Optical Data Storage for Digital Video
Optical data storage is commercially successful in the form of Compact Discs (CDs) for audio and software distribution and Digital Versatile Discs (DVDs) for video distribution. CDs and DVDs look very similar because the fundamental optical technology for both devices is the same. This similarity is also true for the next generation of optical data storage, which may be used for digital home theater recording and HDTV distribution. However, CDs, DVDs and next generation products are different in terms of specific optical components in the drive, in how data are managed and in details of the disk structure used to store the information. These differences allow a larger volume of data to be recorded on each successive generation. Larger data volumes translate into higher quality video and longer playing time.
Parameters for HD Video Storage with Optical Disks
Ø Optical Parameters
Ø Disk Structure Parameters
Ø Data Management Parameters
Optical parameters include laser wavelength, objective lens numerical aperture, protective layer thickness and free working distance. Data management parameters include data rate, video format, HDTV play time and bit‐rate scheme. Disk structure parameters are user data capacity, minimum channel bit length and track‐to‐track spacing.
Optical parameter
Digital information is stored on optical disks in the form of arrangements of data marks in spiral tracks.
beyond a critical writing threshold, changes its reflective properties.
Modulation of the intense light beam is synchronous with the drive signal, so
a circular track of data marks is formed as the surface rotates. The scan spot is
moved slightly as the surface rotates to allow another track to be written on
new media during the next revolution.
Data marks on prerecorded disks are fabricated by first making a master disk with the appropriate data‐mark pattern. Masters for prerecorded CDs and DVDs are often exposed in a similar manner to exposing data marks on recordable optical disks, except that the light‐sensitive layer is designed to produce pits in the master that serve as data marks in the replicas. Inexpensive replicas of the master are made with Injection‐molding equipment.
Readout of data marks on the disk is illustrated in Fig.2, where the laser is used at a constant output power level that does not heat the data surface beyond its thermal writing threshold. The laser beam is directed through a beam splitter into the objective lens, where the beam is focused onto the surface. As the data marks to be read pass under the scan spot, the reflected light is modulated. Modulated light is collected by illumination optics and directed by the beam splitter to servo and data optics, which converge the light onto detectors. The detectors change light modulation into current modulation that is amplified and decoded to
produce the output data stream. A fundamental limitation to the number of
data marks per unit area is due to the size of the focused laser beam that
illuminates the surface. Small laser spots are required to record and read out
small data marks. More data marks per unit area translate into higher
capacity disks, so evolution of optical data storage is toward smaller spot
sizes.
Figure 3 shows a detailed picture of the laser irradiance approaching the surface, where irradiance is defined as the laser power per unit area. Ideally, maximum irradiance is located at the recording material, along with the smallest spot size s. As the distance increases away from the ideal focus, the spot size increases and the peak irradiance decreases. A defocus distance δz of only a few micrometers dramatically reduces peak irradiance and increases spot size. An approximate formula used to estimate the ideal spot size at best focus is s = λ/(sin θ), where θ is the marginal ray angle of the illumination optics, as shown in Fig. 1. Spot size s is the full width of the irradiance distribution at the 1/e2 (13.5%) irradiance level relative to the peak. The value of sin q is often called the numerical aperture or NA of the optical system.
Instead of focusing directly on the recording surface, optical disks focus through a protective layer, as shown in Fig.4 for a simple CD‐ROM. The protective layer prevents dust and other contamination from directly obstructing the laser spot at the data marks. Instead, the out‐offocus
contamination only partially obscures the laser focus cone, and data can
usually be recovered reliably. If the protective layer is scratched or damaged,
it can be cleaned or buffed.
As the protective layer gets thinner, the error rate increases to an unacceptable threshold due to obscuration of the laser beam. This sensitivity decreases as NA increases, due to the smaller defocus range associated with these systems. In addition, the free working distance separates the objective lens from the spinning disk. This separation protects the disk against accidental contact between the objective lens and the disk.
In order to maximize disk capacity, the optical system uses high NA and short wavelength. For maximum contamination protection, the protective layer should be as thick as possible. However, the combination of thick protective layer and high NA is not easily accomplished. High NA systems are sensitive to changes in substrate thickness and disk tilt. Manufacturing variations create thickness no uniformities, which are usually
a small percentage of the total disk thickness. Motor instabilities induce tilt as
the disk spins. Energy from the central portion of the spot is redistributed to
concentric rings, which degrade the quality of the read out signal. This Degrades the read out signal. Tilt causes coma, which is another form of
aberration effect, is called spherical aberration.
