#PhotoCD, XA, … ?
6 messages in this thread
What kind of features? Is this related to multi-session (whatever that
is)?
New features is somewhat vague. Basically it is a new track that isn't
already defined by the CD audio and CD ROM standards.
Moe 2 XA computer data, compress audio data and video/picture data. The
computer data has more error correcting code. XA disc has compress
audio/video or picture data is interleaved with normal computer data.
NOTE: that the audio is not normal CD-Audio since it CD audio track can't
be on the same track as XA.
Single session is what every normal CD-ROM is currently. Basically it has
one table of contents like a book would have. Multi-Session has many table
of contents which can be compared to a having a notebook divided into
section seperated by a plastic color sheet and this plastic color sheet can
be thought of as a new table of contents.
Since with a Photo CD you first add pictures (the very first time) in
Spring 1992, In the FALL after you go to Comdex (Nov 16-20) you have more
pictures to add which you do. This second sessions and all sessions added
after that is multi-session.
Normal CD-ROMs can only deal with one session since it is always at the
beginning of the disk and be only be written at the time of creation. Photo
CD allows the disk to be written to many times giving the different
sessions each session with its own unique TOC (table of contents)
This is very useful since many times there is still a large amount of space
on CDs that could be used since most CDs don't use more than 1/2 the space
on average.
Hope this helps.
Have have read the XA spec a little. This is my interpretation. I beleave
it correct, and suspect I will be made well aware of any errors:
To clear up a detail, XA audio is written as a data track, which is read
into the host system, and sent off to a decoder. The XA audio is
compressed, with a selection of compression rates. The compression appears
to be "lossy", since the higher the compression, the lower the bandwidth of
the playback. I don't have the exact numbers at hand, but I beleave the
numbers to be like this: with minimal compression, you can get 2 hours near
CD stereo; with maximal compression, you get 8 hours voice quality
monaural; then there is one or two steps in between.
XA also allows for the interleave of "groups" (called file numbers (?) in
the XA spec) of sectors. By grouping, you can have an audio group
interleaved with a graphics data group. This allows you to read the
graphics an audio off the disk, with the audio synchronized to the visual.
Since the sectors are interleaved, you can accesses the two groups of data
with minamal head movement. The drive and medai don't differentiate
between these types of data, only the "file number" recorded in the sector
header. It is the applications responsability to request the correct file
number for the data, and to route the data accordingly.
–rick
I have read the XA spec in the past but I was just recalling off the top of
my head. I think your explaination was EXCELLENT it clears up quite a few
problems I had in understanding it. Thanks for the clarification Rick.
Albert,
Thanks for the info. That helps alot to understand PhotoCD and XA. I was
planning to wait until year's end to get a CDROM drive and want to make
sure that it has the capabilities I need.
I have learn so much just reading messages in this forum and if I make a
mistake several people will always kindly correct me too. (GRIN) Good Luck.