Audio CDs not mounting

[Tim]

Joachim Backes
> Weird: Inserting an audio CD, then clicking (inside the desktop) with
> nautilus on the computer icon and then on "CD/DVD Drive: Audio Disc",
> the next nautilus window shows a window with title: "These files are
> on an Audio CD." with a file list like "Track 1.wav", "Track
> 2.wav",...
>  
> You may see the nautilus window at:
> "http://www-user.rhrk.uni-kl.de/~backes/UL/nautiluswindow.jpg"

Yes, I know.  Did you actually read what I said?  It's faking it.  Audio
discs are not mountable, and have never been mounted.

Nautilus is giving you an interface that pretends.  Some other things
can do the same trick.  The clue is when not everything can do it,
because their file chooser gadget only reads the actual directory tree,
and doesn't interact with the handler.  And, when you list all the
"mounted" file systems, it's not present.

There is no file system, nor "files" on a compact audio disc.
There are no wave files.
There isn't even audio data streamed on the disc in a manner similar to
a wave file.

There's a table of contents (TOC) at the start of the disc that says how
far in, and how far for, each track is located.  Then there's a stream
of encrypted audio data.  It's encrypted, in a non-secret way, for error
handling (*).  There's enough spare space in the track audio data that
some people have included a tiny bit of text so that the track title can
be shown on the player.

All of which requires a handler to give you an interface to it.  When
you double-click on a fake wav file in Nautilus, it could start playing
the disc in any number of ways, depending on what command is associated
with the action.  Send a play command to the drive, and let the drive
output audio.  Send a play command to a CDDA player program.



* Audio for a compact disc is a pulse code modulation (somewhat similar
to a wave file), in a sequential continuous left audio, then right
audio, sequence.  *BUT*, it's then scrambled using an old military
encryption scheme, for error handling.  And put onto the disc in that
form.

If it were unscrambled, then missing data would have large dropouts in
the audio.  Imagine this email where the middle paragraph was lost,
you'd have no way to know what was there to reproduce it.  But if you
pre-scrambled it in a defined way, then a large blob in the middle of
that scrabled data going missing in transit is only going to be a few
letters in each word, scattered throughout the message, when
unscrambled.  

It's reasonable easy to work out what a missing letter ought to be, and
fake the missing letter.  Of course, it's not 100% accurate, but far
less noticeable than dropouts on linear data, where the equivalent of
whole words regularly go missing, and larger segments a bit less often.

In the case of an audio signal, if at one moment the audio was 1.435
volts, then there was a missing bit, then the next moment the audio was
1.437 volts, the error correction could reasonably replace the missing
moment with 1.436 volts of signal (half way between the two existing
bits of data).  Rather like repairing a broken step on a staircase, you
have the adjacent steps as a guide to what ought to be between them.

For what it's worth, it's quite common that audio discs play back
through a mass of continual errors, but you never notice it (if your
player isn't crap at error handling).  It's one reason why some discs
are damn near impossible to rip, but apparently play fine (ignoring
deliberately broken discs, by the manufacturers, in an attempt to thwart
copying).

-- 
[tim at localhost ~]$ uname -r
2.6.27.25-78.2.56.fc9.i686

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