Most, but it is not (AFAICS) a required portion of the spec - I'm pretty certain you'd need something similar to the multi-laser strategy that CD/DVD combinatio

Most, but it is not (AFAICS) a required portion of the spec - I'm pretty certain you'd need something similar to the multi-laser strategy that CD/DVD combination drives use. The Blu-Ray Association says[1] that they expect it to be the case in practice, but that it's a market decision for each implementer, which sounds like it's not a requirement. [1] - http://www.blu-ray.com/faq/#bluray_backwards_compatible http://www.blu-ray.com/faq/#bluray_backwards_compatible