Blu-ray: Frequently Asked Questions

With a bevy of products hitting the market, and with movies beginning to line store shelves, now seems a good time for a primer to the Blu-ray Disc format, one of the two high-definition formats looking for a place of prominence in living rooms around the world.

What is Blu-ray Disc?

Blu-ray Disc is one of two formats vying to be the successor to DVD. Like DVD, Blu-ray Disc is an optical disc format. And like its competitor, HD DVD, Blu-ray Disc requires a blue-diode laser to read data stored on a disc.

Which companies back Blu-ray Disc?

The Blu-ray Disc format is governed by the Blu-ray Disc Association, whose members include such consumer electronics giants as Hitachi, LG Electronics, Matsushita Electric, Philips Electronics, Pioneer, Samsung Electronics, Sharp, Sony, and Thomson. The DVD Forum, which manages the current DVD format, is overseeing the course of HD DVD; Intel, Microsoft, and Toshiba are all backing the HD DVD format.

Backing Blu-ray are seven of the eight major movie studios: Lions Gate, MGM, Paramount, Sony Pictures Entertainment, Twentieth Century Fox, Walt Disney Studios, and Warner Brothers. Of that group, only Paramount and Warner are actively supporting both Blu-ray Disc and competitor HD DVD. (Also in HD DVD's corner is Universal Studios, the lone major studio not supporting Blu-ray.)

On the gaming front, Sony's high-profile PlayStation 3 console integrates a Blu-ray Disc drive, for use with both PS3 games produced on Blu-ray Disc media and Blu-ray Disc movies.

How does Blu-ray Disc work?

Blu-ray Discs store information on an optical disc that's the same diameter and thickness as a DVD but holds data just 0.1mm beneath the surface of a polycarbonate cover layer. With a blue laser and a special high-numerical-aperture objective lens, a single-layer Blu-ray Disc can pack up to five times the amount of data that a DVD can.

Whereas DVD uses a 650-nanometer red laser to read information from a disc, the Blu-ray Disc format uses a shorter-wavelength, 405-nanometer blue laser. With Blu-ray, data is packed more tightly: The pinpoint focusing precision of Blu-ray's shorter-wavelength beam and the special lens allow data to be recorded into pits and marks that are less than half the size of those on DVD (a 0.32-micron track pitch, defined as the gap between tracks, for Blu-ray versus 0.74 microns for DVD). Another area of efficiency for Blu-ray lies in the short length of the data marks written to disc: about 111 nanometers on Blu-ray, and about 267 nanometers on DVD.

What is the capacity of Blu-ray Disc?

Blu-ray Disc is available in several flavors. Prepackaged movies and data content come on BD-ROMs, which are available as 25GB single-layer and 50GB dual-layer discs. By contrast, DVD maxes out at 4.7GB on a single-layer disc and 8.5GB on a dual-layer disc. Competing high-def format HD DVD also falls short of Blu-ray's capacity, with its 15GB single-layer discs and 30GB dual-layer discs.

Recordable BD-R and rewritable BD-RE discs offer the same capacity as BD-ROM does. Right now the fastest writable media is rated at 2X--also the current maximum write speed of Blu-ray Disc burners. TDK has demonstrated that it can make a 100GB disc; however, this disc is not part of the current format, nor is it likely to be for some time.

What are Blu-ray's speeds and feeds?

The 1X data-transfer rate for Blu-ray Disc is 36 megabits per second for data and 54 mbps for movies, as opposed to 11 mbps for 1X DVD (data) and 10 mbps for DVD-Video. HD DVD's maximum data-transfer rate is 36.55 mbps for both movies and data.

According to the Blu-ray Disc Association, the rotational speed of Blu-ray exceeds that of DVD--by how much depends upon which type of Blu-ray Disc you're looking at. For Blu-ray data discs, the linear velocity (the speed at which the disc surface is moving past the fixed laser beam) is about 5 meters per second, as opposed to 3.5 meters per second on a DVD disc.

For movie discs, the difference is more dramatic. Blu-ray movie discs have a faster linear velocity because the specification calls for the discs to spin at 1.5X, which translates to about 7.5 meters per second--more than twice as fast as a DVD. Blu-ray Disc players are required to spin a disc at 1.5X in order to guarantee that you'll be able to get the full data-transfer rate of 54 mbps on a movie disc.

The maximum bitrate for Blu-ray Disc movies is 48 mbps for video and audio; the total bitrate on a BD Video player--a dedicated player for use with HDTVs--is 54 mbps, a figure that includes the disc's navigation information and Java-based menus. In comparison, the HD DVD format allows for 36.55 mbps total.

Which video codecs does Blu-ray Disc support?

