Codecs are compression schemes that can be used to store audio and video information on a disc. The BD-ROM specification places requirements on both hardware decoders (players) and the movie-software (content).

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 developed jointly by the same organization (ISO/IEC) as MPEG-2. 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 legal).

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.[8] For lossless audio in movies in the PCM, Dolby TrueHD or DTS-HD 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.[9] 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.

What is a Blu-ray Disc?
A current, single-sided, standard DVD can hold 4.7 GB (gigabytes) of information. That's about the size of an average two-hour, standard-definition movie with a few extra features. But a high-definition movie, which has a much clearer image (see How Digital Television Works), 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.

Blu-ray is 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 to Blu-ray is the sheer amount of information it can hold:

A single-layer Blu-ray disc, which is roughly the same size as a DVD, can hold up to 27 GB of data -- that's more than two hours of high-definition video or about 13 hours of standard video.
A double-layer Blu-ray disc can store up to 50 GB, enough to hold about 4.5 hours of high-definition video or more than 20 hours of standard video. And there are even plans in the works to develop a disc with twice that amount of storage.

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