CD-ROM

When Philips and Sony introduced the Compact Disk, about one hour of audio had to be encoded using uncompressed 16-bit data, sampled at a rate of 44.000 samples per second. The result of this design is that a single CD contains about 650 Mbytes of data.

Soon the computer industry became aware of this digital read-only medium, and turned CD-players into disk drives for CD's containing non-audio data. The CD-ROM (Compact Disk Read-Only Memory) was born, an inexpensive medium for distributing information.

The main advantages of providing information on CD-ROM is that the CD's are cheap to produce in large quantities (around $2 a piece), and relatively hard to copy. The main disadvantage is that the CD-ROM players are slow. The head moves fairly slowly, leading to average access times that are an order of magnitude higher than with hard disks. The data transfer rate is under 200 Kbytes per second, enough for 2 audio channels at 88 Kbytes per second but much lower than the transfer rate of today's hard disks. Modern CD-ROM players are much faster at reading (up to 20 times the standard speed or more), but the heads still move a lot slower than with hard disks.

Because of the high capacity of the CD-ROM, this medium is heavily used for software distribution and for multimedia information bases. Using CD-ROMs for multimedia information poses serious problems, because of the huge amount of data in (moving) images. A single television image contains over 100 Kbytes of information. At 25 images per second (in Europe) this means 2.5 Mbytes of information per second. Not only does this mean that a CD-ROM can contain only less than 3 minutes of video information, it also means that the data transfer rate needs to be more than 10 times the standard speed in order to achieve real-time video playback.

By using advanced image and audio compression algorithms, it is currently possible to store about one hour of video (with sound) on a CD-ROM. The data are stored using an international standard, ISO 9660, better known as the "High Sierra" format. For the compression there is currently no widely accepted standard. The CD-I (Compact Disk Interactive) standard supported by Philips and Sony differs from the DVI (Digital Video Interactive) standard supported by Intel and IBM.

Because of the high capacity of the CD-ROM medium, because it is read-only, and because of the current lack of support for video, interactive applications of CD-ROM are still limited. Dictionaries and encyclopedia are becoming available. They manage to fill a CD-ROM mostly by incorporating many (still) images. Compression of still images is done using the JPEG standard. For moving images, the MPEG-1 standard is used for low-quality (VHS-like) and the MPEG-2 standard for broadcast quality video. The compression is such that the transfer rate of a CD-ROM is sufficient for real-time playback. CD-I uses MPEG-1 for video clips. The amount of processing power to reconstruct the images is considerable. For a short demo of an MPEG-1 clip, click here. (This demo isn't very impressive on monochrome screens. This short, low-resolution demo is still 740 Kbytes. Not all WWW-browsers have MPEG playback capabilities. You may need a special applet or plug-in to view the clip.)