f computer files is popularly based on the CD- ROM (compact-disk, read-only memory), 27 which is wholly similar to the previously described audio-CD, both in terms of looks and storage space (700-800 Mbytes). A key difference is that the CD-ROM is primarily designed for computer data and, hence, it includes the function of error-correction, which is achieved through Reed-Solomon (RS) encoding (see Chapter 11). The CD-ROM can be "burnt" according to three preset modes: audio (for music tracks or copying audio-CDs), and mode 1 and mode 2 for PC data. The CD-ROM has 333000 sectors of 2352 bytes length (which gives a capacity 783.2 Mbytes). In the audio mode, no error correction is used, thus, the full sector length (2352 bytes) can be used for storing audio files. Both mode 1 and mode 2 use a 16-byte header in each sector, for the purposes of synchronization and identification. Unlike in mode-1, mode-2 does not include error correction, which leaves 2352 - 16 = 2336 bytes for payload. In mode 1, the 288-byte trailer for error correction leaves a payload of 2336 - 288 = 2048 bytes. CD-ROMs are characterized by three different possible read and write speeds: A = write-once, B = rewrite and C = read-only. By definition, the 1 X speed rating corresponds to 150 kbyte/s. Thus, a 32 X write-only speed corresponds to 4.8 Mbyte/s. A 700 Mbyte CD-ROM can, thus, be copied in approximately 700/4.8 = 146 s or approximately 2 min 30 The different speed ratings for the A, B, and C functions are then identified under the generic label Ax, Bx, Cx, for instance 12x, 10x, 32x. 28 25 See, for instance: http://en.wikipedia.org/wiki/Izarc;www.izarc.org/ 26 See, for instance: http://en.wikipedia.org/wiki/Comparison_of_file_archivers. 27 See, for instance: http://en.wikipedia.org/wiki/CD-ROM. 28 See, for instance: http://en.wikipedia.org/wiki/Izarc; www.izarc.org/. Appendix G Figure G3 Effect of JPEG compression from 0% to 100% with close-ups shown at right. Images With the development and popularity of digital photography, and the multipl