[mythtv-users] OT DVD ripping software

Stephen Worthington stephen_agent at jsw.gen.nz
Mon Oct 27 16:17:57 UTC 2014


On Mon, 27 Oct 2014 16:25:58 +0100, you wrote:

>AFAIK CDs don't have any error prevention/correction, so you need a tool to
>handle reading errors while ripping. The OS won't report errors.

Actually, audio CDs have two levels of error detection and correction.
The problem is that it is not usually good enough to cover the sort of
damage CDs collect in their old age.  Audio CD error correction is
actually quite good, but when it was designed, no-one knew just how
CDs would age and how bad the damage could get.  Pressed CDs last
pretty well if treated right.  The rest are very vulnerable to light,
heat, compression, damp, mould, ...

I believe data CDs have only the lower level error correction of the
two on audio CDs, which is way less capable than the higher level, and
hence are much more vulnerable to errors.

If there was no error detection or correction on CDs, then the OS
would not be able to detect errors and would just have to accept
whatever data it saw, except when the errors created obviously bad
filesystem errors such as file pointers pointing beyond the end of the
CD.

>But DVDs have a normal file system - I always thought that there is no
>problem concerning the reading quality. The OS and the DVD drive have
>to handle reading errors. And if the disc isn't readable an error will be
>reported.

If you are using the filesystem then yes, errors will be reported.  dd
does not use the filesystem.  It reads sectors directly.

>But dd isn't a good tool for creating disc images???

No.  Unless you check the logs after using dd, you may never know that
a sector had a read error and the data it copied for that sector is
bad.  Until you play the bit of the disk image with the error.  So use
readom which does report errors and is designed for the job of ripping
optical disks.


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