Multimedia Storage Formats

Currently the most popular media for distributing hypermedia applications is the CD-ROM. With its capacity of almost 700 Mbytes it is large enough to store the contents of an encyclopedia, including still pictures in JPEG format (a standard developed by the Joint Photographic Expert Group) and some "video" fragments, stored in the MPEG-1 format (a standard developed by the Motion Pictures Expert Group).

Moving pictures need not be very sharp. MPEG-1 offers a resolution of 352x288 for the brightness and only 176x144 for the colors, and a refresh-rate of only 25 Hz. Still, MPEG-1 is used in the CD-I and other multimedia platforms, and provides about the same "sharpness" as VHS video recorders. A CD-ROM can hold about 1 hour of MPEG-1 encoded video. Decoding MPEG-1 requires a fast (special purpose) processor and introduces a delay of a few tenths of a second, making MPEG-1 ill-suited for direct communication (like in video-conferencing).

A limited number of CD-ROM formats exist, among which the "High Sierra" format is the most popular. CD-ROMs can be freely exchanged between platforms like the PC, Macintosh and several Unix workstations.

MPEG-2 is a new standard (used by some television stations already), offering higher quality images at the expense of higher required bandwidth and more processing power. Standard TV resolution of 720x576 will require 4-5 Mbit/sec, 4 times the speed of a CD-player, reducing the capacity of a CD-ROM to 15 minutes. The new DVD-rom offers up to 17 Gbytes of storage, more than enough to hold an MPEG-2 encoded motion picture with several sound tracks and additional (hypertextual) information.

HDTV resolutions of 1920x1152 or 1440x1152 will require 20-30 Mbit/sec, currently about the maximum transfer rate of affordable large hard-discs, and requiring 1 Gigabyte for 5 minutes of video. No standard hardware platform exists that provides the capacity and speed necessary for such high speed and high quality multimedia applications.

For so-called "virtual worlds" the language VRML (Virtual Reality Modeling Language) is very suitable. It can be used to describe 3D objects and environments by mathematical formulas, which are more compact than any (still or motion) picture format can be.