WinBook Tech Article
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Subject: Information on Multi-Session CD Creation
Keywords: Adaptec, CDRW, CDR
Tech Article Number: WBTA00170123

NOTE: You, the customer, are solely responsible for data security. WinBook strongly recommends that you perform a backup of all personal data contained on your system prior to performing this procedure. Warning: WinBook will NOT be held responsible for any data loss incurred during this process.


What is a Multisession Disc?

A multisession disc is a compact disc to which data is added incrementally in more than one recording, allowing you to add, update, or "delete" files and directories. All the data on a multisession disc, when read on a multisession CD-ROM drive, can be seen as if it were all recorded at the same time (if you linked the data between sessions).

Note: Packet writing, another approach to adding data incrementally to a disc, is not the same as multisession. Packet writing is supported by Adaptec's DirectCD software included with your unit.

On a multisession data disc, each track of data is recorded in a single session which is closed after the track is recorded. When the session is closed, a session lead-out is written which allows a CD-ROM player to recognize the session and read data from it. Writing the session lead-out, and a lead-in preparing for a new session you'll write in future, takes up space on the disc: 22 megabytes for the first session, 13 megabytes for each subsequent session.

When you record the first session on a disc, the names and addresses of the files recorded are written in the file system for that session. When you add more files in a subsequent session, a complete new file system is written for the new session, but it can include references to files recorded in the previous session; this is known as linked multisession. The actual files already on disc do not have to be written again in the new session; only their addresses are included in the new session's file system. These addresses can be carried forward in additional sessions, so that all files recorded in previous sessions are shown as if they were part of the latest session.

When linking data between sessions, you can virtually "overwrite" an older version of a file already recorded on disc by writing a newer version of the same file (with an identical filename and directory path) in a new session. The link to the previous version of the file is lost, but the file itself is still on disc, and you can get it back again if you need it.

Multisession is useful in making incremental backups, or distributing data which is periodically updated. However, because only multisession CD-ROM drives can access all the data on a multisession disc, it's best to consider such discs for use only where you have some certainty or control over what system will be used to read them, or have already tested and know that the systems your discs will be read on do support multisession.


Mode 1 vs. Mode 2

You can turn a single-session disc into a multisession disc at any time simply by adding another session to it (as long as the disc has not been closed). However, it's best to decide in advance whether your disc will be multisession or not, so that you can decide whether to record the first session in CD-ROM format (aka Mode 1) or CD-ROM XA format (aka Mode 2). For historical reasons, some older CD-ROM drives which will recognize a disc as multisession only if it's written in the CD-ROM XA format. You cannot mix CD-ROM and CD-ROM XA formats on the same disc.


Multisession and Audio

You could, theoretically, record audio tracks in multiple sessions. However, an audio CD player will only play the tracks in the first session. Easy CD Creator allows you to add one or more audio tracks to a disc incrementally over several writings without closing a session, until you are ready to close it. However, the disc cannot be played in a normal CD-ROM drive or home or car stereo until the session is closed.


Multi-Volume Discs

It is possible to record multiple sessions on a disc without linking the data between the sessions, so that the data in session stands alone, as if it had been recorded on a separate CD. This is sometimes referred to as a multi-volume disc. In this case, if you need to access sessions other than the last, you will need to use the Session Selector utility in Easy CD Creator Deluxe.


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