Nikkei Electronics Asia -- January 2010
Features
From 25 to 33.4 GB per Layer, Even Larger Blu-ray Discs on the Horizon

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Jan 1, 2010 00:03 Tadashi Nezu

It finally looks like the per-layer storage capacity of Blu-ray Discs will really be increased, from 25 GB to 33.4 GB, thanks to the appearance of a new method of evaluating Blu-ray Disc media quality even at the higher capacity. The new method is likely to accelerate development of discs offering even more storage space.

Read and write at 33.4 GB per layer would be implemented thanks to the use of partial response maximum likelihood (PRML) signal processing. The new method can continue to use the existing Blu-ray optics: a blue-violet laser diode with a 405 nm wavelength, and an object lens with a numeric aperture (NA) of 0.85.

The problem until now has been there was no evaluation technology appropriate for 33.4 GB media using PRML. PRML assumes inter-symbol interference, which makes it difficult to base optical disc quality evaluation on jitter, as is widely done now for Blu-ray and many other optical discs. A source at Sony Corp. states, "At high-density recording, such as 33.4 GB, the relationship between the error rate and jitter collapses, and it becomes extremely difficult to evaluate jitter."

Sony and Panasonic Corp. resolved this by developing the i-MLSE (Maximum Likelihood Sequence Estimation) evaluation index. Details of i-MLSE were announced at International Symposium on Optical Memory 2009 (ISOM '09), held in October 2009. The first of the two key characteristics is that i-MLSE has a strong correlation with the error rate (Fig. 1) even in read/write at 33.4 GB using PRML. The second, according to Sony, is that "i-MLSE exhibits the same relationship to signal quality as conventional jitter." In other words, it will be relatively simple to estimate the read error rate from the i-MLSE, just as can be done now with jitter.

Fig. 1 Confirming Correlation between i-MLSE and Error Rate
The i-MLSE disc evaluation index was calculated for a BD-R with a per-layer storage capacity of 33.4 GB, and shown to have a strong correlation with the symbol error rate.

The problem with i-MLSE is that calculation is complex. With recent hardware advancements, though, says Sony, "It should be possible to process in real time, just like jitter."

Sony plans to propose widespread adoption of i-MLSE via the Blu-ray Disc Association and other avenues. Sony is a core member of the Association, making it likely that i-MLSE will be adopted as a standard.