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Description  |
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FIELD OF THE INVENTION
This invention relates to an optical disc assembly for use as an optical
memory.
DESCRIPTION OF THE RELATED ART
Analogous in some respects to the familiar audio disc, information on an
optical or video disc is stored on the disc surface as laser ablated pits
of approximately 1 micron across, the pits located along a single spiral
track or multiplicity of concentric tracks with a track spacing of
approximately 2 microns. The disc is both written and read by a beam of
laser light focussed to a spot of about 1 micron at the disc surface. In
the write mode, the focussed laser light displaces pits in the disc
surface and in the read mode, the relatively lower energy laser light is
used to monitor data corresponding to the detection of successive pits as
the disc rotates. A reflected beam is monitored and from this, information
that has been recorded is derived.
Two types of discs are currently most common. One type, known as a sealed
disc assembly, has a central hub and outer circumferential ring between
which two layers, a recording layer, typically of 2.5 millimeters thick
aluminum and a cover sheet of Mylar.RTM. or other plastic material are
stretched taut. Separation of the sheets is ensured by spacers and by
tensioning the cover sheet. Tension must be uniform in order that the
cover sheet is not birefringent. The cover sheet must also be of uniform
thickness and high stability. If the cover sheet does not have these
properties then read light which is directed through the cover sheet to be
focussed at the data surface is sensitive to polarization direction and
the focussed spot tends to oscillate in position and possibly change size
as the disc rotates.
In another example of optical disc, a cover film typically of
polymethacrylate overlies and directly contacts the recording surface of
the disc. Other examples of optical recording discs are known but most of
those currently used have a protective layer either directly contacting,
or spaced from, the memory recording surface of the disc.
One of the requirements for using an optical disc as a high density memory
device, storing typically 2 gigabytes on a 30 centimeter outer diameter
disc, is that the stored data should be accessed as quickly as possible.
For a disc of this size, an acceptable access time for moving a read beam
from a rest position to a designated read position to commence reading the
stored data is 100 milliseconds. To ensure a fast access time, an accurate
and rapid servo system is required so that from the moment that movement
of the read beam commences, the position and velocity of the read beam is
accurately monitored and required subsequent movement to read the target
data is continuously and accurately predicted.
One aid to such a servo system which has been used in known optical discs
are the spiral or concentric tracks consisting of stored data.
In one access scheme, repeated, position-characteristic header data or
address fields are stored within several data tracks so that as a scanning
beam radially traverses the disc, the header data is monitored. To access
a track between any two successive header data tracks, the intervening
data tracks are simply counted as a decelerating scanning head moves
across them. As the scanning beam nears the target track, the rate at
which the scanning beam crosses the tracks is reduced still further and
eventually the scanning head is halted at the target track to read the
target data. The data as well as occupying a track at a particular radial
position is also in a predetermined arc of that track and it is important
therefore to know where the disc is circumferentially relative to the
scanning beam. In known optical discs, the data tracks have data sequences
indicative of a circumferential index position on the disc.
As well as providing information from which its position can be determined,
the scanning beam is also used to derive other information from the disc.
Most importantly, it must read the memory data. It must also ensure that
the scanning beam itself remains centrally on a particular track as long
as it is required there. It must ensure too that the scanning beam is
focussed at the surface of the track in order accurately to monitor the
light variations corresponding to both memory-related and position-related
data.
As indicated previously, the memory and position control data is typically
recorded as a succession of pits and lands which together make up data
tracks, the pits and lands causing a certain change in monitored light as
the disc rotates underneath the scanning beam. The monitored change may be
a change in reflectivity, scattering, or diffraction pattern.
One problem in using such data tracks is that initially, before data has
been recorded on the disc, no tracks as such exist to assist in setting
data correctly on the blank disc.
Secondly, any part of the data recording surface used for storing
positional data cannot be used for storing memory data.
Thirdly, the high surface density data although ideal as memory data, is
not particularly adapted to provide positional data for use by a servo
system in controlling the scanning beam. For the latter purpose,
positional data which is more gross and has a lower surface density than
the memory data would be more suitable.
In this specification, it will be understood that data is optically
recorded data recorded in binary form, for example as discrete relatively
light and dark regions, or relatively reflecting and non-reflecting
regions, etcetera. The "surface density" of data is a measure of the
number of such regions in a unit area, a high surface density connoting a
high number of such regions in a unit area. The "grossness" of data is a
measure of the size of such a region.