Sensitivity of the spot to degradation from thickness variations and disk tilt is plotted in Fig. 5 as a function of total protective layer thickness for two values of NA. In order to limit these effects, the substrate is made as thin as possible without sacrificing contamination protection.
The most conservative technology is the Video CD. Its thick protective layer, relatively low NA and long laser wavelength produce a stable system that is not very sensitive to environmental factors like dust and scratches. The ideal spot size is about 0.78/0.5 = 1.6 micrometers. Although the cover layer is thick at 1.2 mm, the sensitivity to thickness
variations and disk tilt is low because of the low NA. DVD technology uses a
shorter wavelength laser, higher NA optics and a thinner protective layer. The
combination of short wavelength and higher NA produce a spot size of about
1.1 micrometers. The protective layer had to be made thinner, because the
sensitivity to thickness variations and disk tilt is too high otherwise. DVDs
are slightly more sensitive to dust and scratches than CDs. The net effect is
not great, because higher NA reduces the focal depth and DVDs have a more
robust error management strategy.
The Advanced Optical Disk and Blu‐Ray systems both use a new blue laser source that emits 0.405 micrometer light. The Advanced Optical Disk system uses the same protective layer thickness as a DVD, and it uses the same NA objective lens. Due to the short wavelength, the spot size for the Advanced Optical Disk is about 0.62 micrometers.
Sensitivity to dust and scratches is about the same as a DVD, as well as the
sensitivity to thickness variations and disk tilt. The Blu‐Ray system uses both
higher NA and thinner cover layer. The spot size is 0.405/0.85 = 0.48
micrometers, which is the smallest spot size of all the technologies. However,
because of the high NA, the protective layer had to be made thin to limit
sensitivity to thickness variations and disk tilt. Therefore, Blu‐Ray disks are
sensitive to dust and scratches. The free working distance is nearly is same for
all technologies except Blu‐Ray. Blu‐Ray systems utilize more complicated
lens systems due to the high NA, so working distance had to be reduced. The
integrity of this reduced working distance is not clear at this time.
Disk Structure Parameters
The spot size created from the NA and wavelength parameters is the most important factor to determine the track‐to‐track spacing and the minimum channel bit length along the track. Several channel bits are encoded into each data mark. The number of channel bits per data mark depends on the modulation scheme. The relatively large spot produces relatively large data marks and correspondingly wide tracks and large channel‐bit lengths. Progressively smaller spot sizes enable smaller track spacing and shorter channel bit lengths.
To the user, all generations of optical disks look very similar. They all are round disks that are approximately 120 mm in diameter, have a central mounting hole and are approximately 1.2 mm thick. Through many years of experience with CDs, this format has proven effective and mechanically reliable. However, the manner in which data layers are arranged on the disk depends on the technology used. For example, the CD uses a simple 1.2 mm thick substrate, as shown in Fig. 6A. Data are recorded on only one side of the disk, through the clear 1.2 mm substrate, which also serves as the protective layer. DVDs, Warner HD‐DVDs and Advanced Optical Disks use the format shown in Fig. 6B, where two 0.6 mm substrates are bonded together and the data are recorded on the bond side of each substrate. DVDs also allow more two layers per side (A, B in Fig. 6B), where the layers are separated by a thin adhesive spacer. The two layers are fabricated before bonding at the same time as the individual 0.6 mm substrates. Like the CD, data are recorded and read through the clear substrates. It is likely that the Warner HDDVD and Advanced Optical Disk
will also take advantage of this multiple‐layer concept. A potential implementation of the Blu‐Ray disk is shown in Fig. 6C, where the protective layers on each side are very thin at 0.1 mm. In this case, data are recorded on the substrate, which does not serve as the protective layer. Instead, a protective layer resin is spun on and hardened or a thin protective sheet is
bonded on each side of the substrate. Because of the thin protective layer, the
Blu‐Ray disk must also be used with a cartridge.
