How Blu-ray Optical Discs Work
Why this could replace DVD discs
by Bob

The Blu-ray Disc Association's logo.The technology for storing data continues to bring us ever-increasing capacity. In 1997, DVD discs were introduced that provided 4.7GB of capacity. It replaced the older CD-ROM technology which only provided 640MB of capacity.  DVD discs revolutionized the movie industry providing an inexpensive media for distributing commercial video and music.
The industry is now going through another revolution with the introduction of Blu-ray Discs (BD). This new technology affects not only the consumer video and audio industry, but also the comercial  industry. As a matter of fact, if Blu-ray becomes the standard in the consumer market, it will surely become the standard in the computer market as well.

The market appears to be moving towards acceptance of this new media. On April 23rd, 2007, the Blu-ray Disc Association announced that since inception, Blu-ray has sold more than one million movies. The majority of these discs were sold during the first three months of this year, where Blu-ray held 70% of the High Definition market.

Blu-ray Disc has gained a large amount of support in the corporate world, with companies such as Hewlett-Packard, Dell, and Panasonic supporting it. Blu-ray Disc was first developed by Sony Corporation in 2002 as a next generation data and video storage format alternative to DVD.

Blu-ray competes with UDO from Plasmon and HD DVD jointly developed by Toshiba and NEC. Blu-ray discs hold 25GB (and 50GB double layer). UDO supports up to 30GB of data (they are working on double layer as well) and is only used by the computer industry.  It is a 5.25 disc and uses a caddy to prevent scratches and dirt from degrading the media.

Both the Blu-Ray and HD-DVD media are the same size as the DVD-ROM disc and use a hard-coat surface to protect the discs. Hard-coating provides a layer of protective material on the surface through which the data is read. HD-DVD supports 15GB on a single-layer disc, and 30GB when using dual layers. You can write a Blu-ray disc at about 9Mbytes/sec (2X) (without verification). HD-DVD supports up to 36.55 Mbit/s. UDO has a max sustained transfer rate - write of 4 MB/s (with verification). Blu-ray computer drives are backward compatible and support DVD and CD media.

The name Blu-ray Disc is derived from the blue-violet laser used to read and write this type of disc. Because of its shorter wavelength (405 nm), substantially more data can be stored on a Blu-ray Disc than on the DVD format, which uses a red, 650 nm laser. A Blu-ray Disc can store 25 GB on each layer, as opposed to a DVD's 4.7 GB. With their high storage capacity, Blu-ray discs can also hold and play back large quantities of high-definition video and audio, as well as photos, data and other digital content.

A current, single-sided, standard DVD can hold about two-hours of standard-definition movie with a few extra features. But a high-definition movie, which has a much clearer image, takes up about five times more bandwidth and therefore requires a disc with about five times more storage. As TV sets and movie studios make the move to high definition, consumers are going to need playback systems with a lot more storage capacity.

If the Blu-ray promoters have their way, Blu-ray will be the next-generation digital video disc. It can record, store and play back high-definition video and digital audio, as well as computer data. The advantage of Blu-ray is the sheer amount of information it can hold:

  • About 9 hours of high-definition (HD) video can be stored on a 50 GB disc.
  • About 23 hours of standard-definition (SD) video can be stored on a 50 GB disc.
  • On average, a single-layer disc can hold a High Definition feature of 135 minutes using MPEG-2, with additional room for 2 hours of bonus material in standard definition quality. A dual layer disc will extend this number up to 3 hours in HD quality and 9 hours of SD bonus material.
Building Blu-ray Discs
Blu-ray discs not only have more storage capacity than traditional DVDs, but they also offer a new level of interactivity. Using the latest consumer Blu-ray systems, users will be able to connect to the Internet and instantly download subtitles and other interactive movie features. With Blu-ray, you can:
  • record high-definition television (HDTV) without any quality loss
  • instantly skip to any spot on the disc
  • record one program while watching another on the disc
  • create playlists
  • edit or reorder programs recorded on the disc
  • automatically search for an empty space on the disc to avoid recording over a program
  • access the Web to download subtitles and other extra features
Discs store digitally encoded data, video and audio information in pits -- spiral grooves that run from the center of the disc to its edges. A laser reads the other side of these pits -- the bumps -- to play the movie or program that is stored on the DVD. The more data that is contained on a disc, the smaller and more closely packed the pits must be. The smaller the pits (and therefore the bumps), the more precise the reading laser must be.

