Hypermedia: A Progress Report Part 3: CD-ROM, CD-I, DVI, Etc.
In this final installment of our three-part series on hypermdia we look at the newest and most promising technologies to be applied to the task of bringing affordable hypermedia to the desktop -- all of wich are based upon the digitally encoded, optically read compact disc (CD). Examining hardware systems and software products should help to clear the industry's murky waters and provide a better picture of what is emerging from the depths of R&D departments around the world.
Describing the fundamentals of CD technologies will help to explain how they can be used as platforms for hypermedia. The easy part to understand and remember is that all of the technologies discussed in this article are based upon the same CD technology introduced in 1982 as a medium for digital music recordings. Co-holders of the compact disc license, Philips and Sony, over the years have extended the CD format beyond its original purpose as a medium for digital audio recording to include new visual and textual applications. These various applications are represented by a veritable sea of acronyms.
Essential differences between each of the "flavors" of CD technology lie not in physical differences like track width, but in the unique encoding, decoding and formatting schemes needed for the types of digital data that are specific to each flavor. Technical specifications for each of the formats are available from Philips and are known by the colors of their covers: CD audio is the Red Book; CD-ROM is the Yellow Book; and CD-I is the Green Book.
Before proceeding to specific formats, let's briefly look at the audio and video digitization processes. Remember that interactive videodiscs use analog, not digital, technology. In contrast, the CD, in all but one of its flavors, is purely digital. (The exception is CD-V, a format intended for music videos that is an amalgam of the analog LaserVision and the digital CD formats. The common denominator between the two is that both LaserVision and CD-based media are optically recorded and played back through laser technology.)
In general, for all CD-based formats everything on the disc -- video, audio or text--is represented not by an analog signal, but by a series of digital numbers. Therefore everything to be included must be converted to digital numbers before the CD is pressed. In the audio domain this process means converting the following types of signals to a digital format: acoustic sound waves produced by musical instruments, voices and environmental entities; analog recording that represent acoustic sound waves; and purely electronic signals produced by synthesizers or other electronic instruments. Luckily, modern digital sound-recording equipment handles these tasks easily.
In the video domain the task is not so simple. Full-screen, full-motion video generates enormous amounts of digital numbers when run through the digitization process. Recall that the digitization of one frame of video to a screen resolution of 640 x 840 pixels with each pixel resolving to 24 bits of color requires 7,372,800 bits. That's 921,600 bytes of data for just one frame out of the 30 needed for each second of full-motion video. Thus in the video arena the numbers required to faithfully represent a moving image get very large very fast. And that's the main technical reason we are still waiting for desktop hypermedia.
In order to get more than about a half minute of high-quality video onto a 4.72" CD, sophisticated computer algorithms (programs) must be developed to compress digital video information prior to its recording onto a disc, and to decompress it on playback. This is not a trivial task; it has occupied some of computer science's best minds for the last few years. Instead of settling for a grainy, jerky image reminiscent of the early days of motion pictures, developers in the field of digital video are aiming for broadcast-quality or better right from the start. …