The only optical disk technology that plans to use a Cartridge is the Blu‐Ray system. The Blu‐Ray cartridge is necessary for contamination Protection, but the working distance of around 0.1 mm and protective layer thickness of 0.1 mm are large compared to the contact recording.
The technology for making disks is very similar to existing DVD technology. Higher‐resolution mastering machines and finer control over the injection molding process should produce the required changes without substantially retooling the industry. The Blu‐Ray system requires the most changes of the three, including a blue laser, detector, and advanced objective lens. Blu‐Ray also requires new disk and cartridge manufacturing technology, which may be difficult to implement in a short time frame.
Data Management Parameters
The logical organization of data on the disk and how those data are used are considerations for data management. Data management considerations have important implications in the application of optical disk technology to storage for HDTV. For example, simply using a more advanced error correction scheme on DVDs allows a 30% higher disk capacity compared to CDs. Data rate, video format, bit‐rate scheme and HDTV play time are all data management issues.
There is a basic difference in data management between CDs and DVDs. Since CDs were designed for audio, data are managed in a manner similar to data management for magnetic tape. Long, contiguous files are used that are not easily subdivided and written in a random access pattern. Efficient data retrieval is accomplished when these long files are read out in a contiguous fashion. To be sure, CDs are much more efficient that magnetic tape for pseudorandom access, but the management philosophy is the same. On the other hand, DVDs are more like magnetic hard disks, where the file structure is designed to be used in random‐access architecture. That is, efficient recovery of variable length files is achieved. In addition, the Original error correction strategy for CDs was designed for error concealment when listening to audio, where DVDs utilize true error correction. Later generations of optical disks also follow the DVD model.
The random‐access nature of DVDs allows very efficient methods for data compression. For example, MPEG‐2 with variable bit rate allows data to be read out from the disk as they are required, rather than supplying data at a constant rate. Slowly moving scenes, like love scenes or conversations, require much less information per frame than a fast‐moving car chase or explosion. In these fast‐moving scenes, the maximum amount of information per scene is limited only by the maximum data rate of the player. For HDTV, acceptable picture quality is obtained by using MPEG‐2 with a maximum data rate of about 13‐25 Mbps for most scenes. During a slow scene, not as many files are accessed, and much less storage area on the disk is used. This architecture leaves room on the disk for the data associated with faster‐moving scenes.
Fixed‐rate schemes, like magnetic tape, supply data at a constant rate, no matter what the requirements of the scene. During fast‐moving scenes, the data stream from the tape supplies an adequate data rate. The tape speed and data rate for these devices are set by the upper limit of the scene requirements. Since the tape does not slow down during slower scenes, the data stream is ‘padded’ at these times with useless information that takes up valuable storage area on the tape. Overall, the random‐access architecture of optical disks is a much more efficient way to use the available storage area. That is, optical disks do not require as many gigabytes of user data capacity for an equivalent length and quality HDTV presentation.
It is not practical to store HDTV on CDs and DVDs with MPEG‐2. For CDs, special multiple‐beam readout or high velocity disk dives could produce the data rate, which is an advantage of the fixed‐bit‐rate scheme. However, the play time would be only a few minutes, at best. DVDs are not capable of the 13 Mbps random data rate to support MPEG‐2. The Advanced Optical Disk exhibits acceptable data rate and reasonable user data capacity for up to two hours of HDTV per side compressed with variable bitrate MPEG‐2. Blu‐ray has slightly higher capacity and data rate. The two‐hour play time for HDTV with Blu‐Ray in Table I is really a specification for realtime recording, which is not easily compressed into an efficient variable‐rate scheme. Blu‐Ray should easily provide two hours or longer of prerecorded HDTV per side compressed with MPEG‐2.
MPEG‐2 is a technique for compressing video data and replaying the data associated with certain rules that are defined in the MPEG‐2 specifications. The action of the optical disk system is not to compress data or interpret the video information rules. Instead, the optical disk system only stores and retrieves data on command from the video operating system. Therefore, as video operating systems and associated compression technology become more advanced, no fundamental changes are required to the optical disk system. MPEG‐4 technology is an advanced video compression scheme that utilizes advanced pre‐filtering and post‐filtering, in addition to a rule‐based algorithm. Estimated improvement in compression is a around a factor of three beyond MPEG‐2.
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