Unlike current DVDs, which use a red laser to read and write data, Blu-ray uses a blue laser. A blue laser has a shorter wavelength (405 nanometers) than a red laser (650 nanometers). The smaller beam focuses more precisely, enabling it to read information recorded in pits that are only 0.15 microns (m) (1 micron = 10-6 meters) long -- this is more than twice as small as the pits on a DVD. Plus, Blu-ray has reduced the track pitch from 0.74 microns to 0.32 microns. The smaller pits, smaller beam and shorter track pitch together enable a single-layer Blu-ray disc to hold more than 25 GB of information -- about five times the amount of information that can be stored on a DVD.

Blu-ray discs are constructed very differently from DVDs.

Each Blu-ray disc is about the same thickness (1.2 millimeters) as a DVD. But the two types of discs store data differently. In a DVD, the data is sandwiched between two polycarbonate layers, each 0.6-mm thick. Having a polycarbonate layer on top of the data can cause a problem called birefringence, in which the substrate layer refracts the laser light into two separate beams. If the beam is split too widely, the disc cannot be read. Also, if the DVD surface is not exactly flat, and is therefore not exactly perpendicular to the beam, it can lead to a problem known as disc tilt, in which the laser beam is distorted. All of these issues lead to a very involved manufacturing process.

How Blu-ray Reads Data
The Blu-ray disc overcomes DVD-reading issues by placing the data on top of a 1.1-mm-thick polycarbonate layer. Having the data on top prevents birefringence and therefore prevents readability problems. And, with the recording layer sitting closer to the objective lens of the reading mechanism, the problem of disc tilt is virtually eliminated. Because the data is closer to the surface, a hard coating is placed on the outside of the disc to protect it from scratches and fingerprints.

Blu-ray discs also differ from CDs and DVDs in the way that data is written to them.
The design of the Blu-ray discs saves on manufacturing costs. Traditional DVDs are built by injection molding the two 0.6-mm discs between which the recording layer is sandwiched. The process must be done very carefully to prevent birefringence.

  1. The two discs are molded.
  2. The recording layer is added to one of the discs.
  3. The two discs are glued together.
Blu-ray discs only do the injection-molding process on a single 1.1-mm disc, which reduces cost. That savings balances out the cost of adding the protective layer, so the end price is no more than the price of a regular DVD.

A BD-ROM disc researcher holds a disc up to the light.

Blu-ray also has a higher data transfer rate -- 48 Mbps (megabits per second) -- than today's DVDs, which transfer at 10 Mbps. A Blu-ray disc can record 25 GB of material in just over an hour and a half.

Unlike DVDs and CDs, which started with read-only formats and only later added recordable and re-writable formats, Blu-ray is initially designed in several different formats:

  • BD-ROM (read-only) - for pre-recorded content
  • BD-R (recordable) - for PC data storage
  • BD-RW (rewritable) - for PC data storage
  • BD-RE (rewritable) - for HDTV recording
Blu-ray Systems for Computer Storage  
Jukeboxes or libraries are used to store and retrieve data stored on optical discs. They have robotics that allow you to select any disc for writing or reading back the data. Archiving appliances make it easy to archive data to a stack of optical discs. The discs have to be placed back on the input stack to read them back into the system.  Here are more details:
Jukeboxes or Libraries
Optical Libraries usually attach to a computer server, but can also be attached through a thin server box directly to the network. 
The jukebox or library includes sophisticated software that helps you write and read the discs in the jukebox. The jukebox has a number of drives. Disc-NSM Corporation has introduced a number of Blu-ray based libraries or jukeboxes for storing computer data. The DISC BD Series is a modular design product family utilizing the Blu-ray technology with 25 or 50 GB per media. The DISC BD Series capacity ranges from 1 TB to 34 TB and provides up to 14 parallel data streams of 9MB/s. WORM and rewritable media are supported, offline data management is maintained by intelligent pack technology.
Archiving Appliances
Archiving appliances can be used to just store data on optical discs. These automated optical writing and printing system utilizes PoINT Archiving software. They can write to DVD or Blu-ray discs. This system is a complete Archiving, Management and Restoration Solution that connects to a Windows PC and can be shared on the network.
This archiving system can be set up to operate completely automatically. The Archiver software periodically monitors network folders and copies or moves files to removable optical media according to Administrator defined filters. Additionally, criteria can be defined to identify and archive multiple versions of the same named file.
If you would like more information about any of the technologies described, please contact us at 1-800-431-1658 or 914-944-3425.

Published by Bob Mesnik
Copyright © 2007 Kintronics, Inc.. All rights reserved.
For more information, please contact us 1-800-431-1658 or 914-944-3425 (outside the USA) or by email infohome at kintronics.com
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