Trends in Optical Disk Mastering

Extract

The commercial life of optical disks started in the late 1970s with prerecorded, read-only video disks. The video information was stored on the disks in the form of pits with varying lengths aligned in a spiral track, a structure which lends itself to easy mass replication. The pits were recorded in a “master” disk by exposing a positive photoresist to a focused beam of blue or UV light from a gas laser and subsequently developing the resist. Now, some 15 years later, mass-replicated, read-only video and audio disks still form the most important application of optical disk technology. Today a large majority of the corresponding masters are recorded using virtually the same technique.

In view of the relative importance of prerecorded disks, it is amazing that so little seems to have changed in the mastering processes. Part of the explanation lies in the fact that the dimensions of the relief structures on the early optical disks were defined by what a player could read using relatively cheap optics, rather than by what could be made in the mastering process. If in the beginning the analog video disk system (Video Long Play or LaserVision) was rather demanding, the digital audio disk (compact disk) (see Figure 1a), introduced later, was a much easier system from a mastering point of view. In that period a trend toward cheaper and faster mastering techniques developed, resulting in a few proposals for direct-effect and direct metal mastering. However, in the meantime very high-quality mastering became necessary for the CD-video system and for preformatted data disks. In the CD-video system, the combination of digital audio and analog video leads in some cases to pits as short as 300 nm (see Figure 1b). On preformatted data disks, wide shallow grooves may have to be combined with narrow deep header pits (Figure 1c).