[TriLUG] why can't I dd an audio CD?
dave at jamsoft.com
Fri Dec 26 09:53:25 EST 2008
----- "Joseph Mack NA3T" <jmack at wm7d.net> wrote:
> I guess silicon was expensive in 1980 (or thereabouts). But
> then they had to handle the pops and squeaks you mention.
> The would seem to require much more Si, but then the pops
> and dropouts are being handled by the CPU (sort of like the
I've read that errors are masked by CD audio players by replaying the previous sample for minor errors or muting the output for longer stretches - both easy to do in silicon. The pops/squeaks I mention pertain to DAE and are primarily caused by marginal DAE implementations in drives. Some drives are much better at DAE (ripping) than others. A clean and new audio CD will play at 1x speed without any audible defects, and the few that are there are easily masked. When you start ripping with DAE at multiples of that speed, it gets more complicated.
> >> I would assume then that video DVDs are also different from
> >> data DVDs in much the same ways.
> > Fortunately they got it right with DVD-Video. It's just a
> > restricted case of DVD-ROM and uses the same, strong error
> > correction. You *can* "dd" a DVD-Video disc.
> hmm Commercial DRM'ed DVD movie
> dd if=/dev/dvd of=foo.out
> about 2M of output
> Error: Illegal request
> Read of scrambled sector without authentication.
> I take it that you're talking about DVD-Videos you make
> yourself then
After you play a commercial DRMed movie with an approved software player, the DVD drive is authenticated at that point and the data may be dd-ed - albeit scrambled. Homemade discs don't need the authentication step. DRMed or not, the DVD-Video and DVD-ROM formats are a series of 2048 byte blocks on the disc.
> I need to know more about the filesystem to understand why
> this is so. I'd assumed since you could mount the audio CD
> as a cdfs filesystem and then copy all the wav files to your
> harddisk, that a CD was just another piece of hardware with
> a known normal filesystem.
You decide whether CD-DA qualifies as a filesystem: it's a simple TOC with a list of sector offsets for each track (song) as track 1, plus the tracks 2-n themselves. When a sector offset is requested, the drive starts playing audio at *approximately* the specified offset. Cdfs is cool but gives the appearance of something not really there on the disc. I have tried cdfs just as a curiosity and never use it to copy audio data. Wonder if reading a track multiple times using cdfs yields .wav files of slightly different lengths, or at least shifted according to where the drive starts streaming bits for the instance of the read?
> so dd is just sitting in a loop fetching consecutive block
> numbers and the hardisk handles finding the blocks (which if
> it's a spare block, will be in the spare area).
You got it.
> So how does a tape drive work? Are the blocks numbered? I
> assumed the blocks were unnumbered and consecutive and the
> drive just kept reading blocks till it found the file or
Tape drives I've dealt with over the years all have the notion of file numbers plus block numbers within each file. The block size can be set to a fixed or variable value. The drive itself usually doesn't know how many files and blocks are on a tape: outboard software has to take care of that by writing a TOC or keeping that info in a file. The drive navigates around by looking for EOF and EOT marks detectable even at high tape movement speeds.
> In an audio CD then the blocks aren't numbered and the drive
> has to pick its way though the CD hopefully picking up the
> next block each time.
The sectors are numbered but in CD-DA mode the request to start playing at a certain sector starts streaming audio with limited accuracy. It could be many samples behind or into the actual start of the referenced sector. That's why the standard specifies two-second gaps between tracks.
> I've always assumed by the time you see your first badblock,
> you've used all your spares and the surface is loosing
> integrity pretty fast.
Yeah, it means the surface is degrading faster than the drive designers expected - a good reason to copy off anything important and toss the thing.
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