Library Technology Guides

Document Repository

Double-speed CD-ROM drives

Library Systems Newsletter [October 1993]

Most CD-ROM drives installed in libraries are limited in speed, both in terms of data-transfer rate and average access time. Data transfer rate refers to the speed at which extended blocks of data can be read off the disk. In general, the faster the transfer rate, the smoother the display—especially video now that multimedia on CD-ROM is becoming a common application. Access time is the average amount of time it takes to move the head to a given piece of data, expressed in milliseconds.

Until recently, drives were limited to 150KBps (150,000 bytes per second) data-transfer rates because of licensing restrictions imposed by CD technology patent holder Philips. The limit has now been removed at the urging of Eastman Kodak, a major Philips licensee, which needed faster data-transfer rates to support its Photo CD products.

Faster drives are now becoming available; in fact, several manufacturers, including Texel, NEC, and Toshiba, are building double-speed drives exclusively. These drives have a 300KBps throughput.

Access time, which is most important when searching for random bits of information, such as in a bibliographic database or electronic publication (e.g., an electronic encyclopedia) has in the past been 400 to 600ms (milliseconds). The double-speed drives generally support 200 to 400 ms access times.

Differences in access time are much more important than the differences in transfer rate stated by the various manufacturers. Double-speed data transfer, while theoretically important for multimedia applications, is still limited in value because most multimedia products are optimized for playback on single-speed drives. The extra video and audio resolution that could be taken advantage of by double-speed drives isn't there yet.

While not a speed measurement, cache size, which has a substantial impact on perceived performance, is also important. A hardware cache buffers the transfer of data from the disk, anticipating that the user may want to read recently retrieved information over again. In some cases, the cache can also act as a read-ahead buffer, automatically looking ahead to the next block of information on the disk. Read-ahead caches might seem most useful for multimedia playback, where continuous streams of data are flowing, but they are equally useful for paging through a database or electronic publication. In this case, the cache has time to get ahead of itself while the user is reading the current piece of information. Some products also have software caches which compensate for the skimpy 64-KB hardware caches in some of these drives (256 KB is the maximum).

The upshot of all of the foregoing is the advisability of specifying double-speed CD-ROM drives for better performance for better database access today and more attractive multimedia displays in the future.

Permalink:  
View Citation
Publication Year:1993
Type of Material:Article
Language English
Published in: Library Systems Newsletter
Publication Info:Volume 13 Number 10
Issue:October 1993
Page(s):80-81
Publisher:American Library Association
Place of Publication:Chicago, IL
Notes:Howard S. White, Editor-in-Chief; Richard W. Boss, Contributing Editor
Subject: CD-ROM hardware
ISSN:0277-0288
Record Number:7474
Last Update:2022-08-06 22:06:38
Date Created:0000-00-00 00:00:00
Views:36