CD-ROM-XA standard
9 messages in this thread
Can anybody here explain in plain, simple words, what the XA standard is? I
understand it is NOT the same as reading multi session Photo CDs – although
many people seem to think so. Thanks, Thomas
Download the file CDFAQ.TXT it has a nice discussion on the XA standard.
Thank you, I will do that. Thomas
Can't really explain it, but you're right, it's more than reading multisession
PhotoCD. In very general terms, CD-ROM(XA) is a superset of the ISO9660
standard (but I don't think it is accepted to be part of the standard by the
ISO) to include some capabilities such as interleaving digital audio and video
between data tracks on the CD. Please, somebody who knows more than I do jump
in here and correct me if I'm wrong.
Kathy <<– The One-Off CD Shop MidSouth, Inc.
The CD-ROM XA spec. is 1) An extension to the CD-ROM specification. 2) An
extension to the ISO 9660 specification, and 3) A specification for storing
audio and video files.
The CD-ROM XA specification was written long before Multisession ever existed.
There is no direct relationship between Multisession and XA. The reason that
they are so often mentioned together is PhotoCD. PhotoCD is the first popular
application to use the CD-ROM XA sector format in the United States. PhotoCD
is also the first popular application to use Multisession. Because of this,
people think there is some connection between multisession an XA, when they are
two entirely different things.
A Multisession disc does not have to use XA format. It can use a standard
CD-ROM format. A CD-ROM XA disc does not have to be multisession. It can be
single session. A PhotoCD disc is always XA format, and can be single session
or multisession.
The CD-ROM XA spec is a proprietary Philips/Sony spec. They are the only
source of a full XA specification. You can get some information on it from THE
COMPACT DISC HANDBOOK by Ken Pohlman.
CD-ROM XA is an Addendum to the Yellow Book Standard (CD-ROM) which defines
streaming, interleaved data with audio compression (ADPCM).
Essentially a CD-ROM XA stream consists of compressed audio (ADPCM) and "other"
data which are interleaved (one sector after another) in a continuous stream.
The major advantages of CD-ROM XA streaming data are:
1) Audio and "other" data (video, etc.) arrive at the computer in synchronized
form. As a result much better synchronication (talking heads, etc.) is
available than with regular CD-ROM, where separate "files" of information are
loaded into RAM, then synchronized by the CPU. It is kind of like the
difference between a city street, with stop and go traffic and stoplights – and
a freeway with onramps and offramps – a big difference in throughput.
2) Lots of multi-channel high quality audio is available (up to 18 hours on a
single CD-ROM XA disc).
3) Because the XA circuitry does the deleaving and audio decompression, the CPU
of the host is not burdened. This means higher performance, and MUCH lower
needs for buffering, either in RAM or on hard disk. This in turn means that
CD-ROM XA brings quality multimedia to low end platforms.
4) Because CD-ROM XA uses Mode Two, Form Two of the CD-ROM standard, it
provides higher effective bandwidth (173 KB/sec vs 157 KB/sec) than regular
CD-ROM. The major difference is in not having robust error correction, as much
higher bit error rates can be tolerated in multimedia data than in say
financial data or executable code. The result is better quality and higher
performace from CD-ROM XA than from CD-ROM.
For the above reasons, CD-ROM XA has been embraced by Apple, Hitachi, IBM, JVC,
Kodak, Microsoft, Nintendo, Philips, Sony, 3DO, Toshiba – and more to come. We
feel that it has the potential to be the cross-platform standard for multimedia
publishing in the future.
Our company, Mammoth Micro Productions, is expert in CD-ROM XA. We would be
happy to provide further information on the subject. We also have a set of
professional stream creation tools on the market – currently being distributed
in the United States by IBM and Sony. For further information contact:
Dan Kennedy
Mammoth Micro Productions, Inc.
1700 Westlake Avenue North, Suite 702
Seattle, WA 98109
(206) 281-7500
(206) 281-7734 FAX
Thanks a lot for that. The questioning goes on: What about drives? What do XA
drives need, that normal CD-ROM drive do not have? Is a multi session photo CD
drive automatically a XA drive? Thanks again. Thomas
<<What about drives? What do XA drives need, that normal CD-ROM drive do not
have? Is a multi session photo CD drive automatically a XA drive?>>
You are very welcome. (for the description of CD-ROM XA)
CD-ROM XA drives are Mode Two drives, while early CD-ROM drives were Mode One
only.
A multi-session "Photo CD" drive will very likely be a Mode Two drive. Most of
the CD-ROM drives being sold this year are Mode Two "XA Ready" drives.
Hope this helps. Let me know if you have further questions.
Tom
There are essentially two types of XA drives:
1) Mode 2 Form 1 data drives. These can read the Mode 2 Form 1 sectors of XA
discs that are used for storing data that does not follow the XA audio or video
data spec.
2) Full XA drives. These drives include audio and video processing to handle
XA Mode 2 Form 2 sectors.
Note that PhotoCD discs only use Mode 2 Form 1 sectors! They do not use the
special Form 2 XA video format. Therefore, a drive that claims to be PhotoCD
compatible may be only partially XA compatible. However, a drive that is fully
XA compatible should be at least single-session PhotoCD comptible IF you also
have the software to decode the PhotoCD picture format.