Offering 25GB per layer, Blu-Ray discs provide the densest form of optical storage available today. Still, it seems Sony has decided that this wasn't good enough. Utilizing a technology called partial response maximum likelihood (PRML) signal processing, Sony has been able to increase the layer density by 33% to 33.4GB per layer.
PRML technology has long been used in traditional mechanical hard drives to increase aureal densities. However, till now it has been unusable with such large optical media due to problems in evaluating disc quality in which error-rate and jitter diverge. Sony and Panasonic worked together to get around this issue by developing the i-MLSE (Maximum Likelihood Sequence Estimation) evaluation index.
According to Sony, "i-MLSE exhibits the same relationship to signal quality as conventional jitter." The main problem with i-MLSE is its complexity, but Sony believes that with current hardware advancements, the calculation could be done in real-time like is done with jitter on current media.
The new discs are to be compatible with current Blu-Ray optics, so all that would be needed for current players to be able to read them is likely just a firmware update.