Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
OPTICAL DISK DATA FOKMAT EMPLOYING RESYNC~ONIZABLE DATA SECTORS
BACKGROUND OF THE INVENTION
This invention relates to optical data storage systems, and more
particularly to a means for organizing data on an optical disk or
platter of such a system in order to provide efficient handling,
storing, detection, and processing of said data.
Over the past two decades or so, there have been two major
trends in the data processing industry that have worked together to
revolutionize the way that information is gathered, stored, and
interpreted. The first trend has been the expansion of
technological sophistication, as exemplified by the microcomputer
chip. That is, computing power, which once required roomsful of
equipment and kilowatts of electrical power to operate, can now be
found in very small silicon ch;ps. The second trend has been the
cost of purchasing such computing power. Particularly in the area
of memory--as costs have dropped and capacities have
increased--there has been an inevitable rush to take advantage of
the new-found memory space and fill it with information. In this
respect~ the demand for more memory and storage space has always
seemed to outstretch the available supply of such memory space.
~ nfortunately, for users with exceptionally large data storage
needs, the magnetic-based storage peripheral devices adapted for use
with high performance computers (i.e~, magnetic tape and disk
drives) have not been able to fill the need for more storage space.
Traditionally, the need for more storage space in such large data
storage systems has been addressed by merely adding additional
magnetic disk drives and/or magnetic tape drives. This has been
costly both in terms of expense (purchase/lease price plus
maintenance costs) and Eloor space. Moreover, even though ~here
~'
. .
have been some significant strides in recent years with respect to
increasing the data storage capacity of these magnetic-based storage
devices, the theoretical design limits of such systems are rapidly
being approached. ~lence, merely adding more magnetic disk or tape
drives is no longer viewed as a practical alternative to the ever
increasing need for storing more and more information. It is
therefore apparent that a new type of data storage system is needed
in order to handle the large amounts of data that information users
need to store.
Optical technology -that is, the technology of using a laser
beam to burn or otherwise mark very small holes on a suitsble mediurn
in a pattern representative of the data to be stored, which pattern
can subsequently be read by monitoring a laser beam directed ~hrough
or reflected off of the previously recorded marks--has been
available in laboratories for some time. ~nfortunately, however,
such laboratory technology has not provided a cost effective
alternative for use in data storage products. This is because the
optical components have tended to occupy entire rooms and the power
associated with operating the laser and associated components has
been enormous. Further, such laboratory systems are not easily
interfaced with existing high performance computer systems. That
is, the techniques used to format and input the data have been
totally incompatible with more conventional formatting and data
processing techniques used in the rnagnetic-based storage systems.
Moreover, the few optical storage systems that have been
commercially introduced in the last few years have primarily related
to the storing of video signals (image storing devices) as opposed
to the storing of digital information. Further, the few digital
optical storage devices that do exist do not represent a viable
alternative or supplernent to the existing peripheral magnetic-based
storage devices for the user of large data bases of information.
A continuing problem that has existed with whatever type of data
storage system is used is the problem of minimizing the errors that
occur during read or write. The number of errors that occur in such
a system is typically measured by a parameter referred to as the
"bit error rate." This parameter is typically expressed as a number
indicating the number of good bits of digital data that can be
obtained for every bad bit of data that occurs. Thus, a bit error
rate of 100,000 (lOE-~5) indicates that 100,000 bits of data can be
read or played back before a bad or incorrect bit of data will be
encountered. In order to provide a viable data storage system, bit
error rates in excess of lOE+12 are generally required.
Numerous Error Correction codes (ECC) and similar error
correcting scheme~ are known in the art in order to improve the bit
error rate of data processing systems. The very e~istence of such
ECC schemes evidences the continuing and recurring problem of
reducing errors that are introduced into such processing systems~
Errors can principa]ly originate from one of three sources: (1) in
the write channel (i.e., the data is written incorrectly); (2) in
the read channel (i.e., the data is read incorrectly); or (3) in the
storage medium (i.e., even though the data is initially written
correctly, the storage medium may change with time 50 as to alter
the data to make ;t incorrect). Of these three potential sources of
error, most known ECC schemes for use with peripheral data storage
systems are directed only to correcting errors that occur in the
read channel. Sources of error introduced by aging media, item (3)
above, are minimized in magnetic-based storage devices by merely
re-writing the data after a prescribed period oE time. (This
technique is co~nonly referred to as "refrèshing.") That is, the
old data is read, stored in a buffer memory, the media is erased,
and new data is then written on the media in p]ace oE the old data.
This reEreshing technique is, of course, only available when
erasable media is used. Optical media, on the otherhand, is
generally not considered erasable because of the manner in which the
marks are placed on the media by the laser beam. That is, once a
hole or pit or other mark is burned or ablated on the media by the
laser beam, it is difficult to remove that hole or pit or mark.
However, over time, the media may "flow" or other changes may occur
thereto so that the hole, pit, or other mark is somehow altered to
the degree that light reflected therefrom might be incorrectly
sensed.
A further challenge facing the user of large information systems
is the manner in which the data stored is accessed. Certain types
of data need only be accessed occasionally, and therefore the access
time thereto is not critical. Other types of data are constantly on
demand, and therefore must be accessed very rapidly if the system is
to operate efficiently. Data bases that are only accessed
sequentially or that only need to be accessed occasionally have been
traditionally stored on magnetic tape. Data bases that must be
accessed quickly, and usually in a random fashion, are stored on
magnetic disks. (Access times are significantly faster with magnetic
disks because the read/write head of the disk can radially move with
respect to any area of the disk and quickly locate a data set within
one or two revolutions of the disk.)
An important factor in determining how fast a given data set can
be accessed is the manner in which the data is formatted. Coupled
with formatting the data is the need to properly index the same so
that a desired set of data can be quickly located. The size of the
index needed unfortunately grows as the amount of data stored
increases. In magnetic disk art, this has generally not been a
major problem because all the magnetic disks are on-line at all
times. Thus, one entire disk surface, or even several disks, can be
dedicated to indexing information. However, in optical storage
systems, it is desirable to have the optical disk removable from the
disk drive, much as a record is removed from a phonograph. In this
way, only enough drives required to access the data that is
continually needed will have to be coupled to the host computer or
CPU. Other data, less coTmonly used, may be stored on a disk and
the disk may be physically removed and stored in a suitable
location, just as magnetic tape is now removed and stored. Hence,
by providing removable optical media, the advantages of both
magnetic tape and disk storage systems may be realized. However,
when such removable media is used> extreme care must be exercised in
defining the formatting and indexing functions so as to preserve
most of the data storage space Eor the storage of user data, not
indexing and housekeeping data.
It is thus apparent that there is a need in the art for an
optical storage system that not only meets the data capacity and
density needs oE the exploding data processing industry, but that is
also compatible with existing and future high performance CPU
systems. Preferably, such an optical storage system will supplement
(rather than replace) existing magnetic-based storage systems. That
is, a few magnetic disk drives and a few optical disk drives coupled
to a main CPU should be able to handle all the exis~ing and future
data storage needs of the high information user, instead oE the
roomsful of magnetic disk and tape drives that such a user must now
have installed. Moreover, it is also apparent that there is a need
in the art for an optical storage system that provides acceptsble
data bit error rates, at least on the order oE no more than one bit
error Eor every 10E~12 bits of inEormation. Also, the overall data
access times must be compatible with the high speed, high
performance computers that are presently available. The optical
storage system herein disclosed is directed to satisfying these and
other needs.
SIMMARY OF T~E INVENTION
It is an object of the present invention to provide an optical
storage system that can be connected to a host CPU via a storage
director that may be shared with other peripheral devices, such as a
magnetic disk drive.
It is another object of the present invention to provide a data
orgainzation scheme for use in such an optical storage system that
allows for the efficient storage, handling, detection, and
processing of data written to and read from a disk of such system.
