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 So what are the differences between Blu-ray and HD-DVD?

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Disc Structure

When we take a look to the specifications above, we immediately see that the capacity is a most important difference. Blu-ray offers a significantly higher storage capacity. The reason for this is the disc itself: while HD-DVD uses a similar disc to the DVD (0.6 mm disc with 0.6 mm protective coating), Blu-ray steps away from the DVD norm and uses a 1.1 mm disc with a protective layer of only 0.1 mm. The recording layer is therefore very close to the surface of the disc. The advantage is that the laser has less material to read through, which allows a higher NA, a lower track pitch, a smaller pit length and therefore more storage compared to HD-DVD.

But this proximity of the information layer also means that the BD disc is more vulnerable to accidental scratches. Furthermore, the lens has to be closer to the surface of the disc than any other optical disc before, which increases the risks of the lens accidentally hitting the disc. This seems so serious that the Blu-ray standard specifically mentions these risks and even recommends adding a 'bumper" to the OPU. In practice, these high constraints will require a very high precision of both BD media manufacturing and drive's tilt control.

At first the BD discs were supposed to require a cartridge or a special coating to protect the disc, but later this idea was abandoned. Furthermore, TDK has managed to develop a hard coating that complies with the Blu-ray specifications for the disc, this should make a BD more resistant to damage and fingerprints than a current DVD.

The HD-DVD standard, on the other hand, offers a lower storage capacity, but through its similarity to the current DVD's, again no hard coating is required (since the data layer is equally distant from both sides of the disc, it is naturally more difficult to damage). It is also much easier to make future optical drives backward compatible.

Note that Blu-ray offers 3 different storage capacities corresponding to 3 different pit lengths (23.3, 25 and 27 GB), and even smaller pit lengths could be added in the future as BD equipment will improve. On the other hand, HD-DVD offers a single capacity (15 GB) with a fixed pit length. This is actually not truly the case, because the pit length changes on a given HD-DVD disc: indeed, if the data area uses minimum pit length of 204 µm, the so called System Lead In and System Lead Out areas use a minimum pit length of 408 um. The purpose of these half-density regions is all the more puzzling that these pits are there even larger the ones on a DVD-ROM, which is pretty strange for a blue laser disc. Toshiba hinted that this large pit size had been chosen to guarantee that this region will be readable even when pits are badly defined on the disc.

Data Processing

HD-DVD uses a modulation scheme called ETM (Eight to Twelve Modulation) inspired from CD and DVD methods: each byte of data is converted into twelve bits which satisfy to a RLL(1,10) code (all '1' bits must be separated by at least 1 and at most 10

'O' bits). All these conversion patterns are stored into ETM tables similar to EFM/EFM+ tables. Some additional rules exist to limit the number or RL2 (which have a smaller amplitude, thus are harder to read) and to limit the DSV (Digital Sum value).

On the other hand, Blu-ray uses a brand new modulation mechanism called 17PP (RLL(1,7) with Parity preserve/Prohibit RMTR). 'Parity Preserve' means that the modulated bits have the same parity (even or odd number of '1') as the original data bits. 'Prohibit RMTR' means that special replacement rules exist to limit the number of RL2 (similar to what exists for HD-DVD). Contrary to all previous modulation mechanisms (8 to 14 for CD, 8 to 16 for DVD), with 17PP incoming data is considered as a bit stream, which means that at each step a variable number of data bits is modulated, based on a few simple rules. In the end, the overhead is equal to the one added by ETM (+50%).

When pits become smaller, scratches become more difficult to handle because they corrupt more bits. Therefore, both blue formats had to propose new error correction methods which could handle errors more efficiently, especially burst ones. HD-DVD standard went for the easiest solution, all the data containers (frames, sectors, ECC blocks) and error correction algorithms (orthogonal PI/PO corrections) are reused from the DVD standard. The only difference is that that one HD-DVD ECC block corresponds to two concatenated DVD ECC blocks, i.e. an HD-DVD ECC block is 64kB large with 20 columns of PI parity bytes and 16 rows of PO parity bytes. As a result, the maximum correctable burst error length is 7.1 mm (more than DVD).

A HD-DVD ECC block

Blu-ray uses a new error correction strategy, also based on Reed-Solomon codes, called LDC (Long Distance Code) and BIS (Burst Indication Subcode). The LDC parity bytes are RS (248,216,33) codes operating on data columns. The BIS blocks contain control and addressing information, which are protected by independent RS (62,30,33) codes. These BIS blocks are organised in 3 'picket columns", evenly spaced between user data. The idea of these 'pickets" is as follows, when errors are detected in two consecutive BIS codewords, it is likely that this was caused by a burst error, thus that lots of data bytes between these two columns are bad. This information can then be used as erasure indications to increase the correction capabilities of the LDC codes.  

A Blu-ray ECC block

On top of this, two diagonal interleaving steps (similar to what is used on CDs) further decrease the impact of burst errors on error correction. All in all, the absolute Blu-ray efficiency against burst errors is similar to the one of HD-DVD (slightly over 7 mm).

Conclusion

Differences