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I copied this from namechamps in a thread from the other forum. It explains beautifully the inner workings of HDM players. A fantastic post IMO. In short, it would be more likely that the current and future BD player makers would go dual-format rather than Toshiba simply for the technical reasons listed below:
Doug
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To expand on what Lee said the big difference is in the optics.
DVD and CD have a numerical aperture (NA) of 0.60. To maintain compatibility HD DVD uses a NA of 0.65 which is close enough that the same lens can be used to focus the infared light (CD), red light (DVD), and blue light (HD). The NA or numerical aperture of a lens in simply a value that corresponds to the maximum angle of light that can enter the lens. To keep it simple a higher NA will have a larger incident angle and as a result a higher the optical resolution, lower focal length and lower depth of field. The limit of a lens in air is an NA of 1.0.
The "problem" with keeping the NA similar to CD/DVD is that moving from red to blue only increases the capacity by a factor of 3x while the jump from 480p to 1080p is a 6x jump in pixels. At the time the only codec was mpeg2 and there simply wasn't enough capacity. Toshiba flirted with the idea of going mpeg2 and 720p. Microsoft showing VC-1 could enabled a 2:1 compression over mpeg2 allowed HD DVD to gain support of the DVD Forum. By using an NA of 0.65 HD DVD allows the use of existing DVD lines. For all intents and purposes an HD DVD is simply a DVD with smaller pits. By using a single lens HD DVD also allows for cheaper optical pickup units.
BD on the other hand was original designed as an mpeg2 system. 3 years prior to it's release as a prerecorded playback system it was being sold in Japan as an HD recorder. Using mpeg2 BD needed a capacity of 25 & 50 GB. The numbers were chosen because at the bitrates used for HDTV in Japan 25GB would record slightly more than one hour (TV) and 50GB would record slightly more than 2 hours (movies).
Now BD and HD DVD use the exact same 405nm blue-violet diode as a laser source. So how does BD gain 2/3 more capacity? Sony changed the NA from 0.6 to 0.85. This creates as focus point closer to the laser source (and hence closer to the top of the disc). This one small change has radical effects on the material science involved.
1) The light spot is now smaller allowing the pits to be placed closer together = higher capacity and bandwidth.
2) The data layer is now located only 0.1mm away from the surface as opposed to 0.6mm for CD, DVD, HD DVD.
3) The same lens can NOT be used for other formats (CD/DVD/HD DVD) because it would be out of focus when looking at a data layer 0.6mm from disc surface.
4) The ultra thin top layer necessitated a harder top coat.
5) The BD discs can not be made on DVD stampers, and other replication equipment.
How do BD players playback DVD & CD?
Two lenses are used (or a floating lens on the PS3). The light from the infrared and red diodes are directed through a 0.6NA lens to allow proper focus at the 0.6mm data layer. The light from the blue diode is directed through a 0.85NA lens to allow proper focus at the 0.1mm data layer.
The interesting fact is that a BD drive that can playback CD and/or DVD essentially has everything it needs (at the optical level) to playback HD DVD also. This does not mean that any BD drive can playback HD DVD because they can't. It means that with little modification and cost a design for a CD/DVD/BD drive could be converted into a CD/DVD/BD/HD DVD drive. The actual material cost in adding HD DVD support to a BD drive would be minimal*. The blue diode light needs to be redirected (via a splitter) through the 0.65NA vs the 0.85NA lens used by BD.
The reverse is not true. A HD DVD only drive will still cost substantially more to build in BD playback. An HD DVD drive would need the additional and higher cost 0.85NA lens and the higher grade actuators to enable the smaller movements necessary to track the smaller BD tracks. While diodes get all the press it is the 0.85NA lens that greatly increases the complexity and cost of the BD optical pickup. While 0.60 NA lenses on CD and DVD can now be mass produced out of plastic the 0.85NA lens in BD drives is a high precision glass optic. The higher NA means the lens has a very small depth of field. If the lens is out of tolerance even slightly it will be either near sighted or far sighted and miss the data layer.
*One potential issue with lower cost of combo drives is that both technologies are proprietary and would need to be licensed. In effect the licensing cost is now doubled.
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