DVD-RAM is a high capacity storage technology that could become the
true successor to the older CD-R and CD-RW formats. In addition to
additional capacity, the format has certain built-in features that
make it much more reliable.
DVD-RAM is a rewriteable data storage format that was designed
by the DVD Forum to provide a reliable and affordable storage
solution to the emerging multimedia world. The ever-increasing
storage requirements in the enterprise market, along with the
advent of digital video and movies at home, are driving demand for
large capacity rewriteable storage. DVD-RAM offers a highly
reliable format designed with future expandability in mind.
In July 1997, the DVD Forum published the specification for DVD-RAM
(DVD-RAM Book 1.0). DVD-RAM is an optical disk technology that
provides 2.6Gb of fully rewriteable data storage capacity on each
disk surface. Drives and media conforming to the DVD-RAM
specification are now available from multiple manufacturers. As a
result, the capabilities of DVD, the new optical data storage
standard, now include both data distribution and data storage. The
DVD-RAM format specification was defined by Working Group Five of
the DVD Forum. Members of Working Group Five, who defined the
DVD-RAM standard are: Eastman Kodak Company, Hitachi Ltd,
Matsushita Electric Industrial Co, Ltd, Mitsubishi Electric Corp,
Philips Electronics NV, Pioneer Electronics Corp, Sony Corp,
Thomson Multimedia, Toshiba Corp and Victor Company of Japan. Since
its founding, in December 1995 (as the DVD Consortium) the DVD
Forum has defined and published specifications for DVD-ROM,
DVD-Video, DVD-Recordable and DVD-RAM. It has also grown to include
more than 120 member companies.As part of its multi-year,
multi-product specification effort, the DVD Forum consults
regularly with other industry groups, and has submitted its
published specifications to international standards organisations.
One of the most important of these informal groups is the computer
industry Technical Working Group, made up of the leading
manufacturers of personal computers and servers. In 1995, this
group defined a set of requirements for optical disk storage
technologies that would replace the 650Mb CD technologies used
throughout the industry. The DVD Forum has responded to the
requirements of the Technical Working Group in the development of
all DVD specifications.The goals of the DVD Forum program in
defining the DVD-RAM Format were: 1) To assure interchangeability
of media between drives manufactured to the defined specifications.
The DVD-RAM Book 1.0 specification was ratified after a three-month
evaluation period, during which nine media suppliers and 13 drive
manufacturers confirmed the technical details and feasibility of
the specification.2) To provide the highest levels of compatibility
between read-only and recordable DVD technologies. Thus, the
rewriteable DVD-RAM format shares common format features with
read-only DVD-ROM. This allows future DVD-ROM and DVD-Video drives
to read DVD-RAM disks.3) To provide reliable and fast access to
data written anywhere on the disk. Zoned Constant Linear Velocity
(ZCLV) recording and multiple techniques for sector header
identification are used to optimise random access data
performance.4) To achieve drive and media cost levels that allow
widespread adoption by manufacturers of personal computer and
consumer electronics products. Phase change media technology and
land/groove recording methodology allow for cost effective media
and drive implementations.The program goals were achieved and
multiple manufacturers brought DVD-RAM products to market in a
relatively short time. The starting point of assured compatibility
with other DVD technology is in the physical structure of DVD-RAM
media. Specifications were defined to match, in all possible cases,
the DVD-ROM media specification. Rewriteable DVD-RAM uses a
land/groove structure and phase change material to record data. The
land/groove combination forms a continuous spiral track, with data
recorded alternately on land and groove. This results in a logical
track pitch of 0.74 µm. With this approach, the physical track
pitch, or distance from a land track to the next land track, is
1.48 µm. Working Group Five also evaluated land only and groove
only recording techniques. With those alternatives, the physical
track pitch would be only 0.8 µm. Such a narrower pitch is more
difficult to manufacture, and would likely be more costly than the
land/groove solution.To maintain nearly constant data density
across the surface of the DVD-RAM disk, the recording surface is
divided into zones. This scheme, combined with the land/groove
technique, maximises disk capacity, with a total recording capacity
per disk side of 2.6Gb. The disk is operated in ZCLV mode. Using
this method, disk rotational speed is kept constant within
recording zones but varied from zone to zone. This results in
constant data rates through-out the disk surface. ZCLV provides for
constant data recording rates, nearly constant recording density,
and optimised random access performance.DVD-RAM media is protected
by a cartridge and may be used as a bare disk. It is strongly
advised to use a cartridge whenever possible, to provide maximum
protection to data at all times. Cartridges with single-sided disks
allow removal of the disk, to enable operation as a bare disk.Phase
change material is used as the recording medium in DVD-RAM. The
phase change material can take on two distinct states, amorphous or
crystalline, with different levels of reflectivity. These different
levels of reflectivity are detected by the optical readers of both
DVD-RAM and DVD-ROM drives, enabling data to be read from the
disk.Prior to recording, the disk surface material is in a
crystalline state exhibiting high reflectivity. The light intensity
of a laser beam is modulated to write data marks on the disk's
surface. The energy of the laser pulse quickly raises the surface
temperature of the disk above the material's melting point. Then
the laser power is quickly lowered to its cooling point, rapidly
cooling the disk surface.The material in the area affected by the
laser pulse remains in amorphous state. This area of low
reflectivity is a recorded data mark. To erase this data mark, a
lower power laser beam is used to return the disk surface material
to their initial crystalline state. The laser causes the surface
temperature to rise to its crystallisation point enabling the
material to revert to the initial crystalline state thus erasing
the data mark.DVD-RAM drives use a multi-pulse waveform write
strategy to form recording marks reliably. The duration of laser
pulses is varied to write different marks. All of the waveforms use
an identical cooling pulse. Data is recorded using the same 8 to 16
modulation code used for DVD-ROM. This enables future DVD-ROM
drives to playback DVD-RAM disks. The challenge of achieving
reliable, high-density recording of data on DVD-RAM phase change
media, while providing for read-only compatibility with DVD-ROM
drives, was met through a combination of techniques contributed by
different members of the DVD Forum. The resulting wobbled land and
groove recording format is detailed below. Three key aspects of the
format are wobble-linked ID detection, sector format and header
format.The DVD-RAM disk surface is divided into 24 annular zones.