It is a further object of the present invention to provide such
an optical storage system that uses removable disk media which, when
inserted into an optical disk drive, offers the advantages of randcn
access storage such as are available with magnetic disks; and, when
removed from the drive, offers the advantages of low cost sequential
storage, such as are available with magnetic tape storage devices.
A further object of the invention is to provide an optical
storage systcm that provides up to four gigabytes of user data to be
permanently stored on a single disk for at least 10 years.
Another object of the present invention is to provide such an
optical storage system wherein error detection and correction
circuitry is used to insure an acceptable bit error rate.
Still a Eurther object of the present invention is to provide an
optical storage system wherein data access times are comparable with
existing high speed magnetic disk storage devices, thereby allowing
the optical storage system to be eEficiently used with existing (as
well as future) high performance CPU systems.
The above and other objects oE the present invention are
realized in an optical storage system that includes a platter or
media upon which the data is written, and a drive into which the
platter is inserted when it is desired to read or write data. A
~L2~
storage controller that provides the necessary interface between a
host CPU and the optical disk drive, advantageously allows other
(existing) types of peripheral storage devices to be used with the
CPU along with the optical storage system. The host CPU initiates
the request to read or write data to the optical drive.
The media or "platter" upon which the data is stored is
physically housed in a cartridge when the platter is not mounted
within the drive. The entire cartridge is inserted into the drive
by the user when it is desired to read or write data therefrom or
thereto. The drive automatically removes the platter from the
cartridge and mounts it for rotation on a suitable spindle
mechanism. The cartridge advantageously protects the platter when
not in use and allows for the easy storage thereof. ~ suitable
platter identification number is optically written onto the platter,
as well as onto the cartridge by other visible means.
The data format on the platter includes bands, tracks, blocks,
and sectors. The platter surface is divided into a prescribed
number of concentric areas that are referred to as "bands." Each
band contains a prescribed number o~ concentric data tracks
therewith;n upon which data may be written. Data is organized on
each track in fixed length logical units referred to as "blocks."
In the preferred embodiment, there may be short blocks (e.g., 128
users bytes) and long blocks (e.g., 7904 user bytes). (As is known
to those skilled in the art, a byte is a set number of data bits,
typically 8.) Each track is physically divided into a fixed number
of equal length segments referred to as "sectors". The sector is
the smallest unit of encoded information, and its boundaries are
predefined by sector marks placed on the platter. User data ;s
encoded and written to specified types of sectors when stored on the
platter. Other types of sectors are used to identify media defects
and incompletely written user data.
~L2~
The optical drive includes means for automatically re~.oving the
platter ~rom the cartridge and mounting the platter on a suitable
spindle for rotation. Several servo systems provide the means for
locating a specific area on the surface of the platter and for
directing the appropriate read or write laser beams thereto. In
order to provide the needed access time, the servo systems include a
coarse and fine seek system whereby a given area of the platter will
be reached quickly using the coarse seek function, such as locating
a desired band, and thereafter the fine seek function can be used to
locate a specific data track therein. Other servo systems provide
the function of tracking a given data track, automatic focusing of
the read and write laser beams, and spin control of the spin motor.
A laser/optical unit includes three laser sources: a write laser
which generates a beam that marks the platter surface; a read laser
which generates a lower powered beam that reflects off the surface
of the platter to detect marks recorded during a write operation and
for subsequent read operations; and a coarse seek laser which senses
carriage position error. (The carriage moves in and out radially
with respect to the platter as controlled by the coarse servo system
in order to locate a desired band on the surface oE the platter.)
The laser/optical unit also includes the mirrors, lenses, prisms,
and other optical components that are needed for generating and
directing the write and read laser beams to and from the des;red
locations.
A read/write channel is also included within the optical drive.
The channel modulates and controls the write laser beam in response
to data signals received from the host CPU through the storage
director. The read/write channel also amplifies, filters, and
converts to digital form, the information received from the read
laser beam reflections. A clock signal is also extracted from the
read data. Appropriate error detection and correction circuitry is
'~
also ;ncluded within the optical drive.
Advantageously, the storage controller used to cormunicate with
the optical drive to and from the host CPU may be any conventional
controller for use with existing peripheral disk storage products,
modified with appropriate software or code. No hardware changes are
required.
In accordance with one specific feature of the present
invention, the optical storage drive employs Dynamic Defect Skipping
(DDS) to eliminate data errors that occur during a write operation.
When a data error occurs while writing to the optical platter, it is
immediately detected by using a read back check beam, and the data
in the error is then rewritten. Consequently, all data written to
the platter is correct. Appropriate tags or flags are used to
identify incorrectly written data so that such data is subsequently
ignored during a read operation.
Advantageously, three of the data bands on each platter are set
aside for housekeeping and maintenance functions. One band is
reserved as an index band in order to keep track of where various
logical records are stored. Another band is a table of contents and
keeps a history of the records stored and their respective status on
that particular platter. The third band is used for maintenance and
test purposes and provides some reference signals, pre-written on
the platter, which can be used during various maintenance operations
of the drive.
In summary, the particular platter format and data organization
scheme used with the present invention, coupled with the protective
cartridge in which the removable platter is housed, and the dynamic
defect skipping and other error detecting and correcting features
included within the optical disk drive, all combine to provide a
versatile, accurate, efficient alternative and/or supplement to
existing magnetically based data storage peripheral devices.
9a
Thus, in accordance with one broad aspect of the invention, there
is provided an optical dri~e system for storing digital data and selectively
retrieving said data in response to commands issued by a host CPU, said
ilOst CPU being coupled to said optical drive system through a storage control
unit, said storage co~trol unit comprising a plurality of storage directors,
a :~irst of which storage directors is dedicated to passing data to and from
a platter removably mounted within said optical drive system, said data
being organi.zed in a foImat in said platter that comprises: a multiplici.ty
of concentric tracks of data written on the surface of said platter; and data
blocks, each of said data blocks comprising a selected sequence of contiguous
data sectors, each data sector comprising a fixed length portion of one of
said tracks of data.
In accordance with another broad aspect of the invention there
is provided an optical data storage system for permanently storing binary
digital data comprising: a platter having a surface that may be permanently
marked with a write laser beam of sufficient power; means for removably
mounting said platter :in optical communication with an optical read/write
head; means for generating a write laser beam that is modulated by a binary
digital data signal, said write laser beam assuming ON and OFF conditions as
a function of the modulating binary signal; means for directing said modulated
write laser beam through said optical read/write head to a selected point on
the surface of said platter for a prescribed time period, the encrgy associated
with the ON condition of said write laser beam within said prescribed time
period being sufficient to permanently mark a spot on the surface of said
9b
platter at the selec-ted point; means for controllably crea-ting relatiye motion
between said read/w~ite head and the~ surEace of s~i~d plat~er so that said
surface may be selectively marked wi~th data; means for organizing the data
marked on the platter so that it may be readily detected and processed; means
for ensuring the correctness oE the data markecl on the sur:Eace of the plat-ter
by said write laser beam; and means :Eor optically reading the data marked on
the surface of the platter.
In a.ccordance with another broad aspect of the invention there is
provided a block format for organizing data written on a storage disk,
1() said storage disk having a multiplicity of concentric data tracks located
thereon on which data may be stored, said klock format comprising a data
block that includes a selected sequence oE contiguous data sectors~ each
data sector comprising a fixed length portion of one of said data tracks,
said data sectors having a selected one of a plurality of signal types
written therein.
