Triple-layer DVD
The latest development in DVD is the single-sided, triple layer illustrated in Figure 20.5 consisting of one DVD and two HD DVD layers. It is capable of either a DL 30 GB HD DVD and single-layer 4.7 GB configuration or single-layer 15 GB HD DVD and DL 8.5 GB DVD. This is a change from the original DVD twin standard that only allowed one HD DVD and one DVD, on the same side of the disc.
DVD encoding process
DVD technology employs a diverse range of advanced techniques including optics, data compression and processing, electro-mechanical servo control and micro processing as well as digital and television technology. Optical technology is used to write data onto a disc and to subsequently retrieve it from the disc. Retrieving information from a disc involves reading the data, bit by bit off a rotating disc using a laser beam that must be positioned accurately to follow a few micron spiral track on a rotating disc. The whole process of reading a DVD disc is controlled and all units are programmed by a powerful processing chip.
DVD video has three main components: video, audio and sub-picture (Figure 20.6). The latter include subtitles, captions, menus, karaoke lyrics, etc. These three components are stored in what is known as streams. In addition there are two other streams: presentation control information (PCI) and data search information (DSI). Literally, a stream means a data flow, something akin to a track on a tape recorder. DVD-video supports one stream of video, up to eight streams of Audio and up to 32 sub-picture (SP) streams. HD DVD and BD formats support up to nine video streams, up to eight streams of Audio and up to 32 SP streams.
The streams are encoded separately to form video, audio and sub- picture packetised elementary streams (PESs) described in Chapter 4.
In the case of DVD-video and HD DVD, before the packets are multiplexed they are broken into 2-KB (2048 Bytes) user data packets (Figure 20.6). Header and correction bits are then added to each packet resulting in 2046-Byte video, audio, etc. PES packets. The header includes ID bits to distinguish between the various types of packets: video, audio, sub- picture, PCI and DSI. The packets are then fed into the multiplexer to form the programme stream to be followed by the framing and forward error correction (FEC) to end up with 2418-Byte recording sectors. Before writing the data onto the disc, it is fed into an 8-to-16 modulator (EFM+), which doubles the recording sector size to 4836 Bytes.
Each packet of data is fully identifiable in terms of data type, stream number and timing information for synchronisation purposes. The packets are then multiplexed to form what is known as a programme stream, which following framing and the addition of data correction bits, is stored on a disk. Unlike digital television broadcasting in which the bit rate of the programme stream is fixed and the data compression has to be adjusted to accommodate it, in DVD application, variable bit rate is used.
In the case of BDs, the video, audio, etc. PES packets from the encoders are chopped into smaller 188-Byte packets to form a transport stream using the same technique as that used for digital television.