CD Extra (Blue Book)
Introduction
The CD Extra is a mixed mode CD-ROM, containing first an audio session, with one
or several audio tracks, then a second session (data track). The first session
is the only one seen by the audio CD players, the second one is only seen by the
computer. You should not confound those CDs with some earlier attempts (before
1995) to put first a data track then the audio tracks, or even to squeeze the
data track in the pre-gap space, before the first track. The CD Extra
specification is described in the Blue Book.
It happens that some people use expressions like "Enhanced Music CD" or "CD Plus" to
designate such CDs.
More recent version of the main operating systems allow you to access both
sessions simultaneously, for instance accessing the audio tracks in a player and
accessing the data tracks in the Explorer/in the Finder.
How to do?
The method is rather simple: you first burn the audio session, without closing
the disk. Some recording software speak of session at once, others speak of
reopening a new session, of not closing the disk. You may have to look in the
manual and to do some tests.
On the other hand, many software packages don't allow you to leave an audio
disk unclosed. This is the case with the Sonic package (ex Easy-CD).
You then burn the data session/track (data sessions contain almost always a
single track), this time closing the disk. Nothing prevents you to register
several data sessions/tracks but it doesn't make necessarily sense.
Some CD-ROM burning software packages recompute the offsets of the ISO catalog
while burning the data track in the second session.
Other refuse to burn the track with an error message stating that the track is
not built for the medium in the burner (in particular, Easy CD Creator).
One should note that this redocking (rebasing) operation is not strictly
necessary, in particular for the Macintosh view in the data session.
A CD Extra with Hybrid Data Track
Depending on the data track you produced, it can be an ISO 9660 track, a HFS
(Macintosh) track or a hybrid (HFS/ISO 9660) track, like an image produced by
MacImage.
You may even use a .DS_Store feature on the HFS/HFS+
side of the data volume.
Doing it with Nero
I tested the sequence with Nero 5.5 and Nero 6.6.
I'm going to upgrade to version 9 to test whether there are any major changes
against those former versions. As I feared it, Nero 9 doesn't allow you anymore
to burn an zero-based image a little further on the disk.
I first created an andio project by dropping afew MP3 tracks. Those files are then
converted transparently to AIFF/WMA files. You could naturally use directly such
uncompressed data files.
The project is then burnt without closing the disk, all other options and
settings keeping the default values.
Then I took a hybrid image produced by MacImage and loaded it in Nero to burn it.
I started the burning.
I had two successive error messages under Nero 6.6 (the second one only under
Nero 5.5):
First error message:
The disk you have inserted is not empty, but the writing mode of your
compilation is 'Start Multisession' or 'No Multisession'. This will
cause the information already on the disk to be inaccessible via
Windows Explorer. Should Nero proceed with with writing to this disk?
This message tells us that there is already a session on the disk and that the
session to be burnt is zero-based (meaning that it should come at the beginning
of the disk.
This error message can be ignored safely. Let's click OK and go to the second
one.
Second error message:
You are trying to write an additional session which is not an ISO/XA
Mode 2 session on an audio CD. The resulting CD will not be of
standard format and might be unreadable on some CD-ROM drives. Do you
still wish to continue?
Nero detyermined that the session to burn is not an ISO/XA Mode 2 session. And
warns us that it could be unreadable on some drives.
I accept this warning also and click on Continue.
That's all.
Those disks could be read under Windows XP and Vista (even 64 bits), in one case
after upgrading Windows Media Player. On the Macintosh side, they were read under
Mac OS X 10.3.9 and 10.5.3.
Some Final Observations
First of all, Nero redocks the data session. That means that the offsets used in
the ISO filesystem, which were zero-based, that is computed from sector 0 on the
disk, were recomputed to account for the presence of the audio session.
Note thta such a redocking is not necessary for the Macintosh view of the data.
nero tells us also that the data session following an audio session should be
an ISO/XA Mode 2 track. This may be the rule stated in the standard (Blue
Book). On the other side, the drives I tested accepted this data disk (with a
Mode 1 data track).
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