General purpose file systems have disadvantages as well as
advantages. For audio-only media, file-based storage would not be as
optimal as the way CD audio works.
CD was designed with full consideration of how it would be used.
Primarily, people put in a piece of music, start at the beginning,
and listen until the end. Even if they skip a few tracks here and
there, there is still over 99% of the time spent just continuing from
where they left off, and less than 1% of the time seeking to some
random point. With this in mind, CDDA was designed to make
contiguous access most efficient. Not only maximally efficient, but
the time delay to find the next audio sample is always constant,
without a gap or jump unless the CD is damaged or the user
interrupts. CDDA uses a spiral track that is contiguous (and starts
near the center of the disc) to enhance the constant access rate.
The data is broken into blocks for error correction and other
organization reasons, but because the track is spiral, the hardware
does not have to seek to find the next block - it is always directly
after the previous block.
DVD has large sections of contiguous material, but it also has to
deal with menus, and even edits to the film for multiple ratings.
Because of the variety of data, and the hierarchy of access, and
completely new ideas like programmed sequences of media clips, DVD is
a more random access media, and it thus forced to use a general-
purpose file system. When reading multiple blocks, the hardware must
often physically seek the laser to a new position, even if the data
is related. You'll note that one of the drawbacks of this design is
the layer switch during longer movies which causes a pause in the
playback on every DVD player available.
We're probably going to see file-based media from here on out, but
this comes at a price. I've seen many DVD players "crash" when
following the links between menus or media clips. That's because the
whole system - from media format to playback system - is more
complex, and thus there are more opportunities for things to be
misinterpreted. I have never seen a CD player get confused, no
matter how complex the programming. There is elegance in simplicity,
especially when the simplicity is so closely matched with the way
something will be used 99+% of the time.
I believe that SACD is also a contiguous spiral of data, but in a
different format than CDDA. As far as I know, it is not file based,
but is stream based, even on the media itself (apart from low-level
blocks for error correction and seeking).
Brian Willoughby
Sound Consulting
On Sep 13, 2007, at 16:10, Harry Sack wrote:
does anybody know why dvd use files for audio and video but audio
cd's not?
What could be the reason for this?
e.g. they could make a file for each track and just put them on an
audio cd and make cd players compatible with this format. So for me it
has always been a mystery why audio cd's work this way.
does anybody knows if super audio cd's still work the same as normal
ones?
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