For the technically minded...

CD-ROM discs are like the compact discs bought in music stores :12 cm in diameter and 1.2 mm thick. The data is encoded in a shiny, metallic film in the form of tiny pits and lands which can be of different lengths but are of standard depth and width: 0.12 and 0.6 microns. When a laser beam passes over the lands, most of the light is reflected back, but pits scatter the light and only a fraction comes back. In this way, the laser beam reads these miniature pits on a three mile long spiral track and decodes them into binary 'ones' and Zeros.

When one subtracts the capacity needed for the internal operation of this technology, about 600 megabytes of usable memory remains. Access to this data is relatively easy compared to more powerful systems because CD-ROM discs can be read in just over one second. Even if it took ten seconds to read an entire encyclopedia, one could hardly complain.

Cost; the AGRIS system is poorly indexed and difficult to use; and that of CABI uses no less than 27 journals. Apart from these institutional weaknesses, anyone who has ever ordered such publications knows how long it takes to receive them, how difficult it is to make a selection based only on bibliographic abstracts, and how useful it is to be able to leaf through a publication in order to identify the information that is needed. Direct access to both abstracts and complete texts is possible with CD-ROM.

Such advantages may also represent a considerable cost saving because existing information systems in Africa are not only unsatisfactory for users but are also quite expensive. Databanks charge very high hourly rates for searches and for providing publications. CD-ROM discs can provide a substantial saving in this regard because they are supplied to users at subscription rates varying between 950 - 1,500 USdollar per year. This is competitive with that of certain indexes or reviews given the quantity of data offered by CD-ROM. Of even more interest is the flexibility of this system in view of the possibility of subscribing to a service that periodically replaces the discs with updated versions (the more frequent the update, the more expensive the cost). Such a subscription service has two main advantages: the first is that the user no longer feels compelled to play against the clock in an effort to minimize on-line costs when using a system billed according to the time connected. The second is that several researchers can work together, evaluating the same information at no extra cost

Several international institutions working on tropical agriculture plan to start using CD-ROM technology in the near future. CABI has already developed a prototype containing 220,000 bibliographic abstracts that can be directly accessed by an IBM or DEC computer. The FAO has also produced a CD-ROM containing AGRIS data. Other related institutions currently working with CD-ROM include the National Agricultural Library (USA) and the CGIAR which plans to produce a disc featuring the many publications of its thirteen centres. The Canadian International Development Research Centre (IDRC), the US Agency for International Aid (AID), and the World Bank have also taken steps to promote the use of this new technology.

Unfortunately, there are no miracle plants in agriculture and there are no miracle technologies for computers. Although the high performance CD-ROM discs are practical, hardy and economic, they do have their shortcomings. The virtually indestructible character of the data that they store, for example, excludes any information that requires constant updating. These Read Only discs mean just that: they can only be read, not modified. This more or less restricts their use to archival or reference purposes. Another limitation to the spread of CD-ROM technology is that its discs are not easy to produce. Data is recorded using the same techniques as the music industry employs such facilities are still relatively scarce, resulting in long production delays. Furthermore, given the cost and difficulty of recording these discs, it will be a long time before Third World countries will be able to store their own information on such discs and so they will continue to be dominated by data that originates, or is processed, in developed countries

Finally, many CD-ROM critics point out that if this technology has definite advantages for storing information, that is not necessarily the case for finding it: compact discs do not enable the kind of interactive searches possible with larger systems that can interrogate several databases at once. CD-ROM users must insert a different disc each time that they want to search a different database. To resolve this problem, compact disc manufacturers are developing a kind of CD-ROM juke box which would enable simultaneous consultation.

It is obvious that CD-ROM is a significant technological step forward, but it remains to be seen if its costeffectiveness will justify its adoption by those institutions concerned. The competition is not about to throw in the towel: existing interactive services have started to lower their consultation rates. Furthermore, progress in the development of magnetic discs may one day increase their capacity to rival that of CD-ROM.

If laser-encoded, compact discs have yet to plough their way into the vast field of tropical agriculture, they hold considerable promise for storing and transporting good information harvests.

References

- Hampshire N (1986). 'CD-ROM leaves the labs for the market place'. 'Times', 23 September 1986. - 'CD-ROM Review', N° 1, October 1986 - 'New Scientist', October 1 986

- Kinney J (1986). 'Agricultural Information Services and the New Technology', Pre-conference seminar of the Seventh Standing Conference of Eastern and Southern African Librarians (SCECSAL), University of Botswana. July 31 - August 1, 1986, pp 13.

- Lambert S. Ropiequet S (1986) 'CDROM: the new papyrus'. Microsoft Press/ Penguin, pp 619

CD-ROM holds enormous potential for the distribution of bibliographic references without resorting to large computers or telecommunications. That is why CTA has decided to participate in a pilot programme organized by CABI in four African countries: Ethiopia, Mali, Kenya and Zimbabwe. CTA will provide the equipment (microcomputers, laser readers and printers) and CABI will provide the compact discs. If the evaluation of this project is positive, CTA plans a larger distribution of such equipment to national agricultural information service