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Posted by gordon shumway on 03/01/06 14:11
"Jeff Rife" <wevsr@nabs.net> wrote in message
news:MPG.1e6c49be9f45589898a3da@news.nabs.net...
> David (sorry@toomuchspam.com) wrote in alt.video.dvd:
>> Well, I know they CAN be copied with the approrpiate software, but I am
>> just curious why that is even necessary. As far as I can tell, CSS just
>> encrypts the data, so that my copy (like the original) can only be played
>> on authorized DVD players. But when I try to do a whole disc copy with
>> Nero, it tells me "This DVD is copy-protected and canot be read". The
>> only
>> thing I can think of is that Nero checks for the copy protection, and
>> willingly refuses to copy it. Or am I missing something?
>
> A simplified explanation:
>
> A DVD-ROM (which is what a DVD-Video disc is based on, just with a
> specific place to put the files) is like any other disc in that it has
> sectors of data. And, like any other disc, in addition to the data,
> there is extra space in each sector for error correction. The error
> correction on a DVD (or a CD, for that matter) is a form of cyclic
> redundancy check (CRC)...basically, a checksum of the data. There are
> other error correction methods used on DVDs as well, and you can do more
> research if you want find out more.
>
> What CSS does is take the *entire* sector that was on the master data
> (including the error correction) and encrypts it. This data is then
> what ends up on the master DVD, and gets pressed to every copy. The
> trouble is that the CRC is no longer valid, and every time the drive
> reads it, it thinks that the sector is bad.
>
> So, unless you decyrpt the data, you can't know if the data is accurate,
> and many DVD drives won't even return the "error-filled" data unless the
> programmer is doing some very low-level calls. Even then, there is
> still no way to know if the data is accurate without decryption, and
> since there is a "soft" error (that is, one that can be corrected by the
> drive because of the extra data on the disc) on about 1 in 10 sectors,
> you basically end up with garbage without error correction.
>
> As for how Nero tells it is copy-protected, I suspect there is just a
> byte in one of the files that says it is. I haven't looked that deep
> into that part of CSS.
>
> But, even if this wasn't set, Nero (or any other program) couldn't copy
> the disc without some sort of CSS decrypter.
Thank you for the good explanation, Jeff. I have always wondered why it
could not just "clone" the disc.
>
>
> --
> Jeff Rife | "The old guy was leading a 'Simon Says' game
> | when he collapsed. On the way down he yelled
> | 'call an ambulance!', but no one moved."
> | -- Wings
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