In accordance with another broad aspect of the invention -there is
provided a method of organizing data that is to be written on a storage
disk having a multiplicity of concentric data tracks, said method comprising
the steps o:E: (a) dividing each data track into a multiplicity of equal length
sectors, the sector boundaries being appropriately marked; (b) grouping the
data to be written on the disk into data blocks, each data block comprising
a selected sequence o:E data sections, each data section being adap~ed to be
written within the boundaries o:E one oE sai.d sectors; and (c) writing a block
of data on the disk such that the block begins and ends on a sector boundary.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features, and advantages of the
present invention will be more apparent from the following more
particular description thereof presented in conjunction with the
following drawings, wherein:
FIG. 1 is a block diagram illustrating how a plurality of
optical drive systems may be coupled to a host CPU through a
suitable control unit;
FIG. 2 is a perspective view of an optical drive system in
accordance with the present invention, and shows how the platter
cartridge is removably inserted thereinto;
FIG. 3 is a mechanical schematic diagram illustrating how the
platter is removed from the cartridge and mounted on a spindle of
the optical drive system;
FIG. 4 is a block diagram of the optical drive system, and
illustrates the principal elements used to read/write data from/to
the platter;
FIG. 5 is a simplified schematic depiction of the read/write
process realized within the optical drive system;
FIGS. 6a and 6b are representations of the data format and track
organization, respectively, used on a platter in accordance with one
embodiment of the present invention;
FIG. 7 is a diagram of the format used within a long block of
data;
FIG~ 8 is a bit diagram illustrating the generation of the sync
bytes of FIG. 7;
FIG. 9 is a block diagram of the control electronics of FIG. 4;
FIG. 10 is a block diagram of the data buffer (DBUF) of FIG. 9;
FIG. 11 is a block diagram illustrating the principle elements
associated with the data path of the control electronics of FIG. 4;
FIG. 12 is a block diagram illustrating the principal elements
used to realize the dynamic defect skipping function;
FIG. 13 is a block diagram of the read DDS bufEer of FIG. 12; and
FIG. 14 is a block diagram of the BS/EM/preamble decoder of FIG.
12~
FIG. 15 is a block diagram of the sync byte generator and
selected circuitry used to generate and detect the sync byte in FIG.
8.
DESG~IPTION OF T~E_PR~F~RRED EMBODDM~NT
The following description is of the best presently contemplated
mode of carrying out the invention. This description is not to be
taken in a limit;ng sense but is made merely for the purpose for
describing the general principles of the invention. The scope of
the invention should be determined with reference to the appended
claims.
Referring to FIG. 1, an optical drive system 20 is adapted to be
coupled to a host central processing unit (CPU) 22 via a storage
control unit 24O ~everal optical drive systems 20 can be connected
to the same control unit 24, if desired. Advantageously, in the
preferred embodiment, the storage control unit 24 includes at least
two storage directors. A first storage director 26 directs data to
and Ercm a plurality of magnetic drive systems 28. A second storage
director 30 directs data to and from the optical drive syst~ms 20~
In this manner, both magnetic and optical storage devices may be
coupled to the same host CPU 22 through the same storage control
unit 24. This beneficial combination--of having both optical and
magnetic storage devices coupled to the same host CPU through the
same storage director--has not heretoEore been commercially
available to applicants' knowledge.
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Advantageously, neither the host CPU 22 nor the storage control
unit 24 need have hardware modifications made thereto in order to
properly interact with the optical drive system 20. Depending upon
the operating system employed within the CPU 22, a suitable
interface program ~software) will generally need to reside within
the CPU in order to pass data to and from storage director 30.
Similarly, appropriate soEtware or firmware control within the
storage control unit 24 will generally be used within the storage
director 30 in order to provide the proper interface signals with
the optical drive system 20. With these software modifications,
conventional CPU's and storage control units may be employed with
the optical drive system 20 of the present invention. The
implementation details associated with the resident CPU software and
the control unit software or firmware are not critical to the
present invention.
In a preferred configuration, the storage control unit 24 may be
an 8880 controller, manufactured by Storage Technology Corporation
of L,ouisville, Colorado. Such a control unit optional]y provides
either two or four storage directors. Therefore, a large number of
disk storage peripherial devices, either optical or magnetic, can be
coupled therethrough to a host CPU 22.
Referring to FIG. 2, a perspective view of the optical drive
system 20 of the present invention is shown. A cartridge 32, having
the media therein upon which the data is optically stored, is
adapted to be inserted into an opening or slot 34 along the front
face of the drive system 20. Operator controls and indicators 36
are also conveniently located along the Eront of the unit 20.
When a cartridge 32 is inserted into the drive 20, the media
housed therein is removed from the cartridge as illustrated in FIG.
3. The cartridge 32 is pushed forward as indicated by the arrow 38
by tl~e user. Once into the unit 20, at a position 40, the cartridge
~2~
32 is opened allowing a tray 42 holding the media or platter 44 to
be slid out therefrom in the direction of the arrow 39. Once
opened in this fashion, an elevator mechanism, schematically
illustrated in FIG. 3 as a plunger 46, lifts the platter 44 away
Erom the tray 42 in the direction indicated by the arrows 47. The
platter 44 is then automatically centered and mounted on a spindle
mechanism 48. An actuator 50 radially positions a read/write
optical head 52 with respect to the mounted platter ~4, thereby
allowing access to a selected area on the surface thereof.
Referring next to FIG. 4, a block diagram of the principle
elements of the optical drive system 20 is shown. The platter 44 is
mounted and centered on the spindle mechanism 48. A suitable spin
motor 50 rotates the splndle 48, and hence the platter 44, at the
desired .otational speed. Signals received from or sent to the
storage director 30 (FIG. l) pass through a control electronics
section 52. The control electronics 52, as its name implies,
provides the control necessary for communication with the storage
control unit 24, including the interpretation of all c~mmands
received from the control unit 24. The control electronics 52,
also, provide the necessary signals for controlling all of the
hardware operations associated with the optical drive system 20.
A read/write channel 54 modulates a write laser diode in
response to data signals received from the control electronics 52.
The resulting modulated laser beam is directed through a
laser/optics section 56 to the surface of the platter 34. Servo
control for the spin motor 50 and the moving elements associated
with the laser/optics 56 is provided by a servo system 58. The
servo system 58 actually includes several servo systems, as will be
apparent from the description that follows. A power/power control
assembly 60 provides the AC/DC power required Eor the operation of
~ 2~
the optical drive system 20. Primary power is secured from a
suitable 50 or 60 Hz 3 phase power source.
In FIG. 5, a schematic representation oE the read/write
operation associated with the optical storage 20 is depicted. The
platter 44, mounted and centered on the spindle mechanism 48, is
spun by a spin motor 50. A write laser diode 60 is modulated by an
input signal 62, which input signal represents the encoded binary
digital data that is to be stored on the platter 44. A modulated
laser beam 64, emitted from the write laser diode 60, is directed
through a beam combiner 66, and is reflected from a suitable mirror
or mirrors 68, through an objective lens 70 to a very small point 72
on the surface of the platter 44. Because the modulating signal is
a binary (two level) signal, the modulated write beam is likewise a
two level signal, having two power states associated therewith
(typically "on" and "off" although any high and low power states
will suffice). The write laser beam 64 has sufficient power
associated with its on or high power state to permanently mark the
surface of the media 44 at the point 72. Because the platter 44 is
spinning or rotating, a track oE data 74 is thereby formed on the
surEace of the media or platter 44.
Conceptually, access to a desired track on the surface of the
platter 44 is achieved by radially positioning the mirror 68 with
respect to the platter 44 so as to pro~ide coarse acce~s to a
desired band (several tracks) on the surface of the platter 44. The
mirror 68 is then controllably tilted about a desired access point
in order to direct the laser beam to a desired track within the
accessed band.
