by Charlie Wyckoff and Jack Berger
CD-ROM drivers are included with most operating systems for reading CD-ROM disks from single CD-ROM drives. Each disk is loaded as required, similar to the way 3.5 inch cartridge disks are loaded and accessed. Most of the commercially available operating systems support single and multiple single drives. These CD-ROM drives and drivers in most cases will read the more common CD-ROM formats, i.e. CD-ROM XA, ISO 9660, High Sierra, MPC 1&2, Photo CD, Multisession, etc.
A new generation of CD-ROM drives have become available which hold multiple CD-ROM disks. They are referred to by a number of names including Jukeboxes, Autochangers, MiniChangers, and CD-ROM Changers. Initially these jukebox style drives found wide acceptance in the audio markets. Once the manufactures outfitted them with SCSI buses, enterprising software companies wrote the drivers to adapt them to the data processing markets. Some jukeboxes can hold upwards to 1400 CD's. A popular 500 CD-ROM jukebox offers over 300 gigabytes of on-line information. The economical jukebox storage technology is used extensively where computer users need access to large libraries of static information such as medical records; technical, legal and medical reference libraries; parts and price catalogs; and many other similar applications.
How Jukeboxes Work
Jukebox style drives select and load CD-ROM disks into their reader mechanisms from magazines or caddies of CD-ROM disks, or, from stacking compartments. The drivers included with most operating systems are not capable of supporting jukebox style CD-ROM drives. The companies who write drivers for such devices require extensive knowledge of device drivers and the targeted operating systems.
In general Jukebox drivers support all the common CD-ROM formats referenced above. Typically each driver transforms each CD-ROM slot in the jukebox into a separate addressable drive device. This device can then be accessed like any other disk drive on the system.
Some operating systems, like Windows NT, have limitations on the number of disk drives which can be directly addressed by the system. Windows NT operating system addresses only 26 devices which is a problem for the larger jukeboxes. The more advanced device drivers circumvent this limitation by assigning logical names to each CD-ROM slot and then mapping calls to those slots through the addressing scheme of the particular jukebox.
Most jukebox drivers operate on server configurations as well as on stand alone systems. Users access the files on the jukeboxes using the same operating system conventions normally used to access other disks on the system.
A number of companies are offering multiple single drives installed in mini towers to compete with the jukebox style changers. These configurations offer higher throughput since there is a head per disk. This solution has three disadvantages:
- Large database configurations get costly.
- Configurations are limited by the number of SCSI addresses available.
- Configurations are limited by the number of devices the operating system will support.
The users must choose which technology best fits their needs.
CD-Recordables in JukeboxesBefore closing, it is important to mention the generation of CD-ROM's. Various brands of CD-Recorders include special record mechanisms for recording on CD-Recordable disks. Most CD-Recorders are stand alone devices. At least one of the large jukebox manufactures offers a record option in addition to read capabilities on the same device. They accomplish this by permitting a mixture of up to four read and record heads which can be mounted in the same changer. The same capability can be achieved by purchasing a stand alone recorder and adding the appropriate number of stand alone readers or jukebox(s). This combination is less costly and more suited to smaller requirements.
Separate drivers are required for recording on CD-Recordable disks. Typically the drivers write the files to a magnetic media disk where they are organized into the required CD-ROM format . In most cases the magnetic media must be of equal or larger capacity than the CD-Recordable disk. The files are transferred from the magnetic media to the CD-Recordable media in streaming mode. The resulting CD-ROM disk can be read on any CD-Reader capable of supporting the format in which the disk was written.
The production of CD-ROM disks from CD-Recordable media is termed disk or CD-ROM mastering. The term originated with those companies who produce large numbers of CD-ROM's for distribution. They generate a master CD-ROM from which all copies are produced. Since CD-ROM's are an economical media for distributing data and programs, this media has become a mainstay in the industry and will be around for some time to come.