Tout savoir sur le format BD-ROM et ses protections 128 bits AACS

In a seminar held last week in Japan, the Blu-Ray Disc Association (BDA) explained the content protection technology used in the BD-ROM format, based on the AACS technology.

In the beginning of the seminar, Mr. Ajima Kosuke of the BDA's Content Protection Group compared the protection scheme applied to the BD-ROM format with the one used by the rival HD DVD-ROM format.

The main difference between the two incompatible formats lies in the copyright protection system (CPS). The HD DVD camp has chosen to use the AACS technology only, compared to the AACS BD-ROM technology which also uses two additional layers of security, the BD-ROM Mark and the BD+.

Below you can an overview of the BD-ROM format and a comparison with the corresponding HD DVD's proposal.

The AACS technology

The Advanced Access Content System provides content protection for BD-ROM AV data. Generally, it defines the rules how to handle such protected content. AACS uses an advanced encryption system with 128-bit keys, as well as a next generation media key block. The technology offers precise revocation of compromised devices and enhanced drive authentication. The technology also supports the managed copy feature, which enables authorized movement of content onto home network and portable devices.

The Image Constrain Token (ICT) is also a feature of the AACS which will be applied to analog video outputs. The technology will downgrade the HD video signal which comes out from analogue outputs of a HD player to a maximum resolution of 960 x 540. Image Constrain Token will be optionally employed by the video content owner so until now, early adopters of the HD technology (HD DVD and Blu-Ray) have not experienced it. Note that any the BD-ROM release that supports the ICT technology should clearly notify it on the disc package.

Another analogue protection supported by the AACS is the Digital Only Token technology. It restricts video content from flowing out of analogue ports of a player, but it is addressed to be used in new business models, which are materially different than the traditional video sell-through. Such video releases would be theatrical versions or HD movie previews. Digital Only Token will be permitted under the terms of the final AACS agreement, expected to be available in Q2 2006.

Audio Watermark is also included in AACS. It is an inaudible signal that a content provider may choose to insert into the audio track of their content. The feature is not supported by current (interim) devices and it is expected to be part of the final AACS agreement.

Last, both HD DVD and Blu-Ray camps have decided to phase-out any analog broadcasting in US and Japan by 2013. The so-called "Analog Sunset" is expected to begin in 2010, where analog output will be limited to SD Interlace Modes only (i.e., Composite, S-Video, 480i component).

All the above features are supported by both HD DVD and Blu-Ray formats. As mentioned before, the BDA has added two additional layers of protection to BD-ROM, the ROM-Mark and the BD+.