Archived from groups: rec.audio.tech (
More info?)
Oh really?
Are you a software engineer too!
The data on a hard drive is truly random access and because of that it can
be in any order. That's why we have things like disk defragmenters. There is
a file system in place that requires that the entire partition on the hard
drive has a format regardless of the amount of data that is stored there. CD
audio is nothing like the data structures that are stored on a hard drive.
The tracks are most definitely in order on the disk and they can be accessed
simply by moving the laser read head a certain amount to catch the stream
somewhere near the beginning of the track. CD audio was developed WAY before
anyone had the idea of putting a file format and computer data on a CD. Sure
lots of enhancements to the format have come along over the years. I'm
talking about first generation CD.
~James.
)
<dpierce@cartchunk.org> wrote in message
news:1123601875.443766.88710@f14g2000cwb.googlegroups.com...
>
> James Lehman wrote:
> > It is my understanding that Redbook Audio is a digital audio data
storage
> > system that does not have the likes of a file system, like you would
find on
> > a data CD. It is more like the grooves on an old LP. The original idea
of a
> > CD audio player was a digital extension of the idea of the LP. It spins.
> > It's round. It has a hole in the middle. The tracks on a disk need to be
> > accessible while the disk is spinning and in any rotation so you can cue
a
> > track from somewhere near the beginning, not necessarily the exact same
> > sample every time; much like dropping the needle in the darker grooves
on an
> > LP. Because there is no file system, it takes some special care to
extract
> > the audio as a pure digital stream.
>
> That's not correct.
>
> CD audio data is collected into well-defined units that comprise
> a hiearchy which certainly qualifies as a random-access, indexable,
> seekable (with 100% repeatability) hierarchy.
>
> The smallest addressable unit in a CD is a subcode block, which has
> its own sync work, instructions, data, commands and error correction
> information. Such a block can define the beginning of a track or index,
> which is uniquely and repeatably accessible, just as would the sector
> on a harddrive.
>
> The lead-in tracks contain track and index tables that can be read in,
> as the vast majority of CD players do. That's why when you put in a CD,
> the player will often display immediately how many tracks it finds. It
> does not have to read the entire disk to gather this information: it's
> all in the index track. Another word for "index track" would
> appropriately be "directory." This area includes not only track
> subcode block index information, but includes track and index timing
> as well.
>
> Once any one entry is known, the drive can be commanded to seek to
> the exact location on the disk and start playing (reading) immediately.
> It will do so from the same sample every time you command it to do
> so. There is no inaccuracy in that respect. You're limited to starting
> reading from the beginning of a sub code block, and not anywhere in
> the middle, simply because the error correction requires an entire
> block to perform its operations.
>
> In all respects, a CD is far more different from an LP than it is
> similar, and for more like a hard drive than it is different.
>
/www.mindspring.com/~benbradley
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