During a read operation, a read laser beam 75 is generated from
a suitable laser source 76. This beam 75 reflects of a mirror 78,
passes through a beam splitter 80, reflects off the beam combiner
66, and reflects off the mirror 68 so as to pass through the
objective lens 70 to the desired point 72 on the desired data track
74. This beam reflects off of the surface of the platter 44 and
follows the same path back through the objective lens 70, the mirror
68, and the beam combiner 66 to the beam splitter 80. At the beam
splitter 80, this reElected read beam 79 is directed to a suitable
detector 82. The detector 82 generates an output signal 84 in
response to the intensity of the reflected read signal 79, which
reflec~ed signal will vary in intensity according to the marks that
have been placed on the media or platter 44 by the modulated write
be~m 64. In this manner, the binary input signal 62, stored as
optically detectable marks on the surface of the platter 44 by the
modulated write beam, may be subsequently retrieved therefrom.
Advantageously, accelerated life tests indicate that data stored on
the media or platter 44 will remain written thereon for as long as
10 years,
Referring next to FIG.6a, a schematic representation of the
format of the platter 44 is shown. The surface of the platter 44 is
divided into a desired number of concentric bands, each band having
a desired number of data tracks located therein. In the preferred
embodiment, Eor example, wp to 716 bands are included on each
platter 44. Each band, as shown in the enlarged portion of FIG. 6a
includes a desired number of data tracks. In the preferred
embodiment, there are 49 data tracks in each band, one track 85 of
which is designated as a home address track. The other 48 tracks
are used to store desired data. The bands are physically separated
by coarse servo tracks 86~ These coarse servo tracks are used in
conjunction with the servo system in order to position the optical
read/write head 52 (FIG. 3) at the correct radial position of the
platter 44. The use of coarse servo tracks in this manner is fully
described in PCr International Application W0 84/01849, published 10
16
May 1984. Advantageously, sector boundary and clock information may
be embedded in or otherwise derived from the coarse servo tracks.
As noted in FIG. 6a, a first data band 88 is set aside as a
Field Engineering (FE) band. The FE band 88 contains data
selectively placed in the tracks thereo during the manufacture of
the platter 44. (That is, much of the data in the FE band is
prewritten on the platter 44, including the servo tracks 86 and the
home address track, during the manufacture of the platter.~ The
purpose of the data in the FE band is to allow the field engineer to
test the reading functions of the optical drive system with data
that is known to have been written correctly without having to
de~ount the platter 44 currently on the drive being tested. Being
able to test the read and other functions without demounting of the
platter is useful in order to identify problems caused by
decentering.
Also illustrated in ~IG~ 6a is a second band 90 designated as
the Index Band. As its name implies, the Index Band is reserved for
the index of the information stored on the platter 44. The index
contains up to two entries for each band on the platter. The Index
Band entries are written with "short" blocks of data. (The
distinction between "short" and "long" blocks is discussed below.)
Indexing data written in the Index Band provides a quick and
efficient method for determining what data has been written on the
platter 44.
A third band 92 on the platter 44 is reserved as the Platter
Table Of Contents (PTOC). The PTOC band 92 contains data which
describes the state of the platter. The entries in the PTOC band 92
are generally written with long blocks.
~ ata is organized on each track in fixed length logical units
called blocks. One track may contain up to 15 long blocks, as
symbolically illustrated in FIG. 6b. A logical data block is a byte
17
sequence whose length is either 136 bytes (a short block) or 7912
bytes (a long block). Short blocks are used primarily for the Index
Band 90. Long blocks are used Eor reading and writing user data,
and are also used in the FE band 9~ and the PrOC band 920 The
maximum number of short blocks for a track is 222, and the maximum
nunber of long blocks per track is 15. As will be explained
hereinafter, the actual number of tracks may vary due to the number
of defects encountered on the platter surface. Eight of the bytes
contained within a short or long block are reserved as
identification bytes. The remaining bytes are available for user
data. Thus, there are 790~ user bytes in a long block, as indicated
in FIG. 6b.
Each track is physically divided into a fixed number of equal
length segments referred to as "sectors." I~e sector is the
smallest unit of encoded information. User data is encoded and
defined into various types of sectors in order to be written to the
platter 4L~. Other types of sectors, as explained below, are used to
identify media or platter defects and inconpletely written user
data.
When the encoded data is written on the platter, a read back
check is employed in order to veriEy that the data has been
correctly written. If a data error is detected, the sector is
rewritten in such a way that during a normal read operation, the bad
sector (the one containing the incorrect data) can be identiEied and
ignored. In order to properly identify defective sectors, an
additional sequence of sectors are thus appended to every logical
data block prior to having it written on the platter. These
additional sequence of sectors may be thought of as subsystem
overhead sectors. The combination of the logical data block (the
user data) and the subsystem overhead sectors is referred to as a
physical data block. The total number of sectors which comprise a
18
physical data block may vary due to the number of media defects
encountered while writing the block of data to the platter~
In the absence of detected errors, the physical data blocks
recorded on a given track have a format as depicted in FIG. 7. This
format may be described as a sequence o~ sectors in the following
order:
1. A block separator sector.
2. Two preamble sectors.
3. One Preamble/Resynchronizable Data Sector (Pre/Resync
Sector).
4. One sector containing a physical identifier or Physical
I.D. (PID). The PID is an 8-byte number supplied by the
control electronics 52 (FIG. 2) whenever data is written to
the platter. This 8-byte numer indicates the band number,
track number, and relative record number within a band.
5. Two hundred forty seven (long block) or four (short block)
resynchronizable data sectors containing 7904 (long block)
or 128 (short block) bytes of user data. This user data
may include a logical ID (LID) and a KEY as well as the
user data. The LID is a 8-byte logical ID supplied by the
access method of the host CPU 22. As such, the LID is a
part of the user record and can be used as an address, and
its use is a user option. The KEY is a string of data of
up to 64 bytes that is appended to user data, if desired,
by the host access method. As such, the KEY is also a part
oE the user record and can be used for addressing purposes
if desired. LID's and KEY's are thus optional identifiers
that may be used to further identify and locate a
particular block of user data.
19
6. An error detection code resynchronizable data sector (34
bytes).
7. Six resynchronizable data sectors containing the error
correction code for the data (204 bytes).
8. A Pre/Resync Sector.
9. A block separator sector.
From the above list, it is seen that a long block format (in the
absence of detected errors) contains 261 total sectors, while a
short block contains 18 total sectors. Those sectors identified in
items 3-8 above are all resynchronizable sectors that have two sync
bytes at the be~inning thereof.
Subsequent blocks are written beginning in the sector
immediately following the last block separator of the previously
~itten data block.
In the preferred embodiment, the block separator sector has a
1.6 M~z square wave written therein. A preamble sector, in
contrast, has an 8 MHz square wave written therein. As indicated in
FIG. 7, the Preamble/Resynchronizable Data Sector, or Pre/Resync
sector, comprises two SYNC BYTES followed by an 8 MHz square wave.
(Advantageously, the 8 M~z square wave can be generated by 2-7
encoding 92, ~9, 2~ [hex] repeating data.)
The physical identifier, or PID sector shown in FIG. 7, is
comprised of two SYNC BYTES Eollowed by 32 bytes of 2-7 encoded data
c~lprising two identical copies of a 16 byte group of data (hex) as
follows:
FF
FF
FF
One's complement Track Nunber.
One's complement Band High.
One's complement Band Low.
One's complement Relative Block ~ligh.
One's complement Relative Block Low.
00
00
00
Track Number.
Band high.
Band low.
Relative Block High.
Relative Block Low~
As those skilled in the art will recognize, FF and 00 are hex
numbers. Tracks are numbered consecutively from the outermost track
of the band to the inner most track of a band. The bands are
likewise numbered beginning from the outermost band to the innermost
band on the platter. Similarly, the blocks of data within a given
track are consecutively numbered. By including within the PID
sector both the one's compliment and the number itself of the track,
band, and block, a positive identification can therefore be made.
