Why is copying a DVD so hard? No, seriously.
November 9, 2006 3:17 PM   Subscribe

Why is copying a DVD so hard? No, seriously.

Just to get this out of the way, I don't need help ripping DVDs. I was just puzzling over two things -- how is it that a DVD is just a long string of bytes on a disk, just like anything else, but it's so hard to get them off? I know quite a bit about computers, but next to nothing about files and I/O and the bits of computing this relates to.

Specifically:
  • How do the film companies introduce stuff into a DVD which causes a $3,000 computer to choke, but my $50 DVD player happily plays it without question? Same with a disk that's a little scratched or worn.
  • What's the difference between the laser in the DVD player and the laser in my DVD drive?
Why doesn't the computer just read whatever bytes are on the disk, or at least, why isn't there a preference for it to do so? For instance, I was just trying to copy a VCD with Toast. It's a little scratched, let's say at about 20 minutes in, the video goes all artefacty and the audio is stuffed for a few seconds. But I don't care. I'll live with it. Toast, however, won't let me. It says there's an error and it can't continue. I'd live with the error if it let me just click "burn anyway".

I guess my fundamental question is, why can't my computer just provide me with an exact copy of every byte on a disk? That seems, to me, simpler than what it's actually doing. Then I would burn it back, and leaving single/dual layer issues aside, I'd have a copy of the DVD.
posted by AmbroseChapel to Computers & Internet (25 answers total) 2 users marked this as a favorite
 
A law called the Digital Millennium Copyright Act makes it illegal for someone to write software that can copy a DVD, even when it's perfectly legal to copy that DVD (like backup, for instance). Specifically what are known as the "anti-circumvention" parts - the parts that say it's illegal to make tools (i.e. software) that can break copy protection.

A lot of people think it's stupid (myself included) and the best way you can change things is to learn more about it and spread the word.

Also, I think movie DVDs are 9.4 GB and DVD-Rs are only 4.7 GB which makes it harder to copy because you can't do a byte-for-byte copy, but if the DMCA weren't around your DVD burner would have come with software to shrink the size of DVDs before burning.
posted by revgeorge at 3:38 PM on November 9, 2006


If you want an exact copy, have a google for CloneCD/DVD.

Just does byte for byte copies.
posted by chrispy108 at 3:47 PM on November 9, 2006


Never found it hard - buy right software, download tweak, put disk in, put blank in, bingo
posted by A189Nut at 3:51 PM on November 9, 2006


>A law called the Digital Millennium Copyright Act makes it illegal for someone to write software that can copy a DVD

Maybe where you live it does.

>I think movie DVDs are 9.4 GB and DVD-Rs are only 4.7 GB


This is why I went to the trouble of posting "leaving single/dual layer issues aside".
posted by AmbroseChapel at 3:52 PM on November 9, 2006


>Never found it hard - buy right software, download tweak, put disk in, put blank in, bingo

And this is why I went to the trouble of posting "I don't need help ripping DVDs".

Please read the question!
posted by AmbroseChapel at 3:54 PM on November 9, 2006


The area on the disc that contains the CSS keys cannot be written to with consumer-level DVD writers or DVD-R discs.

You can, as others have just pointed out, remove the CSS encryption and burn that just fine.
posted by trevyn at 3:54 PM on November 9, 2006


Everything about your cheap DVD player is optimized for playing DVDs. It doesn't have to run Sims Hot Date, or Firefox, or MS Office. It just has to play Finding Nemo. It has chips on its motherboard that have the DVD-decoding algorithms hard-coded into them. This accomplishes the task of decoding a DVD much better and faster than a reverse-engineered software routine--which is what you use when you try to "rip" a DVD to your computer.
posted by deadfather at 3:55 PM on November 9, 2006


The gripe about a bit-for-bit copy is very logical. However here's why it won't work: "protected" discs (movies, etc) rely on a key embedded in the disc that can't be written by consumer units. If you do that bit-for-bit copy, the keys don't match up and the DVD player refuses to play it. So that's why you have to remove the keys with third-party software. The scheme is called CSS and you can Google that up.

Once it's removed, you can easily do bit-for-bit copies... I have a DVD duplicator used for my software company that will dupe any unprotected discs easily.

As far as why a computer DVD might behave differently than a set-top DVD player, I have no idea.
posted by hodyoaten at 3:57 PM on November 9, 2006


What's the difference between the laser in the DVD player and the laser in my DVD drive?

Nothing. But your DVD-writer's laser is much different. As trevyn hinted at, DVD-writers can't make a disc in the same way that a DVD-stamping factory can. The home-recordable DVD-R is an approximation of the original DVD technology. The fact that the movie industry had a hand in creating this approximation should tell you all you need to know.
posted by deadfather at 3:59 PM on November 9, 2006


>This accomplishes the task of decoding a DVD much better and faster than a reverse-engineered software routine--which is what you use when you try to "rip" a DVD to your computer.

Minor quibble, but surely it isn't what happens when I rip a DVD? Decoding a DVD (on the player) means "displaying a particular string of bytes as video" whereas ripping a DVD should just mean "copying the files".
posted by AmbroseChapel at 4:07 PM on November 9, 2006


<marks trevyn as best for that key bit of information>
posted by AmbroseChapel at 4:11 PM on November 9, 2006


Still outstanding, however:

>How do the film companies introduce stuff into a DVD which causes a $3,000 computer to choke, but my $50 DVD player happily plays it


I have software which removes CSS, because, I seem not to have said this enough, I don't need help ripping DVDs.

