Settle back, folks. This gets sort of complicated.
First, let's take a crack at understanding discs, sessions and tracks, working from the bottom up.
A track is a stream of bits with a starting point identified in a Table of Contents (TOC). In audio, a track is usually a song or other identifiable chunk of music or speech. In data, one usually writes a single track with all the data of a session; the capability is there to write multiple tracks, but there is no reason to do so and it wastes space.
A session is a collection of tracks (one or more, maximum 99 per volume) with a TOC at the front and a runout track at the end. If there is more than one session on a disc, the first will point to the start of the second and so on until the last. A session is said to be 'open' if there is no runout track and the TOC still permits adding tracks. Since an open session has no runout track, it cannot be read in an ordinary CD-ROM or CD player; it can only be used in a writer. When the runout track is added, the TOC points to it and a reader can then know when to stop reading; until then, it doesn't know. Each session (after the first) takes about 13 MB - essentially for the runout track. The first session takes about 20 MB, but that is considered when the disc is designated as "650 MB" or some other length.
"Volume" is seldom used in CD recording these days, but it's a useful concept which does show up occasionally. A volume is a collection of one or more sessions, none of which is linked to a session outside of itself. Usually, a disc has a single volume. To create a second volume on a disc, add a session to a disc with one or more existing sessions - and do not link them to one another. The result is as though you had separate discs on one, but access is a problem.
Neglecting the multiple-volume issue, a disc is said to be closed if its last (or only) session has a TOC saying that the disc is closed. On modern CD-ROMs, there is no need to close a disc except to protect it from having a confusing extra session added. It is a sort of write protection against adding sessions. In addition, some older drives can be confused by an open disc, so it's wise to close the disc when you are through adding sessions to it.
Some early CD-ROM drives and all audio players are single-session. They 'see' and use only the first session on the disc. There could be a multi-session audio player, but it is not part of the specification and all portable, mobile and home players are single-session. As a result, once you close an audio session, whatever you write after that will not be recognized except in a CD-ROM drive. That introduces a problem if you want to drop tracks onto a disc in different sittings (i.e., at different times). While the first session is open, you can play the disc only in a writer. But if you close the session, then any sessions you add later will not be playable in an audio unit. You may not like the situation, but those are the rules and that's all, folks!
Critical to all of this is the fact that audio players must be inexpensive. The standard is designed for the lowest common denominator of players, a brain-dead box which can read the TOC to count tracks and their lengths and nothing more. Of course there are fancier versions which can (and do) play tricks, but the design of CD-DA is built around the mindless machines.
Given the above, there are only a few logical ways to lay out multiple sessions on a disc - and many illogical ones with special purposes. Logically, if you're going to put CD-DA onto a disc, you put it into the first session. Put it anywhere else and an audio player won't see it. Data before audio makes sense only for CD-ROM readers, and in that case the only advantage of true CD-DA tracks is that they use disc space better than would WAV files (thanks to error correction). Such arrangements and other non-logical ones can be used on game discs where all rules are made to be broken. They're off-topic here.
They are also off-topic for some programs which may not support creating or copying discs with such layouts. If you are interested in programs for "backing up" game discs and other copyrighted materials, you may find useful information here and usable programs from major publishers, but piracy is not my purpose and you should look elsewhere for expertise on those matters.
Another page in this primer deals with mixing data and other formats including CD Extra, Mixed Mode and Video CD. (Compact Disc Video [CD-V] uses analogue video as on a LaserDisc; it's not relevant here.)
Until packet writing came along, the only useful way to write part of a data disc at a time was in separate sessions, paying the 13-MB penalty for each one after the first. Packet writing is not universal and a disc written in packets cannot be read in DOS or early Mac OS's, so multisession data discs still make sense. All modern CD-ROM drives are multisession and will read such discs without trouble. Although you should be able to mix modes and even ISO 9660 levels, safety says: don't. Data mode 2 (XA) was devised for multisession, but with today's hardware you can use either mode without penalty.
After the first session, you may either import a previous session when creating a new one or not. The simplest course when adding sessions incrementally is to import the immediately previous session automatically. That simply links the old TOC into the new one so that your resulting directory will include the old one. You may have an option at that point to delete files from the first session or to move them around among the folders. In any event, all you are moving are pointers to the data, so no significant space is wasted. Note that if you do not link the previous session, you create a new volume. The old volume (or the old session) is accessible then only with software written for the purpose, such as Adaptec's Session Selector or Nero's volume mounter.
E-mail me at cdrecording@mrichter.com
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