COMPACT TOY BOX
Last month, we examined some of the ins and outs of the CD-ROM format, a primarily data-only application of the Compact Disc storage system. I'll bet a lot of people grumbled because the column had little to do with audio. This month (and next), we'll take a look at yet another incarnation of the Compact Disc. But don't worry; when it arrives, you will be able to listen to this one-as well as watch it, read it, and interact with it.
CD-I (Compact Disc-Interactive) is a special application of the CD-ROM format. Rather than store specific data, such as computer software (on a CDROM) or music (on an audio CD), CD-I will allow simultaneous storage of audio, video, graphics, text, and data, all functioning in an interactive format.
The CD-I system is thus a multi-media extension of the digital audio found on CD-Audio discs. In addition, since CD I players will be able to play regular audio CDs, CD-I can be thought of as an upscale CD-Audio system.
The CD-I format defines both hardware and software standards, much as the CD-Audio format does. Although CD-ROM can also store text, graphics, etc., CD-I specially defines an integration of those functions. Accessing CD-I will be an interactive procedure, a dialog. The system will present the user with alternatives, which he can then use to steer his way through the presentation to the desired information.
CD-I has rigidly defined implementations, just as the CD-Audio format does. The CD-I standard defines the following:
1. How various types of information such as video, audio, text, executable code, and graphics will be distinguished and identified.
2. How each type of information will be encoded, including specification for several formats of high-, medium-, or low-resolution video and graphics, and high to low-fidelity audio.
3. The file layout on the disc, and how files will be accessed.
4. How hardware will read discs and decode the contained multi-media information.
CD-I is specified in detail because it is aimed at the mass consumer market as a user-friendly, standardized system. It is thus important that all future CD-I discs be interchangeable and play in all future CD-I players.
CD-I is a single-medium system, containing both the operating program and the information itself, with no secondary media such as floppy disks required. In addition, CD-I is intended to piggyback on other electronic products, such as stereo systems or television sets, for multi-media presentation.
CD-I is designed to be used in a wide range of applications, from work to leisure, from entertainment to education. This diversity of intended applications posed a considerable challenge to CD-I designers, who responded with a system that incorporated a number of innovations. These diverse applications require the flexibility to select different levels of audio fidelity and of video and color resolution for various applications. This flexibility will allow storage capacity to be maximized and audio, video, and computer text to be presented simultaneously. Depending on the fidelity level selected, it will be possible to divide the audio into as many as 16 tracks, which can play either simultaneously (for multi-lingual or surround applications) or sequentially (for up to 18 hours and 40 minutes of monophonic sound). Multi-media applications will also call for physical interleaving of the three basic types of data (audio, video, and text/binary) to ensure synchronized presentation of the different types. Finally, the disc will have to provide efficient storage for data that degrades gracefully (audio and video) and for data that does not (text/binary). The former requires maximum bandwidth for maximum capacity, while the latter requires extended error detection and correction.
The CD-I data format may be considered a subset of the CD-ROM data format. As shown in Fig. 1, CD-I players will be designed to read CD-I discs, all CD-Audio discs, and certain compatible CD-ROM discs. The CD-I discs will use two physical formats (referred to as forms), as shown in Fig. 2.
Form 1 is intended for text, computer data, and highly compressed visual data; extended error detection (EDC) and correction (ECC) are used. The extended error-correction code used is the same as for CD-ROM Mode 1. In Form 1, user data occupies 2,048 bytes, and 280 bytes are reserved for the extended error-detection and error-correction codes. Form 2 is intended for real-time audio and video; EDC and ECC are omitted, and user data occupies 2,328 bytes.
