When a compact disc is written using a computer, it is possible to finalise or ‘close’ the disc, so that no further information can be stored to it or the disc can be left ‘open’ so that other files and data can be stored to it at a later date if required.
What Happens with an Open Compact Disc?
It the disc is left ‘open’ then the contents of it can normally only be read by another computer and, for example, it would not be able to be used within a compact disc audio player.
The reason for this is that an open disc contains a PMA area (Program Memory Area) which can contain upto 99 sector addresses for each data session on the CD.
A writeable compact disc drive is able to read the Program Memory Area of the disc, however, a standard compact disc drive is not. This is why ‘open’ compact discs are often unreadable by standard compact disc players.
Each write session on the disc has a lead-in, Program Memory Area, Program Area (where the files are stored) and a lead-out which is created to identify the end of the disc.
What Happens with a Closed or Finalised Compact Disc?
When a compact disc is full or if no further data is needed to be written to it then the disc can be ‘closed’.
When a compact disc is closed, the TOC (table of contents) is written within the lead-in section rather than within the Program Memory Area (PMA), then follows the Program Area (PA) and then the lead-out session.
Computer compact disc readers and standard compact disc players are able to read the table of contents within the lead-in area of the disc, which is why they are able to read the content of finalised or closed compact discs.
How Does a Multi Session Compact Disc Differ?
A multi-session compact disc contains a link within the lead-in area of the disc to the lead-in of the next session on the disc.
A computer compact disc drive is able to read and follow the links within the lead-in areas of the disc so that all of the sessions can be identified and the files within them read.
However, a standard CD player is not able to follow the links meaning that only the files or data within the first session can be read.
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