Frequently Asked Questions About CD-R and CD-RW Discs

Please contact Media Sciences if your questions are not answered on these pages.

When should multisession recording be used?

Only when you are forced to use it. Hopefully never, unless multisession is required by a format such as Photo CD. Multisession efficiently supported several recording sessions, enabling one session to use only a fraction of the capacity of an expensive CD-R disc. Media is now inexpensive, and cost savings from multisession recording are insignificant. Packet recording methods are much more efficient, but initially required specialized software and expensive drives. Packet capable software and drives for CD-R and CD-RW are now readily available and affordable.

Multisession might be used to place computer data in one session and digital audio in another session for limited multimedia applications. Single session, mixed mode is better for this application. Mixed mode records data only in the first logical track. The remaining tracks contain digital audio.

A single session begins with lead-in, followed by a program area, and ends with lead-out. The program area contains the desired information. Lead-in and lead-out use 28 MB of the disc for vital navigational information for the read drive. Disc-at-once recording of a single session is optimum because the entire session is recorded in a single, uninterrupted operation.

Multisession discs consist of a chain of single sessions, each having lead-in and lead-out areas. The first session can be read in any drive, but subsequent sessions can be accessed only by a multisession capable drive. Multisession has a high overhead. Lead-in and lead-out of the first session occupy 28 MB that is reduced to 13 MB for all following sessions. Twenty sessions therefore have 275 MB of overhead, and only 378 MB is available for user information. Packet recording is highly efficient because twenty packets have only 0.28 MB of overhead.

Undesirable link blocks can be present between sessions. These are regions containing corrupted data, and are located at the splice where recording of a prior session stopped and where recording of the current session started. Link blocks that are present in an image for replication will cause laser beam recorders to abort the mastering process.

After each session is closed, the lead-in of that session contains the recordable address of the next future session. A table of contents (TOC) in the lead-in contains track locations only for the current session, unless the system is capable of linked multisession recording. When the entire disc is finalized, or closed, a pointer to the next session is missing from the lead-in of the last session, preventing subsequent recording even if physical space is available. At the same time, contents of the hidden Program Memory Area are recorded in the TOC of the last session. Multisession capable drives normally read the TOC in the last session, supporting access to information recorded in all sessions. Drives that are not multisession capable only access the TOC of the first session, limiting access to information in that session.

Multisession recording is usually reserved for special applications such as Kodak Photo CD. Archival storage is best accomplished using single session recording, or packet recording if files are frequently appended, updated, or otherwise changed. Important files that had been saved to a multisession disc may not be accessible at a critical time if the disc has not been finalized, or may not be readable in drives that are not multisession capable.