To differentiate one kind of CD-I data from the other, a subheader is placed immediately after the regular 16-byte CD-ROM sync and address/mode header. The data is directed to the appropriate circuitry for reproduction, display, or processing. For example, audio and video data are directed to separate D/A stages (and, in video's case, to additional modulation stages). Because of its eight-byte subheader, CD-I Form 2 holds eight fewer user data bytes per sector than CD-ROM Mode 2 (2,328 versus 2,336). However, CD-I Form 1 holds just as much user data as CD-ROM Mode 1 (2,048 bytes), because the Mode 1 format includes an eight-byte space which is omitted from Form 1 to make room for the CD-I subheader.
Fig. 1--How CD-I will interact with CD-Audio and CD-ROM hardware and software.
CD-I discs will contain audio, video, or computer data-separately or together.
CD-I players will accommodate all CD-Audio discs and some CD-ROM discs.
Fig. 2--A comparison of CD-ROM and CD-I data formats. All sectors shown are 2,352
bytes long, equivalent to 98 CD-Audio frames or 1/75 S.
The standard for CD-I specifies much more than the standard for CDROM does. The CD-ROM standard defines only physical format capability, which means that the data and file structure on specific discs may allow their use only with specific computer systems. A CD-ROM containing graphics or programs for an Apple computer, for example, could not be used with an IBM PC, and vice versa. By also specifying disc and content identification and file access parameters, designers of the CD-I system have ensured that all CD-I discs will be compatible with all CD-I systems.
In addition, a bridge exists between CD-ROM and CD-I, based on the CDROM Mode 1 format. Mode 1 discs containing text (as opposed to graphics or other specialized information) will be playable on both CD-ROM and CD-I systems. Surprisingly, CD-ROM Mode 2 discs will not be compatible with CD-I, even though (according to Philips) they form the basis of the CD-I system.
The CD-I format calls for a total storage capacity of approximately 650 megabytes. Because a CD-I disc will be recorded with constant linear velocity, a constant readout rate of 75 sectors per second will be achieved. This will result in a data transfer rate of 153.6 kilobytes per second for Form 1 and Mode 1, and 176.4 kilobytes per second for Form 2.
To allow for extended video information, data compression techniques can be used to reduce the storage space required for the encoded audio program. The CD-I format offers four levels of audio quality, to be selected according to the need for fidelity: A CD Audio mode uses linear 16-bit PCM encoding, for the same stereo fidelity as regular audio CDs. Alternatively, eight- or four-bit adaptive delta pulse code modulation (ADPCM) is used for three lesser levels of fidelity: "Hi-Fi" music mode, which approximates LP quality; "Mid-Fi" music mode, which approximates FM quality, and "Speech" mode, for AM quality. The CD-I format also allows for a special ext-to-speech mode using phonetic coding to yield synthesized speech.
The option of trading away some audio quality in order to gain space for other data is an essential part of CD-I's ability to combine audio with video and ext. Obviously, a full-fidelity, full-length audio program, as found on a CD-Audio disc, would consume the entire storage area. Using data-compression techniques on the audio information would free a lot of room for other stuff: 16-bit PCM with a 44.1-kHz sampling rate is extremely data-hungry, whereas four- and eight-bit ADPCM are much more efficient. An audio-only CD-I disc with ADPCM could contain nearly 19 hours of program in "Speech" mode.
Next month, we'll discuss exactly how ADPCM is able to achieve this.
Meanwhile, I'll leave you with this taste of CD-I software: Imagine a CD-I disc about Beethoven. It could contain extensive textual information on the composer, show you pictures of him, and play examples of his music, perhaps while displaying the score. A bibliography might list all of the books written about Beethoven, a discography might list all Beethoven recordings, and a catalog might list all of his compositions-all of that on one disc, with friendly software to guide you to he information you want to access! Or how about Audio magazine on CD-I, complete with text, visual and aural information, and advertisements hat show, describe, and audition their products. Just think, you could even see pictures of the monthly columnists! (Well, maybe we'll skip that particular application.
(adapted from Audio magazine, Jan. 1987)
Also see: Dr. Thomas Stockham on the Future of Digital Recording (Feb. 1980)
= = = =