As further indicated in FIG. 7, the user data is located in
Resynchronizable Data S~ctors that comprise two SYNC BYTES followed
by 32 bytes of encoded user data. (As explained hereinafter, a 2-7
code is used in the preferred embodiment.)
The Error Uetection Code Resynchronizable Data Sector (EDC RDS)
includes two SYNC BYTES followed by two bytes of 2-7 encoded data
whose value is determined by a CRC computation of selected data
sectors. For a short block, the CRC polonomial is xl6 + x15 +
x + x + 1. The seed (initialization) pattern is "5D5D". Thirty
bytes of 00 follow the EDC bytes to fill the resynchronizable data
sector.
The error correction data, or error correction code (ECC),
comprises two SYNC BYTES followed by 32 bytes of 2-7 encoded data
whose value is determined by an interleaved READ SOLoMON computation
on the selected data sectors and on the EDC resynchronizable data
sector.
The format associated with the home address track 85 (FIG. 6a)
~ill now be discussed. This track, as its name implies, is used to
identify the particular band within which the home address track
lies. The data portion of the first block of the home address track
has the following information pre-written therein:
1. A unique 8-byte platter serial number in EBCDIC.
2. The same unique 8-byte platter serial number in ADCII.
3. Two bytes of binary O's (used to terminate the previous
string).
4. The length of a long block (also pre-t~ritten in the FE band
88).
5. The length o~ a short block.
6. The number of long blocks per track.
7. The number of short blocks per track.
8. The n~mber of tracks per band.
9. The location of the index bandO
10. The location of the FE band 88.
11. The number of long blocks per band.
12. The number of short blocks per band.
13. The number of bands per surface.
14. The length o~ the physical identifier (PID).
15. The number of recording surfaces per platter.
16. A media type code.
17. A platter format manufacturing change level number.
Additional information may also be included in the home address
track, following the information given above, such as information
used to identify the media manufacturing process or processes,
information identifying the software or microcode that is used, a
servo track writer ID number (a servo track writer is an apparatus
used during the manufacturing process of the platter to place the
pre-written information thereon, such as the coarse servo tracks
86), the time and date of manuEacture, the media sensitivity, and
similar information.
With the data stored on the platter formatted as described
above, each block of data can be readily identified. Moreover, many
of the sectors used within each block of data are resynchronizable.
That is, they begin with two SYNC B~rES. Hence, a data detection
scheme can lock on to these SYNC B~rES using phase lock techniques
in order to ensure that the subsequent data is accurately detected.
(Conventional phase lock and detection schemes may be used.)
Advantageously, the two SYNC BYTES that precede every
resynchronizable sector do not map into any user data, yet the 2-7
code constraints are maintained. The 2-7 code is so named because
the encoded data has the characteristic that a "1" is separated by a
minimum of two O's and a maximum of seven O's. The 2-7 code may be
summarized as shown in Table 1.
TABLE 1
2-7 OODE
DATA WORD OODE WORD
0100
010 100100
0010 00100100
11 1000
011 001000
0011 00001000
000 000100
The SYNC BYTES are used to establish a known bit position in the
data stream. This position is required to allow the 2-7 decoding to
begin at a code boundary in the bit stream and to determine the byte
boundaries of the decoded data. The SYNC BYTES are located in the
first two byte positions of the resynchronizable sectors as shown in
FIG. 7. These resynchronizable sectors include the preamble
preceding the physical identifier (PID), the PID, data~ EDC, and ECC
sectors of each data block. The SYNC blocks are defined by
performing a 2-7 encoding of the word "BF7A" and modifying the
result by changing a "1" to a "0" at the position indicated in E'IG.
8. This modi~ication advantageously prevents data from generating
SYNC BYTES, but does not violate the 2-7 code rule as defined in
Table 1.
Referring next to FIG. 9, there is shown a block diagrarn of the
control electronics 52 (FIG. 4) of the optical drive system 20. In
the preferred embodiment, two control interfaces 90 and 92 are
provided to enable comnunications with two separate storage
directors of the storage control unit 24 (FIG. 1). Communication
24
with two separate directors is provided to add flexibility to the
particular configuration that will be used with the optical drive
systems. A suitable switch 94 allows communication with either
director to be selected.
A functional microprocessor 96 (FUP) performs the following
functions within the optical drive system:
1. Hardware control.
2. Interpretation/execution of commands from the selected
storage director.
3. Index buffer management.
4. Rapid band search (RBS).
5. Interrupt handling.
6. Status reporting.
Advantageously, this functional microprocessor 96 may be realized
with a commercially available 16-bit processor chip, such as the MD
68000 manufactured by Motorola Semiconductor of Phoenix, Arizona.
Code for the microprocessor 96 may be stored in a suitable
memory device 98, such as a 128K Dynamic Random-Access-Memory
(DRAM). The DRAM 98 may also provide additional memory associated
with the operation of the control electronics, such as index and
other information.
A maintenance microprocessor (MUP) 100 is also used to provide
the functions necessary for communications with and testing of the
optical drive system 20. A 32K random access memory (RAM) 102
provides the necessary storage for the code associated with the
maintenance microprocessor 100. Advantageously, the maintenance
processor 100 may be realized with an identical chip as is the
functional microprocessor 96. A 4K read only memory (ROM), such as
a programmable ROM (PROM) 104, is used to provide the Boot and
start-up code for the MUP 100 and the other start-up functions
associated with the control electronics 52.
~2~
A Data Buffer (DBUF) provides temporary storage of data
transferred for read/write operations and compensates for the
different access rates associated with the optical drive system.
Thus, it is used for matching the speed of data transfer with the
trans~er rate of the host CPU 22. In the preferred embodiment, the
DBUF 106 has a data capacity of one track, or 128 Kbytes. A block
diagram of one possible embodiment of the data buffer 106 is shown
in FIG. 10. Conventional circuitry may be used to realize the
functions indicated in each of the blocks of FIG. 10. Basically,
the DBUF 106 includes two FIFO (First-In, First-Out) buffers 108 and
110 having a memory array 112 placed therebetween. Additional
buffers 114, 116, 118, and 120 are used to help transfer data or
control signals to and from the respective microprocessor 96 or
100. A multiplexer 122 selects the appropriate input data to be
held within the DBUF 106. Control circuitry 12~ generates necessary
control signals for the memory array 112 and the multiplexer 122;
and a FIFO controller 126, in response to control signals from the
appropriate microprocessor, generates an address signal Eor the
memory array 112.
Referring back to FIG. 9, a Dynamic Defect Skipping (DDS) buffer
and control block 128 provides the necessary buffering for rewriting
data after detected defects and to control read transmitted data
during read. The Dynamic Defect Skip Eunction is described more
fully below in connection with FIGS. 11~
Suitable error correction code (ECC) circuitry 130 is employed
in the data path to improve the data error rate during a read
operation. As with all ECC schemes, this process involves properly
encoding the data when it is written with a suitable code that when
read back not only helps identify that an error has occurred but
also provides the necessary information to correct the error in most
cases. In the preferred embodiment, the error correction code that
. ~,.
~2~
26
is used comprises a triple error correcting (255, 2~9) RE~D/SOLOMON
code, interleaved to degree 32. Each of the 32 interleaves has 6
ECC bytes associated therewith for a total of 192 ECC bytes for each
block of data. The location of the ECC bytes within the block ;s as
shown in FIG. 7. Advantageously, the code is capable of correcting
three symbols (bytes) in error per interleave. Because of this
interleaving, this yields a first correction capability of 96
bytes. The error of the code is designed to meet or exceed the
criteria that no more than one uncorrectable error occur in 10E+13
bits transferred.
The circuitry used to encode and decode the error correction
code is advantageously shared between the encoding and decoding
processes.
Referring again to FIG. 9, a serializer/deserializer (SERDES)
circuit 132 is used to take byte-serial channel data and convert it
into bit-serial data to be written on the platter and visa versa.
