Multi-track, multi-session CD-ROM disks

[Carl Fleischhauer]

I have been drafting format descriptions of a number of "forensic" disk image formats for the Library of Congress format sustainability website, including a number of members of Expert Witness format family and two flavors of AFF.  All of these are typically used to image magnetic media.

I also wrote a description of the ISO format for optical media.  It is probably the case that ISO images are not evidence grade according to the careful standards of forensic investigators.

In the course of working on the ISO format, I bumped into discussions of the special issues that arise when imaging what are called multi-track or multi-session optical disks.  Although it may be that you can think of audio CDs as falling into this broad category, I take it that some disk burning systems could put multiple "sessions" on a single piece of media.  Maybe burn one session today and then burn another tomorrow (??).  The Wikipedia article on ISO images reports that the data from multi-track or multi-session disks can be "stored" as a "raw disk image"  in the CUE/BIN, CCD/IMG, and MDS/MDF formats.

I  am not certain that there is value in trying to go further re: multi-track or multi-session disks.  But let me ask: have any archivists run into examples, other than audio CDs?  If so, did you produce (or wish to produce) a disk image for preservation?  Which format?  Which tools?

Thanks for comments and advice.

Carl Fleischhauer

NDI/NIO
Library of Congress

cf...@loc.gov

[Michael Kjörling]

On 22 Dec 2015 15:30 -0500, from cfl...@gmail.com (Carl Fleischhauer):

> In the course of working on the ISO format, I bumped into discussions of
> the special issues that arise when imaging what are called multi-track or
> multi-session optical disks. Although it may be that you can think of
> audio CDs as falling into this broad category, I take it that some disk
> burning systems could put multiple "sessions" on a single piece of media.
> Maybe burn one session today and then burn another tomorrow (??). The
> Wikipedia article on ISO images reports that the data from multi-track or
> multi-session disks can be "stored" as a "raw disk image" in the CUE/BIN,
> CCD/IMG, and MDS/MDF formats.
>
> I am not certain that there is value in trying to go further re:
> multi-track or multi-session disks. But let me ask: have any archivists
> run into examples, other than audio CDs? If so, did you produce (or wish
> to produce) a disk image for preservation? Which format? Which tools?

Multi-session CDs were, as I recall, fairly common in the early days
of home data CD recording. The basic process was that the burn left
the end of the disk "open", and at some point you would write a last
session and "close" the disk. So your summary above is basically
correct.

Multi-session would seem to by necessity imply multi-track (since you
can't leave a track open), but multi-track does not need to imply
multi-session; you could burn a multi-track disk in a single session
and then immediately close it. In fact, I do believe that many
early-consumer-CD era CDDA players had trouble interpreting
multi-session CDs but could handle single-session CDs just fine, which
points toward an actual difference in the on-disk storage format.

_This was sometimes exploited as an advantage_; for example, one
computer magazine I subscribed to back in the day shipped one issue
with a CD with software, and audio samples from a sound card test done
in that issue, using the intricacies of how CDDA and data could be
mixed on the same disk to make a CD that could safely be played on a
stereo system (without risking accidentally playing back the data
track) as well as read on a PC. Naturally, that CD was factory
produced, not burned on a PC, but such tricks definitely were pulled
off at least occasionally in the real world.

--
Michael Kjörling • https://michael.kjorling.se • mic...@kjorling.se
“People who think they know everything really annoy
those of us who know we don’t.” (Bjarne Stroustrup)

[Kam Woods]

Geoffrey Brown has a paper that discusses an historically significant case - mixed-mode CD-ROMs produced by the Voyager Company that include a data track and multiple audio tracks. Relevant text from the paper (p13):

"A small number are mixed-mode CD-ROMs (later called Enhanced CD) with multiple tracks – a data track containing an HFS file system followed by one or more audio tracks. The data-only CD-ROMs present no problems for SheepShaver or BasiliskII in either their physical or image forms. The mixed-mode CD-ROMs do not work on most platforms in their physical form and, prior to our work, were not supported as images."

And link to that paper:

http://www.ijdc.net/index.php/ijdc/article/view/216/285

The paper also discusses specific methods to image these types of disks.