This zoned surface layout keeps the length of a sector and the
recording density almost constant throughout the disk. Each zone
has 1888 tracks (944 land tracks and 944 groove tracks) and each
track is divided into sectors. The innermost zone has 17 sectors
per track, and the number of sectors per track increases by one in
each succeeding zone. Thus, the outermost zone features 40 sectors
per track. Each sector begins with a read-only identification (ID)
field, embossed onto the disk surface. This ID field, the header,
is used to identify the physical location of the sector and is kept
separate from the user-recordable data field, to assure that it is
permanently readable.All types of DVD disks include a control area
at the inner diameter of the disk. This area contains embossed,
read-only data that contains information identifying the type of
media, e.g. DVD-RAM, DVD-ROM or DVD-R. The information allows
drives to identify which type of media is inserted into a drive,
and is designed to support compatibility among the DVD product
family. Data marks are written on both land or groove tracks.Land
and groove tracks alternate on every revolution of the disk,
forming a continuous single-spiral track. This single spiral
feature allows for the continuous recording of large data streams,
such as video data.The embossed sector header includes four ID
fields, each containing identical information to provide redundancy
in identifying sector location. These embossed ID fields are offset
from the track center by half a track width. Two are offset toward
the inner diameter, two towards the outer diameter. The recordable
area of a sector is slightly wobbled, with a period of 1/232 of the
physical sector length. This feature is used to produce a constant
clock signal that allows the drive to find the user data in the
sector in the event that a sector ID cannot be read.The sector
format of DVD-RAM media has data blocks that are identical to
DVD-ROM sectors. Each physical sector consists of a 130-byte-length
header part and 2567-byte-length recording field, which includes
gap field, guard fields, synchronisation signals and buffer fields.
Guard fields protect both ends of the recording data field from
degradation due to repetitive overwriting.DVD-RAM media requires
2697 bytes for each user recordable data sector. This provides for
a gap between each 2418 byte block of recordable area. However, the
sector data configuration itself is the same for DVD-RAM and
DVD-ROM. The recording code (8 to 16) and error correction code
(Reed-Solomon product code) are the same for both types of media.
Thus, the information recorded on the DVD-RAM disk can easily be
played back using the readout circuit implemented on DVD-ROM
drives.The DVD-RAM media header format is designed to assure
reliable recording and playback. As noted earlier, the header ID
information is permanently embossed on the disk. Each header is
divided into two parts. One part of each header is shifted by half
a track towards the outer diameter of the disk, while the second
part is shifted by half a track towards the inner diameter. ID
field offsets are dependent on whether the following recordable
area is on a land or groove track. "Land sectors" start with ID1/2
disk diameter (ID) and ID3/4 are offset towards the outer disk
diameter (OD)."Groove sectors" start with ID1/2 offset towards the
OD and ID3/4 are offset towards the ID. This change of ID offset
sequence between land and groove tracks allows the drive to
reliably switch its tracking servo from land to groove tracking and
vice versa.DVD-RAM has both rewriteable data areas and embossed
data areas, while DVD-ROM only has an embossed data area. The
embossed data for DVD-RAM is located at the innermost part of the
disk, which is a part of lead-in area. The lead-in areas of all DVD
disks contain information that identifies the type of media, such
as DVD-RAM, DVD-ROM or DVD-R. The data structure of this area is
common across the DVD-family, in order to ensure future
compatibility.The rewriteable data area is divided into 24 zones,
each of which has an associated spare area. The spare area is used
for the replacement of any defective sector in the user area.
Defective sector addressing is handled using four defect management
areas, two of which are located in the lead-in area and the others
are located in the lead-out area. Those four areas contain the same
information, providing an additional level of redundancy to improve
reliability. Each defect management area has lists of defective
sectors and their replacement sectors.Defect management is
performed by drives in order to provide an error free storage
environment for file systems. DVD-RAM media is designed to support
rewriting of data more than 100,000 times in the lifetime of a
disk. This is achieved by implementing two methods for randomising
the location and the polarity of data in recordable sectors. The
DVD-RAM format specification calls for a different start position
of written user data each time a data sector is overwritten. The
recording start position is randomly shifted by 0 to 15 channel
bits, and the lengths of the guard fields are changed by 0 to 7
bytes.The second overwrite technique is based on the fact that a
recorded mark to code a data bit, or "1" is represented by the edge
of the mark. (This principle is called mark edge recording.) Since
the leading and trailing edge of a mark are treated equally, the
polarity (mark or space) of a recorded mark is unimportant and does
not effect the ability to retrieve data. By applying random
inversion, the location of marks and spaces becomes almost uniform
when averaged throughout the overwrite cycles. This uniformity
greatly improves the overwrite cyclability.
( Hitachi
1999Compiled by Agith Ram