The SERDES 132 circuitry comprises the elements enclosed within the
dotted line 132 of FIG. 11, which figure depicts the data path
associated with the control electronics 52 (FIG. ~). As is evident
from an examination of FIG. 11, the SERDES circuitry provides the
desi-red encode/decode function. The translation from data to code
words and back follows the pattern indicated in Table 1. The decode
function perfo~med at block 137 (FIG. 11), takes as inputs the 2F
clock signal and the detected data fron the phase locked loop
circuits and generates corresponding data bits. The decode Eunction
of the decoder 137 also provides error checking to detect the
presence of "11" "101", or "00000000" patterns in the coded data.
These bit patterns violate the rules for a 2-7 code.
As will be explained hereafter, there are a number of special
patterns which must be inserted into the serial encoded bit-stream
for formatting and for defect skipping purposes. These patterns
27
include block separators, exception marks, and special resync
characters. Thus, pattern generation circuitry 134 (FIG. 11)
provides these characcers to the data stream at the output o~ the
2-7 encoder 136.
A synchronizer circuit 138 provides the means to detect the
beginning of a Resynchronizable Data Section (RDS~ by decoding the
special 2-byte field at the start of every RDS. (An R~S is that
sequence of data bytes, such as is shown at 139 and 141 of FIG. 7
that is written into a data sector, such as 139' or 141' of a block
of data). The circuit 138 also provides the means to acquire
phase-sync for the 2-7 decoder and bit-sync for the data path.
In the preferred embodiment, the 2F frequency used in the
generation of encoded data is derived from a crystal oscillator 140
running at 48 MHz. Other clocks used in the data path are all
derived from this frequency. The clocks are distributed throughout
the control electronics 52 (FIG. 4) as a lF clock signal (24 MHz)
and as four 12 MHz clock signals, each derived-from the lF clock and
offset rom each other by 90.
Referring back to FIG. 9~ an RS-232 interface 142 is provided
within the control electronics in order to allow communication with
the optical drive system 20 through the use of any suitable
diagnostic tool on either a local or remote basis. An operator
panel 144 provides the controls and indicators necessary for
operator use in completing a power up/down sequence, or in a
load/unload of a cartridge 32. An FE panel 146 provides to the
field engineer a manner of controlling and monitoring the operation
of the optical drive system 20 so that proper diagnostics and tests
can be run. A floppy disk 148, preferably an 8-inch floppy disk,
provides storage for the maintenance processor 100 microcode,
diagnostic microcode, and error log information.
. "
28
As mentioned previously, an important feature of the present
invention is the ability to correctly write data on the platter
during a write operation. Correctness on the data is assured by
performing a read back check. That is, immediately after the data
is written on the platter ~ it is read back therefrom. The data
read back is compared with the data written to the platter and if
any differences exist then the data has been incorrectly written and
is so marked. This process is repeated as many times as is
necessary (within reason) in order to insure the correctness of the
written data~ This process is referred to as Dynamic Defect
Skipping (DDS) and is functionally illustrated in FIG. 12. The data
to be written is loaded into a write buffer 150. Frcm this buffer
150, the write data is made available to the serializer 133 and a
comparator circuit 152. Data read from the platter immediately
after it is written is likewise made available to the comparator
circuit 152 and to a read buffer 154. If the compare function
perEonmed by the comparator circuit 152 indicates that the data read
back is the same as the data written to the platter, then
appropriate control signals are generated to allow the read data
held in the read buffer 15~ to be transferred to the data buffer
106. If, however, the comparison of the write data to the read data
indicates that an error has occurred, t~len appropriate control
signals are generated to flag that particular sector as containing
incorrect information. Any subsequent attempts to read a sector so
flagged will cause the data therein to be ignored (not made
available to the data buffer).
The particular flag used to indicate whether a given data sector
is good or not is referred to as an Exception Mark (EM). An
Exception Mark is a 2.0 MHz square wave signal and is derived from
the ~8 MH2 write clock, as are the preamble signal (an 8 MHz square
wave) and the block separator (BS) signal (a 1.5 MHz square wave).
:~2~
29
As indicated previously (in FIG. 7) each block of data starts with a
block separator sector followed by two preamble sectors. (A block
separator sector is a sector having a block separator signal written
therein. Similarly, a preamble sector is a sector having a preamble
signal written therein. Thus, as a short hand notation, a sector is
identified by the signal written therein, e.g., an exception mark or
EM sector, an RDS sector, a BS sector, etc.) The phase lock loop
circuits used to detect the data require that at least two sectors
of preamble precede the data. If any sector in these initial three
sectors--the block separator or two preamble sectors--is bad, then
the sequence must be restarted. A defective data sector is marked
by a preamble/exception mark sequence. That is, immediately
following the detection of a defective data sector, the following
sectors are written:
Preamble sector
Exception mark sector
Preamble sector
Preamble sector
Rre/Resync Sector
Rewrite of sector before defective sector
Rewrite of defective sector
In the rare case of defects larger than a sector, additional
preamble sectors are written (prior to the exception marks) until a
good preamble sector is read back or until ten defective sectors are
encountered. In the case of a good preamble sector after n
defective sectors, n exception mark seCtOIS are written to mark the
length of the defect. If a defect exception mark is read back~ an
incomplete block sequence is written and the entire block is
rewritten. In the case of 10 defective sectors, an incomplete block
sequence is written and the entire block is rewritten. The reason
for this limit is the limited size of the read defect skip bufer
15~1.
An incomplete block sequence is defined to be a consecutive
sequence of an exception mark sector and a block separator sector.
If any sector is determined to be bad, a sequence is restarted. If
the end of the track occurs before the sequence can be written
successEully, error recovery procedures must be invoked from the
host CPU or the storage control unit in order to recognize that a
block that has been ~ritten could not be completed and could not be
marked incomplete because of the end of the track. If an incomplete
sequence i5 due to the end of a track, the sectors following the
incomplete block sequence are padded with preamble sectors.
For the special case of the defect in the first Resynchronizable
Data Section, the rewrite begins with the deEective block, not the
block prior to the defect.
Decisions concerning correctness of sectors are made at the
start of each sector. This means that only approximately the first
3/4 of the prior sector has been checked. An error beyond this
point is detected one sector later. Because the rewrite starts one
sector prior to the defect, no failure in the defect skip mechanism
will occurO This means that a defect growth is less than
approximately 3/4 of a sector.
The particular write methods or processes used in connection
with Dynamic Defect Skipping are summarized in Tables 2 and 3. In
these Tables, a preamble, separator, or exception mark sector is
termed "deEective" if it is determined to be of marginal quality
such that it may become unreadable in the life of the media. In
contrast, a resynchronizable data section (RliS) sector is termed
"defective" if any bit is in error during the read back check. Any
sector may be termed "defective" if the input signal conditioning
phase lock loop circuits determine marginal data is read back.
31
TABLE 2
WRITE METH~D
l. If the track is not empty, go to 9.
2. If 200 sectors written attempting to write inconplete block
sequence, go to 35.
3. Write Exception Mark sector.
4. If Exception Mark defective, go to 2.
5. If 200 sectors written attempting to wrice incomplete block
sequence, go to 35O
6. Write Block Separator sector.
7. If Block Separator defective, go to 2.
8. Go to 12.
9. If track full, go to 34.
lO. Write Block Separator sector.
ll. If Block Separator defective, go to 9.
12. Set ~Irewrite defective sector only" flag.
13. If track full, go to 34.
l4. Write Preamble sector.
15. If Preamble seccor defective, go to 9.
16. If track full, go to 34.
17. Write Preamble Sector.
18. If Preamble sector defective, go to 9.
19. If track full, go to 34.
20. Write "sync field, 92H, 49H, 2411, . . . " sector.
21. IE sync not acquired or Preamble not detected, go to 9.
22. If track full, go to 34.
~3. Write Resynchronizable Data Section.
