6.1.1.4.

CD-ROM standards

For the delivery of multimedia from CD-ROM, the CD-ROM drive must first conform to ISO9660, a refinement of the Yellow Book Standard for CD-ROM ( Table 6.2 ), and then at least to the Multimedia PC (MPC) Level 1 standard defined by the MPC Marketing Council. The Level 2 standard is preferable for advanced CBL (Table 1.2).

A drive conforming to this minimal specification should be suitable for most multimedia CBL applications, though a satisfactory data transfer rate for digital video could be a problem with some of them. Dual-speed CD-ROM drives are considerably better for this and for multimedia in general. Pre-MPC drives, however (which were still being manufactured until quite recently), almost certainly will not deliver sound or video satisfactorily. To enable the computer to treat the CD-ROM drive like a read-only floppy drive, a DOS device driver is required - MSCDEX.EXE- which will translate the High Sierra directory structure into something DOS can handle. This is usually bundled with CD-ROM drives, as well as recent versions of MS-DOS. It is good practice to use the most recent version available (currently the one included with MS-DOS 6.2).

CD-ROM standards are varied and complex, and have been formalised as a series of "colour book" standards, reflecting (believe it or not) the colour of the covers of the books in which the standards are formally specified ( Table 6.2 ). Data on a CD-ROM are organised into 24-byte frames, and frames are organised into tracks. On a standard CD-ROM, these data are played at 75 frames per second. Frames can store various kinds of data - sound, text, graphics, photographic stills, animation and video - and conventionally each of these types of frames are located in separate tracks. However, the more recent CD-ROM standard, CD-ROM.XA (XA = extended architecture), specifies ways in which different types of data can be "interleaved" in the same track, thereby permitting access to both simultaneously (eg sound with video).

CD technology was developed for the music industry, not for data storage. The speed of data access and retrieval is not fast by hard drive standards, even in dual speed drives, so not surprisingly CD-ROM drives are the main bottleneck in interactive multimedia systems. Dual speed drives are becoming more widely available, and they certainly improve things at reasonable cost, but it may be some time before applications are able to take full advantage of their capablities.

Data management on CD-ROMs is specified in terms of sub-channels, modes and forms. Sub-channels are blocks of data (there are eight of them), modes (1, 2 and 3) specify the type of data in each of the eight sub-channels, and forms (1 and 2) define the respective allocation of storage space in each sector to error detection / correction and user data. CD-ROM drives vary with respect to their ability to support these various standards, but most new drives nowadays support both Mode 2 and Form 2.

Most new drives also support CD-ROM.XA, the standard recently re-vitalised by the development of Kodak's Photo-CD format, which requires components of the "extended architecture" specified for CD-ROM.XA. The two main advantages of CD-ROM.XA over conventional CD-ROM relate to sound: (1) audio data is interleaved with graphics, which means narration or background music can be mingled with images; and (2) audio data is compressed under ADPCM to increase storage space, but without loss of quality.

Photo-CD, however, does not make use of the entire XA specification. For example, it has no plans to support the ADPCM encoding of digital audio data - so "Photo-CD compatibility" does not necessarily mean full "CD-ROM.XA compliance", even when the former includes multi-session capability. It is true that Kodak has plans to add sound capability to Photo-CD so that "sound captions" can be added to image collections, but this will be Red Book (PCM) sound, not ADPCM.