[TriLUG] why can't I dd an audio CD?

[Joseph Mack NA3T]

On Thu, 25 Dec 2008, David Black wrote:

> Here's another answer from someone who understands the details better than I:
>
> http://www.xiph.org/paranoia/faq.html#play

This is a lot more complicated than I'd expected. I don't 
understand enough about CDs to digest his posting. Neither 
can I see why Philips/Sony back in the early days would have 
picked such a complicated (non-standard?) format. It can't 
have been insanity - there must have been technical reasons 
for it.

I would assume then that video DVDs are also different from 
data DVDs in much the same ways.

Hats off to the CDparanoia people. They worked it all out.

They posting talks about the difficultly turning bits on the 
CD into audio and about the format on the CD, but it doesn't 
directly address the problem of why you can't just dd all 
the bits, no matter what the represent audiowise.

It must be to do with the absence of positioning and 
synchronisation information. When you dd a partition in a 
hard disk, the hardware must be told "go to sector 1 which 
is at sector x/cyclinder y/head z, now go to sector 
2..sector n" and when it gets to the x,y,z position, checks 
the sector number found against what it expects. I guess I'd 
assumed that the heads were told to go to a list of x,y,z 
positions and read whatever was there. Clearly dd must have 
some understanding of the low level format of the hardware, 
and must look for the low level formatting on the disk 
(blocks?) that mke2fs (etc) look for, to put down the inodes 
etc.

If this were the case, you wouldn't be able to dd a 
completely blank harddisk that didn't have any 
sectors/blocks on it, or a disk that had been passed through 
an erasing alternating magnetic field.

Still a lot I don't understand here.

Thanks Joe

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
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