24. If Resynchronizable Data Section defective, go to 300.
25. Reset "rewrite defective sector only" flag.
~2~
32
26. If last Resynchronizable Data Section written is not last
Resynchronizable Data Section in this block, go to 22.
27. If track full, go to 34.
28. Write Preamble sector.
29. If Preamble defective (actually testing end of prior RDS),
go to 300.
30. If track full, go to 34.
31. Write Block Separator sector.
32. If Block Separator defective, go ~o 30.
33. Stop, block successfully written status.
34. Stop, unable to write block, track full status.
35. Stop, unable to write incomplete block sequence at start of
track status.
TABLE 3
DEFECTIVE RESYNC~ONIZABLE DATA SECTION EN00U~TERED
300. IE track full, go to 334.
301. If defect has spanned 9 sectors, go to 326.
302. Write Preamble sector.
303. If Preamble sector devective, go to 300.
304. If track full, go to 334.
305. Write Exception Mark sector.
306. If Exception Mark sector defective, go to 326.
307. If number of Exception Mark sectors marking this defect
is less than number of defective sectors, go to 304.
308. Write Preamble sector.
30~. If track full, go to 334.
310. Write Preamble sector.
311. If Preamble sector defective, go to 326.
316. Write sync field, 92H, 49H, 24H, . . . sector.
317. If sync not acquired or Preamble not detected, go to 326.
318. If l'rewrite defective sector only" flag set, go to 323.
319. Back up dat pointer two sectors.
320. Set l'rewrite defective sector only" flag.
321. If rewriting last data RDS in block, go to 30.
322. Go to 23.
323. Back up data pointer 1 sector.
324. If rewriting last data RDS in block, go to 30.
325. Go to 23.
326. IE track full, go to 334.
327. Write Preamble sector.
328. If Preamble defective, go to 326.
329. Write Exception Mark sector.
330. IE track full, go to 334.
331. If Exception Mark sector defective, go to 329.
332. Write Block Separator sector.
333. If Block Separator sector defective and track not full,
go to 329.
334. Stop, successful incomplete block status.
The function of the write data compare circuitry 152 (F'IG. 12)
is to locate deEective areas on the platter or media 44, or to
isolate and identify other conditions that may cause data to be
defectively written, by performing a byte-by-byte compare of written
and read back data. When a mismatch occurs, an indication is sent
to the write control circuitry 156. This indicator lnitiates a
defect mark sequence at the next resynchronizable data sector
boundary. The requirements of the data ccmpare circuitry 152 are
that the write buffer 150 hold the data until the read back data is
ready. Each read back data byte is latched as it is input into the
read buffer 154. The data compare circuitry 152 performs a
byte-to-byte c~mparison. The write control circuitry 156 is
signaled when a mismatch occurs.
The primary function of the write ~DS buffer 150 is to hold the
data until the read back check function has determined that data
need not be rewritten. The size of this buffer is determined by
three factors:
1. The delay between the inputting oE the data to the
serializer and comparing the data read back to the written
data. This time is estimated to be on the order of 2
Microseconds in the preferred embodiment.
2. The data buffer latency (e.g., how fast data can be
retrieved from the buffer). Data buffer latency is
considered to be negligible.
3. Ihe DDS algorithms. The algorithm requires rewriting of
the resynchronizable data sector before the
resynchronizable data sector with the defect~ Therefore
two sectors must be buffered for this factor. In the
preferred embodiment, consideration of these factors yields
a total write DDS buffer size of three RDS's, or 96-bytes.
A read control section 158 (FIG. 12) supervises data block
deforma~ting, including finding the start of the block and
separating data RDS's fr~n non-data RDS's and defective data RDS's.
This control requires that several decisions and actions be taken
within an RDS time. The requirements of the read control circuitry
158 may be sumnarized as:
1. To find the start of a data block;
2. To manage the read DDS buffer (full or empty);
3. To count the exception marks in order to determine a given
defect size;
. To adjust the read DDS buffer point for defect/reread;
5. To set/reset the phase lock loop control lines as required
(high gain, filter unlock, etc.); and
6. To initialize the ECC syndr~ne generators at the start of
each data block entering the data buffer.
The read control section 158 must exhibit flexibility in order
to allow rapid changes to take place as unexpected read errors
occur. A high speed microprocessor is the preferred means to
achieve this flexibility. An 8 X 305 based design is a suitable
microprocessor that may be used for this task because it is a
c~nplete single chip microprocessor that is cGnnercially available
fran Signetics and it operates at the required speed. I~is is the
same type oE microprocessor, advantageously, that may be used to
realize and control the ECC functions (block 130 in FIGS. 9 and 11).
,
36
The write control circuitry 156 (FIG. 12) controls the data
block format, including the preamble, postamble, dynamic defect
marking and rewrite, and incomplete block marking. As with the read
control circuitry 158, the write control circuitry 156 requires that
several decisions and actions be made within an P~S time. (An RDS
time, in the preferred embodiment, is 11.3 microseconds). The
requirements of the write control circuitry may be su~marized as
follows:
1. To control the stating and ending of data blocks;
2. To determine/control the writing of special RDS's ~such as
exception marks and block separators;
3. To manage and write the DDS buffer (full or empty~;
4. To adjust the right DDS buffer pointer for defect or
rewrite;
To resynchronîze the data compare hardware after a defect;
6. To synchronize/reset the miscellaneous write path hardware
at the start of each block; and
7. To monitor for end-of-track while writing.
Flexibility in the write control circuit is also important.
Flexibility allows rapid changes to take place as unexpected
problems occur. For this reason, the write control function in the
preferred embodiment is controlled by a high speed microprocessor.
Advantageously, the microprocessor used for this function may be the
same microprocessor used to compute correction vectors and offsets
during a read, which would otherwise be idle during writes.
Input/output ports, the capabi~ity to run the microprocessor at byte
~rate, and additional control memory are the only conditions required
to utilize the ECC microprocessor for this function. In the
preferred embodiment, the ECC microprocessor is realized with an 8 X
305 based design which executes instructions in 200 ns. As
indicated, the same microprocessor used to realize the read control
, ! ; '~
37
circuitry 158 may be used for this function. A suitable
connercially available from signeticsO
The read DDS buffer 154 serves the primary function of delaying
data from entering the data buffer 106 until the read DDS
microprocessor 158 can determine if the data has been rewritten
because of a defect detected at write time. Hence, the read DDS
buffer 154 must be approximately 374 bytes (11 RDS x 34 bytes) in
length. Because the data buEfer sometimes cannot accept data (e.g.,
to analyze KEY information, or for error correction) additional
buffering may be required. The time required for these functions is
variable, but should be no more than 300 microseconds in the
preferred embodiment. Thus, all of these factors indicate that a
total buffer size of approximately 1300 bytes be used. When a
defect/rewrite occurs, the read control 158 senses this fact, stcps
data ingating, adjusts the read DDS ~uffer write counter, and
restarts the data transfer at the beginning of the rewritten data.
The implementation of the read DDS buEfer is depicted in the
block diagram of FIG. 13. Byte-wide data from the deserializer is
latched into the read DDS buffer by a "byte strobe" signal on line
160 received from the deserializer. lhis line is a divide-by-8 of
the phase lock loop (PLL) generated bit clock synchronized by the
sync bit decoder 138. Data will norma]ly enter the read DDS buffer
154 at a 333 ns rate, with the exception Eor two missing cycles
every RDS. The maximum Erequency of the PLL (in and out of lock
case) is 5.4 MHz, and this rate corresponds to a 317 ns byte rate.
