Hitachi have developed a prototype 100GB four layer Blu-Ray disc. TDK and Panasonic had also created such discs, but they required specially developed optical heads to read the disc. Hitachi however say that have used a standard optical head that has been slightly modified to read and write data. It is believed exsiting drives will only require a firmware update to be compatible. Hitachi are trying to stabalise the signal quality before considering releasing the discs.