Bruce et al, "Video Disk with Updating Feature", IBM Tech, Disc. Bulletin,
Vol. 22, No. 3, Aug. 1979, p 1183 describe an optical video disc with a
magnetic layer which can also serve as a protective layer, the magnetic
layer being used for indexing of the optical data. Bruce et al teach
mounting magnetic and optical read heads in conjunction. The video disc
data is high surface density data and the magnetic layer is low resolution
data. In Bruce et al, the top layer is made updatable to permit the
alteration of identifying data as data is logged on the underlying optical
disc surface. Since the identity of data on the Bruce et al cover is to be
changed periodically, a recording medium which is erasable and re-writable
is needed. Hence, Bruce et al use a magnetically recordable top layer.
SUMMARY OF THE INVENTION
According to one aspect of the invention, there is provided an optical disc
assembly comprising an optical disc and a protective cover, the optical
disc adapted to receive thereon an optical recording consisting of a
series of laser ablated pits recorded as a first series of concentric data
tracks, said pits being of a predetermined size and surface density, the
protective cover having thereon a permanent optical recording consisting
of a series of visible marks, the size of the pits being an order of
magnitude smaller than the size of the marks and the surface density of
marks being an order of magnitude smaller than the surface density of the
pits in the tracks.
Preferably, the marks are relatively light-absorbing printed regions but
can alternatively be relatively rough or embossed regions on said cover.
Preferably, data on the protective cover is positional data used to map the
underlying optical disc. To this end, the cover can have an outer marginal
region in which is stored relatively high surface density, radially
extending encoding marks and an inner marginal region in which are stored
lower surface density radially extending sector marks, the sector marks
including a unique index mark. Said unique index mark can provide a
circumferential start position from which sector marks signifying the
circumferential start position of memory data packets stored or to be
stored on the optical disc are referenced. The rate at which a dedicated
scanning beam traverses the more densely positioned encoding marks can be
used to compensate for variation in disc rotational speed.
The cover data can further comprise a series of data tracks in an annular
area radially inwardly of the area overlying an annular read/write zone on
a recording surface of the underlying disc. Positional data in the data
tracks of said annular area of the cover can be used initially for setting
the position of memory data to be recorded on blank portions of the
annular read/write zone. Such data can be used also for monitoring and
controlling radial movement of a scanning beam for memory data access.
Radially inside said annular area, the cover can have recorded thereon a
reference or closure track and disc identifying data. The inside reference
track can be used to ensure that a blank disc is accurately centered
before memory data is first written. The closure track thus acts as a
reference to ensure that memory data is not eccentrically loaded on the
optical disc. The identifying data marks can be a simple bar code to
differentiate the particular optical disc from others.
The positional information can be optically recorded on the protective
cover surface in any of a number of ways. Firstly, it can be a series of
dark and light areas produced for example by selectively printing a dark
film on the cover. When the positional data scanning beam traverses the
disc cover, the positional data is monitored as successive changes in
reflectivity. The reflectivity of the cover sheet must not however, vary
by more than 2 percent otherwise the transmission of a scanning beam for
scanning the memory surface is non-uniformly affected with the attendant
risk of reading memory data incorrectly.
Secondly, the data can be a series of rough and smooth areas produced for
example by selectively etching the protective cover through a mask. When
the positional data scanning beam traverses the disc, the positional data
is monitored as successive changes in scattering.
Thirdly, the data can be a series of pits and lands produced by etching.
When the positional data scanning beam traverses the disc cover, the
positional data is monitored as successive changes in a diffraction
pattern.
Typically, the positional data on the cover is recorded as linear elements
of the order of 50 microns wide.
In using the disc assembly, at least one positional data scanning beam is
used which is distinct from a memory scanning beam used to read the memory
data at the optical disc recording surface. Because the scanning beams are
focussed at different levels and at different lateral positions and
because the data bits which they read are an order of magnitude different
in size and surface density, the light which is reflected from the memory
surface into a positional data channel, and light reflected from the
positional surface into a memory data channel, do not cause error.
The positional data recording surface is preferably the outer surface of
the protective cover although it could be the cover inner surface. Because
positional data stored at the cover surface is relatively more gross and
has a low surface density in comparison to memory data stored at the
optical disc surface, then the outer protective surface of the disc may
have dust or contaminants adhering to it without incurring error.
One method of recording the positional data is by photolithography to
produce regions which are relatively dark.
