The success of the compact disc format owes much to standardization. Without a clear standard. The downside of a standard, however is that it can impede progress. This trade-off is clearly evident in CD ROM when compared with present and future alternatives. Particular emphasis is placed on the emerging user recordable forms of optical storage and on the CD and non-CD versions of these technologies. Within the recordable format, both write-once and eraseable are considered, and appropriate applications for the CD versions of these technologies are identified.

Limitations in Current CD ROM Technology

            Today’s CD ROM drive is clearly limited as a CPU peripheral. These limitations were need for maximizing compatibility with CD Audio and by cost limitations. The major limitations are as follows:

1.     Limited access time (typically 0.5 to 1.0). access time is influenced by the limited rotation rate that affects the latency period. In particular, the CLV (constant linear velocity) nature of the drive significantly slows degrade data retrieval time also.

      2.  Data transfer rate limitations [150 kbytes/second (continuous)]. This is set once

           again by the rotation rate and, in addition, by the error correction encoding and

           decoding time. Rotation rates vary from 200 to 500 rpm and are set to give a CLV

           of about 1.2-1.4 m/s.

3.     Format and encoding limitations. The selected modulation coding scheme for CD

     ROM, Eight-to-Fourteen modulation code (EFM), is designed to allow for

     significant tolerance buildup in minimum “feature” separation. This stacking up of      

     tolerances potentially results from spatial errors induced from the master disc to a

     final worst case user disc in a user drive. It also derives from the optical numerical

     aperture (0.45) spot resolution limits in today’s products.

4.      Error correction. The CD ROM standard calls for “two-layer” error correction

Scheme; the Cross-Interleaved Reed Solomon code (CIRC) system defined into all CD Audio players allows error correction from a raw 10^-3-10^-4 region to a user level of 10^-9-10^-10. an error rate of 10-9 would result in occasional hard errors.

Future Possibilities: Prerecorded Optical Data Products

Given the trade-offs described above, it is obvious that a higher performance read only optical product is relatively straightforward to achieve. Using typical commercial 130-mm write-once drive specifications, it si simple to derive a pro forma specification for such a device. Table 1-1 lists expected performance parameters; the price for such a device would be higher than for CD ROM drives.

Table 1-1 Predicted Read-Only Optical Disc Performances Compared with Today’s CD_ROM

Technologies Issues

Disc media for such advanced read only devices have already been discussed: generically’ “O_ROM” (Optical Read-Only Memory), or more specifically, “Data ROM”. The major CD-ROM drive and computer is far too slow at 171 Kbytes/sec using a Mode 2 disc

The solution to supporting motion video on CD-ROM lies in image compression, of which numerous techniques and technologies exist. The image compression algorithm used in Intel DVI, for example, operates by storing only those parts of a frame which are different from a previous one. Compression ratios achieved therefore depend upon the type of sequence being compressed.

Database

            Database have become synonymous with computer technology, where in recent years information on the printed page has been transcribed to magnetic storage media.

            Many useful on-line database are in the Gigabytes range. These can sometimes be stored on CD-ROM by “daisy chaining” typically four to eight drives, from which it is then possible to access data randomly as from a single drive.

            Database protection remains an issues for many publishers, and the notion of installing a CD_ROM system on-site increase the threat of illegal copying or unlicensed access. Encryption methods provide one solution to this, requiring the user’s computer to be fitted with decryption peripheral.

            Product pricing is often a complex problem for database publishers, and traditional methods of charging users are usually perceived as unfair. Flat payment for a complete database installation, or yearly subscription fee, are two methods which are likely to leave either publisher or user dissatisfied.

            A fairer costing system naturally takes into account the amount of service provided to the user. Return On Investment(ROI) owned by Personal Library Software Inc., is a hardware/software system which monitors each access to a database calculating a fee commensurate with usage.