Lecture
All of the compact disc standards are better known by the colors of the books in which they are described. In 1980 a series of standards called the Red Book was adopted, covering audio compact discs. According to this document, the sampling rate used when reading audio signals from a CD-ROM must be 44.1 kHz. The amplitude resolution is represented by a 16-bit value. Since the standard defines stereo sound, not one but two 16-bit values must be read every second.
It is worth noting the basic difference between audio compact discs and compact discs holding digital data. Clearly, missing (misreading) any single pit on the disc surface can destroy the integrity of the recorded data. That in turn leads to incorrect information being reproduced, or to the executable modules of the recorded programs failing to run.
Audio compact discs are another matter. Here, missing several pits on the disc has practically no effect on the sound quality. The reason is that in this case the principle of interpolation comes to the rescue. As is well known, interpolation makes it possible to find intermediate values from values that are already known. For example, suppose three consecutive values, 10, 13 and 20, were recorded on an audio disc. Suppose the second number was not read for some reason (because of dirt or damage to the disc surface, say). Linear interpolation on the numbers 10 and 20 gives 15 [(10+20)/2]. Of course, the value obtained does not match the original one, but in this case, for the encoding of an analog signal (music), that is of no particular importance, since a listener will not notice it by ear.
The first standard for compact discs holding mixed information, called the Yellow Book, was adopted in 1985. It was one of the computer industry's first steps toward multimedia technology. Under this standard all discs were divided into two categories: Mode1 and Mode2. Media in the first category were recorded with error-correction bits, and the transfer rate for useful data was 150 KB/s. For discs in the second group it was higher – 170 KB/s – because the correction bits were absent.
Mode2 in its original form was never actually implemented. Audio and video information was stored in different parts of the disc, with the result that the laser beam had to keep "running" from one area of the disc to another. Although the standard defined the error-correction process used when reading data from a CD-ROM, it did not at the same time give a sufficient specification for the structure of the stored file, which was defined more clearly by the ISO 9660 standard released in 1988.
The Green Book standard, adopted in 1986, covers interactive compact discs, CD-i (CD-interactive). It introduced the concept of headers to simplify work with continually interleaved video and audio information. In the Green Book standard the design of Mode2 was formally reworked. Mode2 compact discs were divided into two subgroups: Form1 and Form2. The first, as with the Mode1 category of the Yellow Book, specified error correction by means of additional bits and a data transfer rate of 150 KB/s. The second subgroup allowed a read rate of 170 KB/s because error-correction codes were absent.
The XA (Extended Architecture) standard was developed in 1990 jointly by Philips, Sony and Microsoft, and it established compatibility criteria between CD-ROM discs conforming to the Green Book and Yellow Book standards. It defines a way of indexing multimedia information: graphics, text, bitmap images and sound. A disc conforming to the XA standard can be played back on a CD-i interactive disc reader compatible with the Green Book standard, or by means of a CD-ROM drive that conforms to the Yellow Book standard, supports XA operations and is controlled by a special software driver.
Finally, in 1991 the Orange Book standard appeared, covering compact discs that can be written to repeatedly.
The standardization expert group MPEG (Moving Picture Expert Group) developed the MPEG-1 standard, which addresses the compression of full-motion video. It should be noted that this standard does not define a data storage format. Data in it can be played back on a CD-i interactive disc reader equipped with an MPEG decoder. Another option is to store MPEG-compressed full-motion video on a CD-ROM device conforming to the Yellow Book standard.
The White Book standard, adopted in 1993, introduced certain interactive capabilities that allow information to be searched quickly by individual frames in random-access mode. The first discs conforming to the White Book standard, called Video-CD, appeared in 1994. At present some Video-CD compact discs can be played back on computers by means of MPEG decompression if a board that performs MPEG conversion in hardware is installed. However, many CD-ROM drives do not read information in continuous mode, which makes it impossible to play these discs even after an MPEG board has been installed. In addition, the processor must be no slower than a 386/25.
All compact discs for modern multimedia systems, including CD-i interactive compact discs and Video-CD, are recorded in the Mode2/Form2 standard, that is, without the use of correction. The resulting gain of 20 KB/s in speed is used to improve the quality of the video image. In this class of applications the absence of error correction does not affect quality, which can by no means be said of business applications.
One type of CD-ROM that allows information to be appended is the so-called Photo CD. Writing information to a disc in one go is called a session. Accordingly, writing repeatedly is called multisession. It must be kept in mind that each session requires its own table of contents, so the more sessions are used, the less information fits on the disc. Drives that handle multisession discs and can play Photo CD discs have already appeared.
Kodak developed Photo CD devices that can store up to 100 frames of pictures taken on 35 mm film. The idea is that the consumer can have pictures scanned using Kodak equipment and afterwards play them back on any drive. In practice the disc can hold five different versions of the same slide at different resolutions of a 24-bit palette.
Using compression (without loss of resolution), the data of five images can be packed into a 6 MB file. Thus a compact disc with a capacity of 600 MB can hold up to 100 photographs.
Fast drives are recommended for working with Photo CD, because individual frames can be as large as 18 MB.
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