The New Face to Digital Storage Media Blu-Ray Disks and Drives

This article introduces the concepts of new digital storage media Blu-ray disks and drives, their evolution and new technologies behind it. Blu-ray Disc(BD) has sufficient capacity to include bonus materials and interactive sessions for an unparalleled entertainment experience. For PC users, archiving photographs, video or other data on the hard disk, Blu-ray Disc is the only recordable/re-writable format that provides such a quantum leap in optical storage . Additionally, Blu-ray Disc is based on today’s most advanced technological breakthroughs and offers full backwards compatibility. The Blu-ray Disc has a revolutionary protective cover layer utilizing an advanced hard coating technology, offering superior robustness, better than CD and DVD. This coating prevents fingerprints and scratches and will provide ease of use by improving cleansibiliity without the risk of damaging. The drive features recording and playback capabilities of all popular optical storage formats including CD, DVD and Blu-ray Disc, which allows users to select their media of choice depending on the preferred application .The BD drive features a 2x (72 Mb/s continuous data-rate) read and write speed on BD-ROM, BD-R and BD-RE media.

The Objective of Blu-ray

The standards for 12-cm optical discs, CDs, DVDs, and Blu-ray rewritable discs (BD-RE Standard) were established in 1982, 1996, and 2002, respectively. The recording capacity required by applications was the important issue when these standards were decided (See Fig.1.1.1). The requirement for CDs was 74 minutes of recording 2-channel audio signals and a capacity of about 800 MB. For DVDs, the requirement as a video disc was the recording of a movie with a length of two hours and fifteen minutes using the SD (Standard Definition) with MPEG-2 compression. The capacity was determined to be 4.7 GB considering the balance with image quality. In the case of the Blu-ray *1) Disc, abbreviated as BD hereafter, a recording of an HDTV digital broadcast greater than two hours is needed since the BS digital broadcast started in 2000 and terrestrial digital broadcast has begun in 2003. It was a big motivation for us to realize the recorder using the optical disc. 

* 1) The spelling of "Blu-ray" is not a mistake. The character "e" is intentionally omitted because a
daily-used term cannot be registered as a trademark.



In a DVD recorder, received and decoded video signals are compressed by an MPEG encoder and then recorded on the disc. To record in the same fashion for an HDTV broadcast, an HDTV MPEG-2 encoder is required. However, such a device for home use has not yet been produced. In the case of BS digital broadcasts, signals are sent as a program stream at a fixed rate, which is 24 Mbps for one HDTV program. In the program stream of BS digital broadcast there is a case that the additional data stream is multiplexed, and it is desirable to record and read the data as is. Fig.1.1.2 shows the recording capacity with the data transfer rate and recording time parameters. Two hours of recording requires a recording capacity of 22 GB or more. This capacity is about 5 times that of DVDs, which cannot achieve this capacity by merely increasing their recording density.



To obtain this capacity a number of techniques have been developed such as: employing a blue-violet laser, increasing the numerical aperture of objective lens, making the optical beam passing substrate thin, 0.1mm, and evenly thick, using an aberration compensation method of pickup adapted to the substrate thickness and dual layer discs, improving the modulation method, enhancing the ability of the error correction circuit without sacrificing the efficiency, employing the Viterbi decoding method for reading signals and improving the S/N ratio and the inter symbol interference, using the on-groove recording and highly reliable wobbling address system, developing high speed recording phase change media, etc. In addition, the convenient functions of a recording device have also been realized in the application formats. 

In addition to high picture quality, the introduction of core and new functions is indispensable for the spread of the next generation package media. For example, during the switch from VHS to DVD, digital recording and interactive functions were newly introduced. Consequently, it is anticipated that the specifications of BD-ROM will provide a high performance interactiveness and a connection to broadband services, reflecting the demands of the movie industry (Fig.1.1.3).



Recently, the digital HD broadcast started, and PDP and liquid crystal displays with large and high picture quality screens are spreading for home use. The recording of HD digital broadcasts and HD packages with BD-ROM are considered to accelerate this tendency and expected to be the trigger factors for the rapid spread of HD (Fig.1.1.4).



