Special Coverage: Blu-ray

Codecs

DVD uses the MPEG-2 video codec (enCOder/DECoder) developed by the Moving Picture Experts Group (hence its name) to compress video data. This reduces storage and bit-rate requirements so that a standard-definition, full-length movie -- and then some -- can fit in 4.7 GB and spool out to the display at less than 10 Mbps.

Newer, more advanced codecs reduce these requirements further by throwing away even more data and concealing the loss more cleverly. Examples of these new codecs include MPEG-4 Advanced Video Coding (AVC, a.k.a. H.264) and Windows Media Video 9 (WMV9), the latter recently adopted as a standard by the Society of Motion Picture and Television Engineers and renamed VC-1.

In general, the more compression, the lower the quality, but advances in codec algorithms mitigate this tendency. In fact, the BDA recently completed a round of subjective testing using a slightly modified version of H.264. At bit rates of 8 to 12 Mbps, they discovered that viewers found BD to be superior to D-VHS tape; at 16 to 20 Mbps, they found it to be equal in quality to the original D5 master (a lightly compressed digital-videotape format commonly used to store movies after they've been telecined from film).

Blu-ray titles can use MPEG-2, H.264, or VC-1, and all players must be able to decode all three video codecs. Early titles from Sony Pictures and Lionsgate use MPEG-2 because there are many available encoders that have reached a high level of sophistication and speed, making them a good choice for getting content to market quickly.

Like HD DVD, Blu-ray titles are not required to include any specific audio codec. Blu-ray players must be able to decode standard Dolby Digital, DTS, and PCM at sample rates of 48 or 96kHz. The new advanced audio codecs, such as Dolby Digital Plus, Dolby TrueHD (what Blu-ray calls Dolby Lossless), DTS-HD, and DTS-HD Master Audio (which, like Dolby TrueHD, is also lossless), are optional. As of this writing, there is no way to digitally output these advanced codecs to an outboard decoder; that must wait until HDMI 1.3 is implemented in both players and receivers, which will likely be sometime in 2007. Meanwhile, they can be decoded in the player and sent from the multichannel analog outputs if the player's manufacturer provides that capability.

If Dolby TrueHD is included on a disc, it must also have a standard Dolby Digital track that can be used if the player can't decode the lossless stream. Dolby Digital Plus and the DTS advanced codecs already provide the ability to down-convert to standard Dolby Digital or DTS, which means that players will be able to deal with any audio format on any disc.

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