The name Blu-ray Disc is derived from the blue-violet laser used to read and write this type of disc. Because of this shorter wavelength (405 nm), substantially more data can be stored on a Blu-ray Disc than on the common DVD format, which uses a red, 650 nm laser. In comparison to HD DVD, which also uses a blue laser, Blu-ray Disc has more information capacity per layer (currently 25 GB, but test media is up to 33 GB). Several manufacturers have released 50 GB recordable BDs and rewritable discs. All supporting studios have either already released or have announced release of movies on 50GB discs.
The Blu-ray Disc is a similar PDD, another optical disc format developed by Sony (which has been available since 2004) but offering higher data transfer speeds. PDD was not intended for home video use and was aimed at business data archiving and backup, although currently it is gaining popularity as an HD video format medium. The UDO format is aimed at similar purposes. Blu-ray Disc is currently embroiled in a bitter format war against the HD DVD disc.
For video, ISO MPEG-2, H.264/AVC, and SMPTE VC-1 are player-mandatory. (This means all BD-ROM players must be capable of decoding all three video codecs.) MPEG-2 video allows decoder backward compatibility for DVDs. H.264, sometimes called MPEG-4 part 10, is a more recent video codec. VC-1 is a competing MPEG-4 derivative codec proposed by Microsoft (based on Microsoft’s previous work in Windows Media 9). BD-ROM titles with video must store video using one of the three mandatory codecs (multiple codecs on a single title are allowed).
Initial versions of Sony’s Blu-ray Disc-authoring software only included support for MPEG-2 video, so the initial Blu-ray Discs were forced to use MPEG-2 rather than the newer codecs, VC-1 and H.264. An upgrade was subsequently released supporting the newer compression methods so the second wave of Blu-ray Disc titles were able to make use of this. The choice of codecs affects disc cost (due to related licensing/royalty payments) as well as program capacity. The two more advanced video codecs can typically achieve twice the video runtime of MPEG-2. When using MPEG-2, quality considerations would limit the publisher to around two hours of high-definition content on a single-layer (25 GB) BD-ROM.
For audio, BD-ROM players are required to support Dolby Digital AC-3, DTS, and linear PCM (up to 7.1 channels). Dolby Digital Plus, and lossless formats Dolby TrueHD and DTS HD are player optional. BD-ROM titles must use one of mandatory audiotracks for the primary soundtrack (linear PCM 5.1, Dolby Digital 5.1 or DTS 5.1.). A secondary audiotrack, if present, may use any of the mandatory or optional codecs.For uncompressed PCM and lossless audio in Dolby TrueHD or DTS-HD Master Audio formats, Blu-ray Discs support encoding in up to 24-bit/192 kHz for up to six channels, or up to eight channels of up to 24-bit/96 kHz encoding. For reference, even new big-budget Hollywood films are mastered in only 24-bit/48 kHz, with 16-bit/48 kHz being common for ordinary films.
For users recording digital television broadcasts, the Blu-ray Disc’s baseline datarate of 36 Mbit/s is more than adequate to record high-definition broadcasts. Support for new codecs will evolve as they are encapsulated by broadcasters into their MPEG-2 transport streams, and consumer set-top boxes capable of decoding them are rolled out. For Blu-ray Disc movies the maximum transfer rate is 54 Mbit/s (1.5x) for the combined audio and video payload, of which a maximum of 40 Mbit/s can be dedicated to video data. This compares favorably to the maximum of 36.55 Mbit/s in HD DVD movies for audio and video data.
Originally, Blu-ray Disc drives in production could only transfer approximately 4.5 MB/s or 36 Mbit/s (54 Mbit/s required for BD-ROM), but 2x speed drives with a 9 MB/s or 72 Mbit/s transfer rate are now available. Rates of 8x (288 Mbit/s) or more are planned for the future. First devices used AT Attachment but newest ones support Serial ATA.
