Constructing a Disc (Sessions)

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Each time you write to a CD is considered a session - as in writing session. There are two ways to employ sessions in recording your discs: single session and multi-session. As the names indicate, a single session disc is written to once, while a multi-session disc may be written to multiple times, or until the disc reaches it's maximum storage capacity.

A session is a recorded segment of a compact disc that contains one or more tracks (data or audio). These segments are always preceded by a lead-in and closed by a lead-out file. The lead-in and lead-out files contain information about the recorded data, such as a table of contents.

The session method you choose to use will depend on your recording requirements and the amount of disc capacity you wish to use.

Single Session

Single Session recording makes the most efficient use of available disc space, so more data can be stored on a disc using single session than multisession. A single session disc contains data written during one (and only one) session. Once all the tracks have been written, the disc undergoes a process called fixation where the lead-in and lead-out are written. Once a single-session disc has been completed, you can never add more data to the disc.

Table 2.2: Single Session Track Structure

Lead in (TOC) Track 1 Track 2 ... Track 23 Track 24 Lead Out


If you want to record data to the same disc over the course of time you should consider creating a multi-session disc. Data recorded in each session is linked, permitting subsequent recording sessions to refer to data from previous sessions. The term for adding a session to a multi-session disc is append. When you append a session, GEAR will read the previous session on the disc and create a virtual image of it - remember from earlier, a virtual image is like an address book that contains the location of your files. By this token, references to your previously recorded files are incorporated with the new files you are appending in a single table of contents (TOC) which resides on the disc.

IMPORTANT: One drawback to using multi-session is the disc space required as overhead for each append session you conduct. On average, each time you append to a disc, an additional 15 MB will be needed to create session buffers and the new lead out and lead in files which include the TOC.

It's through the TOC that a computer is able to locate information on a disc. If information is present on a disc, but not it it's TOC, it is inaccessible. If you ever choose to delete information from a session, the old data will not actually be erased, instead the reference to that data will be removed from the new TOC so the old file can no longer be read. The structure of a multisession disc will look similar to this:

Table 2.3: Multi-Session Track Structure

Session 1 Session 1 Session 1 Session 2 Session 2 Session 2
Lead In Track 1-X Lead Out Lead In Track 1.1-X Lead Out

A true multi-session CD-ROM drive will automatically skip to the most recent session and present all linked sessions as one. As the user you will never be aware that multiple sessions exist on the disc. This makes multi-session particularly well suited for document archiving or regular file updates of catalogs.

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