Write-once and rewritable storage share the same ancestry. Read-only storage has a different purpose. In fact it is only ‘storage’ by courtesy; its function is the distribution of information from one originator to a number of users. The greatest impact of read-only storage, however, is in the wider field of publishing. Read-only disks can supplement and sometimes replace both on-line database and the printed page in a wide range of commercial, administrative, and scientific applications.

            CD-ROM is by far the most important form of read-only optical disk, although not the only one. It is a very close derivative of the Compact Audio disk which has been so successful in the entertainment field. CD-ROM has no writable direct equivalent. Read-only disks equivalent to various existing WORM drives have been proposed, one under the trade name OROM. One other type of read-only disk exists that can store data: the videodisk. This was designed as a medium for video films in the entertainment field. However, it is not likely to succeed in the computer industry in view of the competition from CD-ROM and its derivative.

 Recording Formats And Standards

CD-ROM is by far the most important form of read-only optical disk. One of the reason for its success is its precise specification; every CD-ROM disk can be read on every CD-ROM drive.

Tracks and paths

            We saw in the last section that data is represented as a sequence of pits and spaces forming a continuous spiral path. The observant may have wondered why the word ‘path’ has been used rather than ‘track’. The reason is that in the entertainment industry a ‘track’ is a logical rather than physical section of the disk contents, of varying size; the close relation between Compact Audio and CD-ROM has led to the word being used in this sense in the context of CD-ROM. This is not a problem, because ‘track’ in the sense of ‘path’ is not an important concept in CD-ROM, but it can cause confusion to those dealing with both CD-ROM and WORM disks.

Disk layout

            CD-ROM disks use the constant linear velocity (CLV) format. The spacing of signal elements along the spiral path is constant; the rate of rotation of the disk is varied to match the path radius, so that data is read at a constant rate. The spiral path (which runs outwards) is divided into sectors of equal length and hence equal data capacity. Sectors are identified solely by their sector number. The sector number is in fact compound, made up of ‘minutes’, ‘seconds’ and sectors.

            Each sector can hold 2048 bytes of data; these are coded in a complex way to provide for error correction and detection. The disk is normally limited to 270000 sectors, which at 2048 bytes per sector can hold 552960000 bytes.

Data retrieval   

            Data is normally retrieved by its sector number. However, there is provision for the contents of the disk to be divided arbitrarily into up to 99 tracks. Tracks can be of any length, and the contents of a tracks can be retrieved by specifying its track number instead of the address of the first sector.

Strength And Weakness

            As a computer peripheral, CD-ROM competes with magnetic media where these are used for the distribution of data and software. However, its main significant is a competitor to more diverse methods of publishing; in particular to printed paper, microfiche, and on-line computer database.

            The greatest strength of CD-ROM is its high data density; this allows the equivalent of hundreds of printed books or floppy disks to be carried by a single disk. CD-ROM are made by a mastering process, so the cost is high for a single copy, but falls rapidly with quantity.

            A further strength of CD-ROM is its resistance to damage and wear and its high data integrity. We have seen that an error rate of 1 in 10 can be achieved. This is far better than is economically possible for any other publishing medium.

            In comparison with the non-electronic publishing media CD-ROM offers fast random access and high data rate. CD-ROM data can be manipulated by computer. This does not necessarily mean that the computer can understand and act upon the data itself; this is unlikely to be possible if the data is in image form, whether representing text or graphics.

            To offset these strengths, CD-ROM is incompatible with the normal types of WORM and rewritable drives; it is compatible with the forthcoming these will have a real impact on the market.

            The same point more forcibly where the data is to be read by the human eye, and the user does not already have computer. In this case the comparison is not between the cost of different peripheral devices but between complete ‘delivery systems’. Since every human eye is equipped with delivery system for printed text at no cost, CD-ROM has to offer considerable advantages in other directions to offset the cost of even the simplest delivery system.

            Finally, a word about read-only disks to be read on multifunction drives designed for standard writable disks. Many of the above points apply, except of course that of drive compatibility. In general, these drives offer higher performance than CD-ROM drives, but at a substantially higher cost.