|New Training Technologies (ILO - UNEVOC, 1995, 95 p.)|
Sound and video we enjoy every day are generally analogue: radio, TV, videotapes, videodisks, cameras, camcorders. By contrast, sound and video recorded on and played from audio CDs, CD-ROMs, CD-ls are digital; and so are images recorded on and played from photo-CDs.
Analogue sound and video are of high quality, but one cannot handle them in order to copy them, send them via computer networks, combine them with digital data (text, presentation techniques). This can be done after transforming analogue data into digital data, which are stored on CD-ROMs, hard disks or diskettes (e.g. optical ones), or on CD-ls and photo-CDs. They can therefore be used in developing learning material (after having settled the copyright problems, if necessary).
Digital sound has the high quality everyone knows from using audio-CDs. Compressed digital images and video do not quite equal analogue images and video, but progress is quick and they may soon reach the same level.
The problem is mainly due to the fact that fast recording of digital sound, images and video creates huge files; here is an example:
· a videodisk contains 54,000 images per side, which means 36' of video in the PAL or SECAM system (25 images/min) or 30' in the NTSC one (30 images/min);
· an equivalent digital image, full screen (1024x768) with only 256 colours represents a file of 786,432 bytes (786 Kb); if 65,536 colours are needed, the size of the file is doubled;
· in a PAL or SECAM environment, we need 25 times more disk space for one second, which means that only 30" of full screen full motion video would be recorded on a CD-ROM (600 Mb);
· but thanks to the compression techniques (e.g. MPEG, DVI, etc.) we are able to save about 140 times more, which means 72' instead of 30" on the same CD-ROM.