In another alternative particularly for an optical disc formed with a
protective film contacting the memory data recording surface, the film can
be embossed using a master disc negative.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the invention will now be described by way of example,
with reference to the accompanying drawings, in which:
FIG. 1 is a cross-section, not-to-scale, through part of an optical disc
assembly according to the invention;
FIG. 2 is a schematic plan view of part of a protective cover of the FIG. 1
optical disc assembly;
FIG. 3 shows a detail, in perspective, of the cover and the underlying disc
of the FIG. 1 assembly; and
FIG. 4 is a block schematic view of a control system for use with the
optical disc assembly.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIGS. 1 and 2 in detail, there is shown an optical disc
assembly 10 for use as an optical memory. The assembly has a 30 centimeter
diameter, 2.5 millimeter thick optical disc 12 of aluminum. Overlying the
disc is a cover sheet 14 of Mylar.RTM. which functions to protect a memory
recording surface 16 of the underlying disc 12 from dust and contaminants.
The 100 micron thick layer 14 and the aluminum recording disc 12 are
spaced apart by spacers 18 and are uniformly tensioned between a metal rim
20 and a central hub 22.
At an exposed outer surface 24 of the protective cover 14 is stored
positional data and at the protected upper surface 16 of the aluminum disc
12 is stored memory data. The memory data consists of a series of pits and
intervening lands, the pits 15 typically being 1 micron across and of 1.5
micron spacing and forming concentric data tracks. In contrast, the
positional data on the cover exists as a series of printed marks 17 of
about 50 microns in width.
To write on a blank disc, that is, to form the pits at the memory surface
16, the optical disc assembly is rotated on a turntable under a write head
(not shown). The write head includes a 10 megawatts gallium arsenide diode
laser which is driven to provide 50 to 100 nanosecond pulses. Light from
the laser is focussed at the memory surface 16 of the optical disc 12 and
is guided to a desired radial positon on the optical disc by an
optomechanical drive. Then at selected angular positions of the optical
disc assembly, the diode laser is energized momentarily. The action of the
laser beam is locally to displace regions of an optically active film 21
on the substrate to create pits about 1 micron across and 50 nanometers
deep, the displaced material being partly relocated and partly vapourized.
The optically active film 21 is a 150 nanometer dye polymer layer which
overlies firstly a 40 nanometer aluminum reflecting layer 23 and secondly
a 1000 nanometer plastics smoothing layer 25. The diode laser is driven on
and off in accordance with a digital data signal when the laser beam is
focussed at a selected region of the disc memory surface.
The encoded data at the optical disc surface is subsequently read using a
read head shown generally as 26 in FIG. 2. The read head has a lower power
laser, typically a helium-neon laser with an output wavelength of 633
nanometers and continuous wave power output of 0.25 megawatts. The
digitally encoded signal is monitored by analyzing the change in
diffraction pattern of diffracted light as discontinuities corresponding
to the boundaries of pits pass under the read beam. Other read out methods
can be used depending on the type of optically active film used.
Particularly for use in high density optical memories in which typically
more than 2 gigabytes are stored in a 30 centimeter diameter disc, there
is a need to access memory data quickly and accurately. To access such
data, the optical read head 26 moves radially outward from a reference
position as the optical disc assembly 10 rotates beneath the head.
Simultaneously, positional data is read from the disc assembly using a
second read head 48 physically mounted with the read head 26. This data is
processed to ensure that the read head 26 moves quickly towards a position
at which the target item of memory data can be read.
By this invention, the positional data is mainly or wholly recorded on the
surface 24 of the protective cover 14 and not on the memory surface 16 as
is common in known optical disc systems.
In the embodiment shown in FIGS. 1 and 2, positional data is encoded
optically as variations in surface reflectivity of the protective cover.
As shown in the plan view of FIG. 2, the positional data occupies a number
of zones.
Firstly, there are two series of optical encoding radial marks occupying
marginal bands 28 and 30 of about 0.1 millimeter width at the outer edge
of the cover. The first band 28 has 5,000 marks 32. A fixed positional
reader 34 is used to count the rate at which the marks pass under it in
order to regulate the disc speed. Radially within this band is a second
band 30, the sector mark band, the sector marks 36 indicating the position
of address or header data within the encoded data within the memory
surface. Typically the sector mark band has 128 marks one of which is a
primary index mark 38. A second fixed positional reader 40 is used to
monitor sector data.
Inside this band is an annular area substantially in vertical registration
with an annular read/write area of the underlying memory surface 16. For a
30 centimeter disc there are 300 positional data tracks within this area
corresponding to 30,000 memory data tracks on the underlying memory
surface when fully recorded.