The ~aximum input rate to the read DDS buEfer is thus 317 ns. A
control block 162 of the read DDS bufEer is a synchronized state
machine clocked by the 24 MHz write clock. It performs the
following Eunctions:
1. Monitors requests for reads and writes.
2. Arbitrates read/write requests (write occurs first).
3. Generates control lines to R~ and address multiplexer.
4. Monitors buffer for full/empty.
5. Flags bufEer for overflow.
The control block 162 appends memory cycles adjacent to each
other if data can be supplied or received at adequate rates. This
achieves a maximum data rate to or from a R~M 164 of 8 Megabytes.
The minimum required data rate for a 3~Mbyte throughput is a 6-Mbyte
RAM input/output rate. The 8-Mbyte rate can be expected due to the
fact that the decision concerning if data can be released from the
read DDS bufEer 154 is made on a 32-Byte basis. Table 4 identifies
the input/output lines of the read buffer circuit 154 oE FIG. 13.
39
~2~
TABLE ~
INPU'I`/OUTPUT (I/O) SlGNALS OF READ DDS BIJFPER
I/O Lines: (Into Read DDS Buffer from Deserializer)
DIN 8 TTL signals corresponding to
data byte received from the media.
BYrrE STROBE TTL signal indicating valid data is
on DIN lines.
I/O Lines: (Into Read DDS Buffer from Read DDS UP)
i~D CNTR LIMIT 12 TTL signals corresponding
]0 to maximum RAM address which can be
shifted out of Read DDS Buffer.
STOP INGATING TTL signal used to stop data from
being input to Read DDS Buffer. This
occurs when a defect is detected and the
write counter must be updated.
I/O Lines: (Into Read DDS Bllffer from Data Buffer)
WORD STROBE TTL signal indicating the data word on
DOUT has been accepted.
I/O Lines: (Read DDS Buffer to Data Buffer and ECC Syndrome Gen)
DOUl' 16 TTL signals corresponding to two data
bytes from media.
WORD READY TTL signal indicating valid data is on
DOUT lines.
As has been described previously! ccrtain speclal conditions (such as
,.,."~
'10
the s-tart of a block, platter clefects, etc.) are indicated on the platter
by writing two unique flags. The block separators (BS) sector is a 1,5
M~-lz square wave. The excepti~on mark (EM) sector is a 2.0 ~lz square wave.
These frequencies arc outside the range o~ data. ~Data frequencies range
from 3 M~lz to 8~lz). A third special function frequency, the preamble, is
used to synchronize the PLL and thus is the highest frequency recorded (8
~Iz). The preamble resides~ in -the data range. These frequencies are written
in burst and in a integral number of RDS's in length. In order to detect
the presence of these special conditions as indicated by these frequencies,
the Block Separator/Exception Mark/Preamble Decoder circuit 135 is employed.
A block diagram of this special decoder circuit is shown in FIG. 1~.
I`he requirements of the Decoder Circuit 135 (referred as the BS/
EM/Preamble decoder in the FIGURES.) may be summarized as follows:
1. To reliably discriminate the special frequencies associated with the block
separator, exception mark, and preamble from each other and from the data.
2. To determine while writing if the special frequency being written will be
able to be decoded for the life of the media.
3. To provide a method of setting the detection threshold. This allows some
level of de:Fects to be tolerated during a write (to allow imperfect media
to be used) and a greater level of defects to be tolerated during read ~to
compensate for aging the media). During error recovery, repeated attempts at
different tllresholds can be used -to recover otherwise unreadab]e data.
. To ensure that incorrect decoding ;s extremely unlikely. That is, the ECC
scheme cannot correct dynamic defect skipping failures. [hus, the marking
system used ~exception marks) to indicate that an incorrect write has
occurred must be very reliable.
:,
~2~
41
Referring to FIG. 14, prior to PLL synchronization, data enters
a shift register 170 clocked by the 48 MHz system clock. The data
is synchronized to the clock and a 2-3 majority decode is
performed. The majority decoder 170 filters noise Erom the data.
This filtered data is monitored Eor transitions in a transition
detector 172. Each transition resets a window counter 174. The
window counter 174, also clocked by the 48 MHz system clock,
generates timing windows in which the next transition can be
expected iE one of the special frequencies is received. The next
transition, if it occurs inside one of the windows, causes a latch
corresponding to that condition to be set. This latch, identified
as the window decoder 176 in FIG. 14, enables a counter
corresponding to the condition to increment. Three such counters
are employed: a block separator counter 178, an exception mark
counter 180, and a preamble counter 182. A transition outside oE
the windows or no transition before the end of the longest window
causes the window decoder 17~ to be reset, and this action disables
the respective counter. If one of the three special frequencies
occurs for a long enough interval, the counter corresponding to that
Erequency reaches its maximum value. The counter latches in this
state until the counter is preset by a restart line 184. The
variable threshold requirement is implemented by allowin~, microcode
or so~tware to control the value to which each value is preset to
when the restart line is pulsed. The restart line needs to be
pulsed at the beginning of each resynchronizable data section.
While searching for the start of a block, a signal may be derived
from the coarse seek band modulation.
While reading, this signal is derived frGm a counter clocked by
the 48 MHz system clock and synchronized by the sync byte decoderO
The input/output lines to the BS/EM/Preamble Decoder 135 are
summarized in Table 5.
42
Referring next to FIG. 15, a block diagram of the sync byte
generation and detection circuitry is shown. Many oE the elements
shown in FIG. 15 are also found in FIG. 11, but the particular
arrangement of the elements in FIG. 11 helps clarify the function
performed. Parallel write data from the DDS Buffer 150 (FIG. 11) is
directed to a serializer register 133. A Sync byte generator 196
selectively intersperses the sync word "BF7A" with the data in the
serializer 133. The serial data Erom the serializer 133 is sent to
the 2-7 Encoder 136 where it is encoded according to the pattern
shown in Table 1. This data is then gated through an AND gate 198,
the output of which is coupled to a serial write data bus 155.
Logic circu;try 200 determines when the particular transition ~bit)
of the unchanged sync word is present (see FIG. 8) so that this bit
may be surpressed, thereby generating the desired sync byte.
The sync byte is sensed by monitoring read bytes as they pass
through the 2-7 decoder shift register 207. Sync byte detect logic
204 is unfigured to generate a sync detect signal whenever the
prescribed sync byte bits are present. This sync detect signal is
used to organize the read data into correct parallel data bytes.
43
TABLE 5
INPUT/OUTPUT (I/O) SIGNALS OF BS/E~/PRE~MBLE DEOODER
I/O Lines (into Decoder)
RAW DATA ECL signal corresponding to
signals read from media.
48 MHz EC1 signal derived frcm
system crystal clock.
RESTART TTL signal corresporlding to
start of each RDS.
BS THRESHOLD 9 TTL signals corresponding
to desired BS threshold.
E~ T~ESHOLD 9 TTL signals corresponding
to desired E~ threshold.
PRE THRESHOLD 9 TTL signals corresponding
to desired preamble threshold.
1/0 Lines (From Decoder)
BS DETECT TTL signal indicating a block
separator ~DS has been
detected.
EM DETECT TTL signal indicating an
exception mark RDS has been
detected.
PRE DETECT TTL signal indicating a
preamble RDS has been
detected.
Advantageously, by organizing the data into resynchronizable
data sections, which section.s allow the clocking signal to be
resynchronized every 32 bytes, the propogation oE errors through the
data due to an unsynchronized clock signal is ]imited. This Eurther
` ;
reduces the number of errors that can occur when data is being read
from the platter.
~ -ile the invention herein disclosed has been described by means
of specific embodiments and applications thereof, nunerous
modifications and variations could be made thereto by those skilled
in the art without departing from the spirit and scope of the
present invention. It is therefore to be understood that within the
scope of the appended claims, the invention may be practiced
otherwise than as specifically described herein.