All Blu-ray Disc players can play video compressed in MPEG-2, the same compression format used currently on DVD (albeit in a form optimized for high-definition content). In addition, the Blu-ray Disc format supports MPEG-4 (H.264) AVC and VC-1, two next-generation video compression/decompression schemes that handle video more efficiently than MPEG-2 does. Blu-ray movies may be encoded in any of these three formats. Many first-wave Blu-ray Disc movies are encoded in MPEG-2; some studios are opting for that format simply because they're familiar with the MPEG-2 codec and they already have the tools necessary to use it.

Which audio codecs does Blu-ray Disc support?

Blu-ray Disc supports a slew of audio codecs--but not every player is required to support every flavor within those codecs. According to the Blu-ray Disc Association, every Blu-ray Disc player must be able to reproduce at least two channels of audio from any of the six codec streams indicated in the Blu-ray spec: Lossless PCM, Dolby Digital, Dolby Digital Plus, Dolby TrueHD, DTS, and DTS-HD.

Sound confusing? That's because it is. "In other words," explains Andy Parsons, chair of the U.S. region of the Blu-ray Disc Association's Promotional Committee, "if a DTS-HD stream is present, a player must be able to reproduce at least two channels of the legacy or 'core' DTS signal that can be derived from [the DTS-HD stream]." The same is true for Dolby TrueHD. The only "optional" aspect of any codec in the BD spec, adds Parsons, is whether a player can make full use of the more advanced audio possibilities, such as a full 7.1 channels or lossless DTS-HD or Dolby TrueHD.

Although the Blu-ray Disc spec does not require more than two channels, Parsons says "it's highly likely that all BD players will be able to output at least 5.1 channels when they are available in the title."

Do home-theater Blu-ray Disc players have any minimum storage requirements?

The first wave of players have no such minimum. Today, so-called persistent memory is optional on Blu-ray. However, as of June 2007, new Blu-ray Disc movie players will require a minimum of 256MB of persistent memory storage, in the form of flash memory. If the player has an Internet connection, the minimum required local storage will be 1GB of memory.

Do all discs have a hard-coat covering or something equivalent?

Yes. As mentioned earlier, with Blu-ray Disc the data resides close to the surface. The top of the substrate is 0.1mm beneath the cover layer surface, which means the substrate itself is 1.1mm thick. "A greasy fingerprint could be destructive, since you're focusing so close to the fingerprint," says Parsons.

As a result, the Blu-ray Disc specification requires that some variation of hard coating be applied to every disc. The spec has a minimum hardness rating that the disc must achieve to survive normal handling by users. A coating is one method of achieving that hardness requirement, and vendors can take different approaches (for example, a spin coating on the disc's surface).

Do you need an Internet connection for Blu-ray Disc?

An Internet connection is not required for Blu-ray Disc. Aside from the PlayStation 3, none of the first-generation players have an Internet connection (Pioneer's BDP-HD1 has an ethernet connection, but this is solely for streaming content from a PC to your TV).

What is BDLive?

BDLive is the Blu-ray Disc's nomenclature for online interactivity. No players--including Sony's PlayStation 3--currently support BDLive. For BDLive, the spec calls for an ethernet connection and a minimum of 1GB of local storage on the player.

What is the programming language of Blu-ray Disc?

The Blu-ray Disc format uses BD-Java, or BD-J, as its application layer for designing menus and presenting interactive content on the disc. (The HD DVD equivalent is Microsoft's XML-based HDi.) Typically, movie titles with the most interactivity (Twentieth Century Fox's The League of Extraordinary Gentlemen is an early example) will be authored with BD-J.

Some Blu-ray discs are authored in BD-MV (or HDMV, for High Definition Movie Mode). These discs are easier for content producers to author, as they don't use BD-J; nevertheless, they are still DVD-like, with navigation data such as the pop-up menus that are quickly becoming one of the perks of both next-gen high-def video formats.

All Blu-ray Disc players will handle BD-Java and BD-MV.

What copy protection scheme does Blu-ray use?

Both HD-DVD and Blu-ray Disc use AACS (Advanced Access Content System) to protect content stored on a disc. Currently in its preliminary version, AACS lacks provisions for managed copy--which means early players probably won't be able to handle making legal copies of disc content to another device when that functionality becomes available.

Blu-ray Disc also has two additional layers of security that HD DVD lacks: BD Plus and ROM Mark. According to Parsons, "BD Plus is a different layer of content protection that should not have any direct impact on [AACS and managed copy]. All of the members of BDA are fully committed to the idea of managed copying. It's just a question about implementation. It's a case of getting a final copy of AACS in our hands [before we] announce anything."

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