At the 2007 International CES, Hitachi, Ltd. has demonstrated playback of a Blu-ray Disc (BD) with four recording layers at its booth. Featuring 25 GB capacity per layer, a disc can record up to 100 GB data.
There have been other academic reports of creating four- or six-layer disc media and demonstration of picking up signals from those discs using test players embedded with a special optical head, but, according to Hitachi, "This is the first demonstration of quadruple disc readout using a player based on an optical head generally used in current BD drives on the market."
Minor reform and update of optical system and firmware
An optical disc drive based on the "GBW-H10N" supporting 4x speed BD recording, which Hitachi-LG Data Storage, Inc. announced in July 2006, was used for the demonstration. The four-layer disc used in the demonstration was an outside company's product, but a Hitachi spokesperson said, "We cannot tell the name of its supplier."
On this disc, so-called 3T patterns (the shortest pattern among BD recording marks) are marked in series (repetition of a 3T mark next to a 3T space) on a layer zero (L0), which is the furthest layer from the optical head, followed by 4T patterns marked on a layer one (L1) above it, 5T patterns on a L2 and 6T patterns on a L3. When the player picks up signals from this disc's each layer and displays the signals' waveform using an oscilloscope, frequencies change depending on the layer being read so the demonstration observers can recognize it as soon as the readout layer switches. "We could have used random patterns, but that would not allow observers to recognize when the readout layer switches and we chose these patterns," said the spokesperson.
Hitachi reportedly made some alterations on firmware and the optical system inside the head to make the player compatible with four-layer BD playback. Frontend signal processing and other circuits are the same as those used in the "GBW-H10N."
What made it difficult to develop this technology was the mechanism of how to detect which layer to read. The company has not specified how the pickup lens actually detects the readout layer, but reportedly explored a wide variety of methods including a very basic way, in which the pickup lens finds a targeted layer by scanning each layer in order of shorter distance from the optical head.
Asked about the drive's commercialization, the spokesperson answered, "Including the technology standardization, nothing has been scheduled yet." The four-layer disc seems to be manufactured by coating a L0 layer with L1, L2 and L3 layers one by one. This media manufacturing process also appears to be technically very difficult under current circumstances.