Kam

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[Alexander Duryee]

Carl,

I wouldn't worry about "the careful standards of forensic investigators" - there's no appreciable difference between consumer tools and those in use by law enforcement.  As optical media is a read-only format - it takes real effort to contaminate a disc - there's no need for special tools or standards to generate "forensic" images.  The Forensic Toolkit, for example, runs ISOBuster under the hood, and generates the same BIN/CUE file combination as would standalone ISOBuster.

I don't recommend using cdrdao to image hybrid discs, due to CD Audio playing fast and loose with error correction.  cdparanoia (OSX/Linux) or Exact Audio Copy (windows) will create more reliable images.

Cornell University just published a whitepaper regarding their work on preserving and accessing complex digital art objects (mostly on optical media) that you may find useful: https://ecommons.cornell.edu/handle/1813/41368

Happy holidays,

--Alex

[Carl Fleischhauer]

Alex, thank you for the citation -- I was aware of the report but hadn't stirred myself to look!  I see you played a big role in this project. 

My own curiosity just now is about the selection of disk image formats.  Regarding this small-and-narrow topic (compared to the report's fascinating and extensive discussion of content preservation), what I read there is more about tools than the imaging format.  Footnote 11 (p. 9) indicates that the favored tool was Guymager. I am not a practitioner, but it looks like Guymager produces "dd" image files as well as a couple of the "forensic" types (normally associated with magnetic media).  Am I correct then, to conclude that the Cornell project produced dd images?

By the way, I see you tweeted today about your "raised hackles" when people use say "forensic CD image."  I have no doubt that my query inspired this comment, although I tried very carefully not to say that.  I have always fretted that, in any case, the common usage of "forensic disk image" was misleading.  "Forensic" is not an attribute of the disk image, but rather it is the case that disk images support forensic work in a helpful way, and (for magnetic source materials) the types that some folks call "evidence grade" (a term I prefer) do a better job of supporting forensic investigations.  Meanwhile, I quite agree with your assertion that for optical disks, the "evidence building" stakes are much lower.

Best from Carl

[Alexander Duryee]

Carl,

It was pointed out to me that cdrdao runs cdparanoia as part of its extraction process.  As such, disregard my previous comment regarding it - it should be sufficient to create preservation copies of mixed-mode discs.

I believe that the preferred output for that project was BIN/CUE - the BIN file representing the actual data from the disc, and the CUE file containing the structural metadata necessary to make sense of the BIN (typically session/track markers).

Regarding my remarks off-list - my apologies if my frustration seemed directed at anyone.  I was venting general frustration at the misapplication of law enforcement tools and paradigms in digital preservation.  While the forensic methodology and tools have proven very powerful for the preservation of magnetic disks, their applications are limited beyond that scope.  For example, few criminals use CD Audio as a storage format, so tools developed by law enforcement are ill-suited to transferring that format.  Hence, it vexes me to see "doing forensics" held as the gold standard of preservation, particularly when tools and methods from other communities of practice would be more suitable.  As you said, "evidence-grade" would be a more accurate term to describe the process output, as it answers the question of "is the preservation object an accurate and comprehensive representation of the original?".

Happy new year,

--Alex

--

[Carl Fleischhauer]

Thank you for the added information and happy 2016!  Carl

[Simon Spero]

CD-DA is a fairly high capacity covert channel for steganography; the audio format is not  compressed (in the size reduction sense).

A much lower bit rate channel can exist by the deliberate introduction of correctable errors (which are potentially visible to paranoia but which are not preserved in the extracted audio).

Or you can just let mentally disturbed individuals bring CD-RW's into the SCIF and not bother checking what comes out.

Simon // or have data sent in your wife's name- no wait, wrong Manning

[Tyler Thorsted]

Carl,

I am curious what you have learned since this thread was started. I recently came across a disc made by a film scanning studio. The disc reports it was created by "SILVERLAB MULTISESSIONJ PICTURE-CD". There was four sessions burned, one for each roll of film scanned. Once mounted the disc showed all four tracks together as one disc, but when I tried to use dd or ddrescue, I would only get back the first session as an ISO. I was only able to get all sessions imaged into the CCD/IMG format.

Tyler

[cfle...@gmail.com]

Alas, I have not pursued disk image topics recently (I'm a retired guy now!).  Luckily many others on this list are very much at work and up to speed: let's see what we all learn when some reply.   Best from Carl