Part I: CD Anathomy

Lead-in Area, Data Area, Lead-out Area, and TOC

The sequence of sectors of the same format is joined into the track, the minimum possible length of which is 300 sectors, while the maximum possible length can consume the entire disc. The first and last tracks of the disc are called the Lead-in and Lead-out areas and are used for special purposes, although most contemporary drives can do without them (with regard to recorders , they are obliged to do so).

Lead-in Area is an auxiliary disc area, which actually represents track number 9, always preceding the first PMA track. Each session of a multisession disc has its own Lead-in area. The size of the Lead-in area according to the standard is 9 MB (60 seconds, or 4,500 sectors). The Q subcode channel of the Lead-in session contains the TOC, where among other useful information, there are special pointers specifying either the Lead-out area address (closed disc), or the Lead-in area address of the next session (opened disc). The contents of the Lead-in area are unavailable for reading at the software level (drives from MSI provide this capability). Visually, the Lead-in area looks like a uniformly light shining ring.

Caution  

Not every shining ring is the Lead-In! The actual Lead-In is always located at a distance of 23 mm from the disc edge, and is preceded by arbitrary trash.

Lead-Out Area. This is an auxiliary disc area designated by the track number AAh and terminating any closed session. The Lead-out area serves as a kind of indicator of disc and/or session termination. It helps the optical head to avoid being darted out of the disc limits. CD recorders must correctly process discs with open sessions. Normal CD-ROM drives and audio players, however, are not obliged to do so.

Caution  

The lack of a Lead-out session (as well as specifying an incorrect address for it) might damage some drive models (for instance, PHILIPS).

The capacity of the Lead-out area of a single-session disc is, according to the standard, 13.5 MB (6,750 sectors, or 1.5 minutes). The capacities of the Lead-out areas for the second and subsequent sessions of multi-session discs are reduced to 4 MB (0.5 minutes, or 2,250 sectors). The contents of the Lead-out area are unavailable at the software level (the only exception are MSI drives). Visually, the Lead-out area looks like a shining ring.

TOC ”Table Of Contents. This is an auxiliary area of the disc written in the Q subcode channel of the Lead-in area of the disc (it looks like a shining ring near the inner edge of the disc). Multisession discs have several independent TOCs, one TOC per each closed session. The TOC of an open session is stored in a special area in the PMA and, according to the standard, is available only to CD recorders. However, some models of CD-ROM drives can also read the TOC from the PMA.

The TOC contains information on the starting addresses of the disc Lead-in/Lead-out areas and the attributes of all its tracks (for instance, track type: audio or data. If the track type is data, then the data mode ”Mode 1, Mode 2, etc., absolute starting address of the track and its corresponding session number). Besides this, the TOC also contains part of the ATIP and pointers to the location of its continuation.

The TOC is unavailable for reading directly or at the sector level,. However, to extract its contents in the raw form, it is possible to use the following SCSI/ATAPI command: READ TOC/PMA/ATIP (opcode: 43h) with format field == 2h .

The TOC should not be confused with the file system. In fact, the TOC and file system have nothing in common! File systems of CDs are stored directly in PMA and are easily available for reading at the sector level.

Program Area. This is the disc area located between the Lead-in and Lead-Out areas and containing information tracks with music or data. This is the main area of the disc, fully available at the sector level, including pauses between audio tracks. Most Data-CDs contain a single data track storing all of the necessary information recorded in a specific file system. However, file systems go beyond the limits of the topic under discussion. With regard to Audio-CDs, they have no file system at all. Rather, they use the TOC for this purpose, placing each song into a separate track.

If the Lead-out area is followed by a Lead-in area, the disc is described as multisession. Each closed session has its own Lead-in, Lead-out, and TOC, and the pointer contained in the Lead-out area contained in the TOC can include both the actual address of the Lead-out area of the current session and the address of the Lead-out area of the next session! The number of sessions is, in principle, unlimited. However, because of the pass-through numbering of tracks, the number of sessions cannot exceed 99. The session can be independent (the TOC specifies only tracks within a session) or connected (in this case, the TOC contains the addresses of tracks from previous sessions). However, not all drives recognize the existence of sessions. For example, a vast majority of audio players see only the first session and ignore all others. Because of this, it is possible to create discs that cannot be read on CD-ROM drives installed in computers, but can be read normally by standard CD players.

The session is considered to be closed if its data area is framed by Lead-in and Lead-out areas. Unclosed sessions can only be read by recording devices, which require access to the PMA. The pointer in the session s TOC pointing to the Lead-out area can contain either the actual address of the Lead-out area of the current session (closed disc), or the address of the Lead-in area of the next session. Recording is limited by the available disc space, the space in the PMA and the number of tracks (tracks are numbered using pass-through numbering, from 01 to 99). Sessions can also be connected at the level of the file system. The sessions mechanism allows for the modification of the CD-R by writing a new session.

Fig. 1.24: CD structure