Blu-ray Format

The features of the BD format are summarized below:

* BD achieved a recording density five times greater than that of DVD, which uses a red laser, by adopting a blue laser and a high NA objective lens. 
* A light transmission protection layer as thin as 0.1 mm, peculiar to BD, is determined to achieve a tolerance similar to the conventional one against disc inclination even in the case of a high-NA objective lens.
* Because the protection layer is thinner than a conventional disc, an error correction method superior to conventional method is used, making up for the weaknesses caused by a lesser thickness. 
* A dual layer BD disc has also been defined for a future increase in capacity. Doubling the capacity will enable wider applications in the future. 
* BD has the same shape as conventional CDs and DVDs. Considerations have been made for a chucking system which could provide mechanical interchangeability with conventional discs.
* The modulation technique for the signal is (1-7) modulation. To withhold the DC component of the signal, a new system called 1-7PP has been developed.
* As with conventional CDs and DVDs, BD also plans to provide all types of playback including the read-only type, write-once type, and the rewritable type. The rewritable type has already been standardized, and the format is defined so that all three types are interchangeable. 
* The track format is groove recording. This is a result of consideration for interchangeability. 
* The address format is the wobble type, which has meandering grooves. By simultaneously applying two types of modulation, an extremely high reliability has been achieved. Similarly to the data error correction method, BD also has a built-in function for a stable operation.
* The image recording method of BD is MPEG-2. This was chosen from the viewpoint of interchangeability of a current contents format. 
* Sound recording method is the same as the most widely used one.
* BD employs the MPEG-2 transport stream recording method. This enables flexible editing of a digital broadcast that is recorded as is and where the data can be edited just by rewriting the playback stream. Although it is quite natural, a function for high-speed and easy-to use retrieval is built in.

BD parameters are defined with a good balance of conventional and new technologies in the best possible manner considering requests from the current market demands, interchangeability, and expandability in the future.



Dual layer Disc

standards, dual layer system is also defined for rewritable type discs. To achieve the dual layer disc, many new techniques such as super-thin recording films and assembly processes are needed. It can be said that continuous developments since 1994 produced results, i.e. the dual layer BD 1) 2). BD provides large recording capacities such as 46.6/50/54 GB (23.3/25/27 GB per layer) or more. In addition, since recording and playback can be done from one side, user does not have to turn the disc over in the drive



Contents Protection System and Interface

Since BD assumes that the HD contents of digital broadcasts are being recorded, contents protection is more important than with conventional DVDs, on which only SD contents are recorded. Thus a new contents protection system (Content Protection System for Blu-ray Disc Rewritable) optimized for MPEG-TS recordings of digital broadcasts has been adopted.
In the BD contents protection system, DES (Data Encryption Standard) with a key length of 56 bits has been adopted, and Triple DES with a key length of 112 bits has been adopted for the key generating process. In addition, for the exclusion of illegal devices (System Renewability) and for the prevention of illegal copying, RKB (Renewal Key Block) information and a Disc ID unique to the disc are written in ROM area of the disc. Each recorder (player) has a device key, which differs depending on the manufacturer or the machine. An encryption key is generated by combining the device key and RKB. Therefore, illegal devices can be excluded by updating the RKB information. Furthermore, illegal copying is also prevented by using a disc ID unique to the disc to generate the encryption key, because even a bit-by-bit copy cannot generate the encryption key .In addition to such highly-secure coding during recording, by implementing the contents protection technology in the interface to output played-back contents, illegal copying and retransmission to the general public through Internet can be prevented.

Roadmap of the Blu-ray Disc Standard

The rewritable type (BD-RE), the first standard, was issued in June 2002. The data transfer rate is the standard rate of 36Mbps (1x). The Blu-ray Disc Standard will be expanded from this fundamental rewritable type to cover all types. The hard-coating technology has made the use of BD possible without the cartridge. A bare disc is being planned to be specified as a standard in the read-only type (BD-ROM), the write-once type (BD-R) and the re-writable type (BD-RE) V2.0. These three types of physical specifications will be completed by October 2004. The storage capacity of all types of Blu-ray Disc will be 25GB (for single-layer disc) and 50GB (for dual-layer disc) or more. User data can be recorded on BD-R at the double-speed rate (72Mbps) as well as the standard rate. BD-RE will be able to record at the rate of 1x to 2x as V2.0, rewritable at 1x by V1.0 drive already manufactured.



Conclusion

The development of further high speed Blu-ray Disc is desirable. The anticipated speed is much higher that previously achieved by conventional optical discs. Although the recording speed of the media itself limited the development in the past, the limiting factor for the use of Blu-ray is the capacity of the hardware. The transfer rate for data will advance from double speed to quadruple speed (144 Mbps).
Through the above-mentioned expansion, the first generation of Blu-ray family standards will be complete. For further development, additional speed and capacity will be necessary. In the future, the speed will be raised to eight times or more. Assuming that the limit of the disc rotation speed is 10,000 rpm, 12-time speed at the outer diameter is possible. This corresponds to a hard disc with a transfer speed of about 400 Mbps. Further, a large capacity standard of several hundred GB is expected following the CD (650 MB), DVD (4.7 GB), and BD (25 GB) or more. To achieve this, both the hardware approach such as multi-value/multi-layer and the signal processing at the base of the Blu-ray system, which provides the highest density recording at present, and a software approach such as coding will be needed.

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