Sony announced at the "ISOM'04" convention that they've developed a "BD/CD Dual Format Disc," which is exactly what it sounds like. It is both CD and Blu-Ray layers o­n the same disc. This means two things. You can either have a disc with o­ne "Blu-Ray side" and o­ne "CD side," or a disc with o­ne "Blu-Ray layer" and o­ne "CD layer."Needless to say, this announcement is another nail in HD DVD's coffin.

Sony announced the "BD/CD Dual Format Disc" at this year's ISOM (International Symposium of Optical Media) conference, which is an integration of Blu-ray Disc media and CD media. It was proposed to help spread the usage of Blu-ray Disc media. This was accomplished because the respective structures of Blu-ray Disc media and CD media are similar. Conventional CD media, when looking from the light's incident direction, consists of a 1.1mm thick disc substrate, recording layer, reflective layer, and protective layer. o­n the other hand, Blu-ray Disc media has a 0.1mm thin cover layer, recording layer, reflective layer, and 1.1mm disc substrate. In short, the construction of the latter is the opposite of the former.

Sony proposed the following two new structure types: (1) A structure that consists of the laser light for CDs hitting a different recording/playback layer than that of Blu-ray disc lasers. This structure would be made by putting a CD recording layer o­n the back of the Blu-ray disc recording layer. Laser light for reading CD information would enter from the opposite side of disc substrate. To the end user, this would create o­ne side of the disc being "Blu-Ray," and o­ne side being "CD." (2) A structure that consists of the CD and Blu-ray disc information being recorded/played from the surface. There would be a Blu-ray Disc recording layer before the disc substrate, and a CD recording layer inside of the disc substrate.

The advantage of (1) would be simple applications in CD/Blu-ray Disc ROMs, rewritable media, and media for "after-recording." Sony has also said that it is possible to have two Blu-ray disc layers o­n o­ne side and o­ne CD layer o­n the other side. This differs from the structure of (2), where laser light for the CD write/read layer would enter in areas where there is no Blu-ray Disc recording surface. The advantage of this structure would be saving time from not having to turn the disc over, as well as being able to use the other side of the disc for a label surface.

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http://nikkeibp.jp/wcs/leaf/CID/onair/jp/elec/337888