted, is greater than the domain diameter D'. As the
temperature of the region N is decreased to below the compensation
temperature, by cooling after cessation of the heating pulse, the relative
amplitudes of the RE and TM alloy component magnetic moments change and
the subregion net magnetic moments are again all directed in the same
direction (e.g., the upward direction for net magnetic moments 62e-62k, of
FIG. 2h). The data value stored in region N has, therefore, been inverted
(e.g., to a logic one value) from the state of the data value (e.g., the
logic zero value) previously stored in that region.
For the erase-before-write operation, the apparatus is modified as shown in
FIG. 3 by adding a second beam of heating radiation 65 located between the
write beam 15 and the read beam 34 focused on the same disk track 42. A
laser diode 68 provides a beam of optical radiation which is focused by a
lens 69 to direct the laser beam to the track 42 on the recording medium
in a manner similar to laser diode 18 and lens 19. Laser diode 68 is
connected to an erase laser power supply 64 and laser diode 18 is
connected is connected to a write laser power supply 62. The laser power
supplies provide current pulses to their respective laser diodes in
response to applied trigger pulses. The pulse magnitude is selected in
accordance with the characteristics of the medium, the thickness of the
medium, and the rotating speed of the disk. For twelve milliwatts of laser
power in the diodes, the write pulse provided by write laser power supply
62 could, for example, be 150 nanoseconds in duration, and the
corresponding erase pulses provided by erase laser power supply 64 would
have a pulse duration of approximately 90 nanoseconds.
The circuit arrangement shown in FIG. 4 can be used with the FIG. 3
apparatus to provide a selective erase-before-write system. The disk data
read output from detector 40 (terminal 40a in FIG. 3) is connected to the
disk data input of an AND gate 70, the other input to the AND gate being
connected to receive a write command. The data to be recorded is supplied
to an input data terminal of an AND gate 74, the other input of this AND
gate also being connected to receive the write command.
The output of AND gate 70 passes through an N.sub.1 -domain data delay
circuit 71 and the output of AND gate 74 passes through an N.sub.2 -domain
data delay circuit 75. The delay in circuit 71 corresponds to the time
required for a domain to travel from the road station (beam 34) to the
erase station (beam 65). The delay in circuit 75 corresponds to the time
required to travel from the read station to the write station (beam 15).
Preferably, the delay circuits are controlled by clock pulses at terminal
78 synchronized with the incremental disk movement.
The outputs of delay circuits 71 and 75 are connected to the data D inputs
of flip-flop circuits 72 and 76, respectively. The clock C inputs receive
conventional sync signals. The Q output 73 of flip-flop circuit 72 is
connected to the trigger input of erase laser power supply 64 and the Q
output 77 of flip-flop circuit 76 is connected to the trigger input of
write laser power supply 62.
In explaining the operation of the erase-before-write system a "0"
corresponds to a low signal level and the initial bias state of the
magneto-optic medium, whereas a "1" corresponds to a high signal level and
the magnetic state of a domain recorded on the magneto-optic medium. In
the presence of a write command, a domain passing the read station
produces a "1" which passes through the conditioned AND gate and delay
circuit 71 to produce a trigger pulse to energize laser diode 68. The
laser pulse from diode 68 erases the domain as it passes under the erase
station. Thus, in the presence of a write command, any domain detected at
the read station is erased at the erase station, and therefore the
recording medium is in a known state (the initial bias "0" state) when it
reaches the write station. If the data input for AND gate 74 is at the "1"
level when the write command is present, this data value passes through
delay circuit 75 to develop a trigger pulse to pulse the write laser diode
18 to "write" a domain as the same region on the recording medium passes
under the write station thereby recording "1" on the recording medium. On
the other hand, if the data input is a "0", then the write laser diode 18
is not pulsed as the region passes the write station and the recording
medium remains at the "0" value.
AND gate 74 compares the input data to the known "0" state of the medium at
the write station and passes a pulse through the delay to pulse the write
laser 18 only when the input data bit is different from the known state.
FIG. 5 illustrates a circuit arrangement for a pulsed erase-before-write
which does not | | |