Radially inward of area 42 is a reference band which includes firstly, a
closure track 44 and secondly, disc identifying data 46. The closure track
is used following positioning of the disc assembly on a turntable to
assess whether closure of data tracks can be expected. If the disc
assembly is eccentrically positioned, then the extent of this is monitored
and, if it is too great for the memory read head 26 to compensate for,
then the disc is repositioned using the centering hub 22. The identifying
data 46 is recorded simply as a bar code, individual elements of which
extend radially, the code itself extending circumferentially.
Data in the annular area 42, the reference band containing the closure
track 44 and identifying bar code are read by the read head 48 which
shares a common mechanical drive with the memory read head 26 but has an
optical system which is at least partly distinct from the optical system
associated with read head 48.
The read head 48 has a rest position adjacent to the center of the disc
assembly. In this position, the corresponding scanning beam monitors the
identifying bar code and the closure track for assessing disc identity and
eccentricity. Then when an item of data at the memory surface is to be
accessed, the two heads 26 and 48 move radially outwards at high speed
with the control system of FIG. 3 being actuated.
As previously indicated, the optical recording on the protective cover 14
takes the form of periodic variations in reflectivity. To obtain this, a
pattern is printed on the top surface of the cover using photolithography.
The printed material is a developed photoresist which is compatible with
Mylar.RTM., and is highly stable and abrasion resistant. The film is less
than 10 microns thick to produce a contrast in reflectivity compared to
the unprinted Mylar.RTM. of less than 2 percent. A low contrast is
important since if contrast is too high, then accuracy of reading of the
memory data can be detrimentally affected. As an alternative to developing
a photoresist film through a mask, the pattern is screen printed.
As an alternative to printing onto the top surface of the cover sheet,
selected regions of the cover sheet are roughened to cause a variation in
scattering of a positional read beam. To do this, a mask is formed over
the protective cover which is then etched by sputtering or reactive ion
etching through windows in the mask.
In yet another alternative, the cover sheet is scribed with a needle such
as a phonograph type diamond stylus.
For a disc which, in contrast to that described in FIGS. 1 and 2, has a
thick film typically of polymethylmethacrylate in contact with the memory
surface, the positional data on the cover surface can be formed by
embossing. To do this a master disc is formed by scibing a copper sheet
and then nickel plating the scribed surface. The master disc is then
pressed onto the film of polymethyl methacrylate after heating to render
the film deformable. This technique is well known in the production of
phonograph records.
Referring to FIG. 4, there is shown in schematic form a control system for
use with the disc of FIGS. 1 and 2. Essentially in use when accessing a
data item, positional data on the protective cover is initially used to
obtain a coarse evaluation of the actual position and movement of the
physically combined read/write and coarse monitoring heads 26 and 48
respectively, so that the required movement of the read/write head 26 to
access the data item can be predicted. Initially, the tracks on the cover
sheet are counted as they are successively crossed by the coarse
monitoring head 48. As the read/write head nears the target track, data in
the cover tracks is read to confirm that the head position are assessed
from the data track count is correct. The coarse monitoring system is then
switched out and a fine, high resolution monitoring system using the data
channel from the memory surface is utilized. As shown in FIG. 4, the
encoder band and the sector band on the cover are also monitored and the
data obtained are used respectively to control the disc drive and the
timing of data writing and reading at the memory surface by the read/write
head 26.
Details of the monitoring and control system are not shown since they exist
in known optical disc monitoring and control systems in which control of
read/write head movement is based on positional data present on the memory
recording surface. However it will be recognized that a monitoring system
used to interrogate data on the protective cover need not be so complex
nor so difficult to set up as one for obtaining data from the memory
surface since no separation or discrimination between the positional data
channel and the memory data channel is required. Further the cover read
head need not have such an accurate beam focussing mechanism as the disc
read head. In addition, for cover data, a relatively slow counter can be
used to monitor track crossings and data analysing circuitry need not be
so complex since there is little risk of confusing the identity of
radially adjacent tracks on the cover.
Multilayered optical discs have been proposed by for example, Thompson -
CSF. However, these known discs differ from the currently proposed disc in
that the multiple recording surfaces are of the same type, the recorded
data at each surface consisting of a combination of positional and memory
data, and the data at the several surfaces being encoded at the same bit
size and surface density.
It will be recognized that whereas in the embodiment described, positional
information is recorded only on the protective cover and not at all on the
memory data surface, some positional data could be stored at the memory
data surface. Also, the positional data on the cover sheet, further gross,
low surface density data may also be carried. For example, this can
include a disc serial number, and a list of optical disc sectors which
should not be used owing to their being flawed.
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