But Sony has some kind of other technology, and there's "RipGuard" as well, and there's been something of an arms race between ripping-software creators and movie publishers of late. The movie people author disks which deliberately contain "bad sectors", in some way that is bad for my computer, but not bad for my DVD player?
posted by AmbroseChapel at 4:16 PM on November 9, 2006


Your DVD player and your computer handle error cases differently, because they're trying to perform different tasks. Your DVD player might decide it best to skip over a block it can't read, to prevent the video from stalling, but your computer might try to error-correct that data in order to get a perfect copy. What Macrovision et. al. are trying to do is find such error cases that almost all standalone DVD players will handle fine, but that will foil a computer trying to rip it.

Of course, not all ripping software or DVD drives have the same quirks, and many of the error cases that cause problems can be fixed once those cases are known to cause a problem; that's why it's an arms race, and there is no winner.

It also doesn't make much sense to try and emulate a standalone DVD player exactly, because then you don't get things like robust error correction or 16x ripping or whatever.
posted by trevyn at 4:31 PM on November 9, 2006


The firmware in your computer's DVD drive is designed to make a best effort attempt to read a sector and then if it fails, emit an error. The read operations all occur under the control of a driver which is written to behave in the way you're experiencing.

The firmware in your consumer DVD player is, er, a consumer DVD player. These are intended to make a minimal effort to read a sector and then if it fails, try to play some more video.

Obviously, it should be relatively easy to make your computer's DVD drive read deliberately or incidentally damaged media in roughly the same bulletproof way a consumer DVD player does. The fact that Toast (and the underlying block device layer in the operating system) doesn't do so isn't your computer's fault or Hollywood's fault or the DMCA's fault, it's Roxio's (and Apple's).
posted by majick at 4:36 PM on November 9, 2006


Even for a non-protected disk that is scratched you can't tell it to "provide me with an exact copy of every byte on a disk" since it doesn't know what to write for the bytes that can't be read because of the scratch. Your DVD player will always try to playback what it can read since the worst that can happen is it displays some incorrect video on the screen. Your computer on the other hand will not accept just any garbage data in.

On preview, the "bad sectors" added by the movie companies act just like scratches in that both the DVD player and your computer read some bad data in but only the DVD player ignores it and keeps on reading.

On preview again, Trevyn is right.
posted by metaname at 4:36 PM on November 9, 2006


ripping a DVD should just mean "copying the files"

I think you want it to mean that, but it doesn't. Ripping, unless you are using some very old tools, decodes in the same step. Because what's the use of a "direct copy" if you can't play or write said direct copy?

>How do the film companies introduce stuff into a DVD which causes a $3,000 computer to choke, but my $50 DVD player happily plays it

It's very technical stuff. They hire some clever people and exploit differences between the two. DVDs (and CDs) contain redundant "error-checking" information, and that's where they usually introduce extra copy-protection. By finding things that the DVD player will ignore but your DVD-ROM drive won't, they can trick your computer. But since it falls outside the way DVDs are supposed to work, this voodoo science isn't perfect.
posted by deadfather at 4:37 PM on November 9, 2006


Why is copying a DVD so hard?

The direct answer to your question is: because all the companies involved in making DVDs and DVD burners are deliberately trying to prevent you from doing what it is you're trying to do.
posted by Steven C. Den Beste at 4:48 PM on November 9, 2006


Oh for god's sake, SCDB.

I'm honestly starting to believe that some people only read the title of the post and just skip the more inside part. What other explanation is there for the stupid "answers" I've got so far?

For the next person who comes along:
  • I don't need help ripping DVDs.
  • I know that the movie companies are deliberately trying to make it difficult for me.
  • I know that commercial DVDs tend to be dual layer 9GB disks.
  • I said all of this in my original post.
But then, you won't read this post either, will you?
posted by AmbroseChapel at 5:11 PM on November 9, 2006


But on the other hand, much thanks to trevyn, deadfather, metaname, majick and hoady for all the useful information.
posted by AmbroseChapel at 5:13 PM on November 9, 2006


>Ripping, unless you are using some very old tools, decodes in the same step. Because what's the use of a "direct copy" if you can't play or write said direct copy?

This is a key point I hadn't figured out.

I can't burn a copy of the disk which contains the decoding key, so ripping software has to decode it and write the decoded version to disk.

So in a way I was right -- copying is easier, except for the fact the copy wouldn't work.
posted by AmbroseChapel at 5:19 PM on November 9, 2006


There are basic linux tools that will let you burn discs byte-by-byte, but I do not know of Windows/Mac equivalents.
posted by doomtop at 5:39 PM on November 9, 2006


I'm sure you know all the DMCA stuff and the CSS keys, but this comment:

How do the film companies introduce stuff into a DVD which causes a $3,000 computer to choke, but my $50 DVD player happily plays it without question? Same with a disk that's a little scratched or worn.

says to me you have a crummy DVD drive. Mine reads discs faster and more reliably than my DVD player. Consider doing some research getting a new one. These days even good ones are super cheap.
posted by chairface at 5:40 PM on November 9, 2006


To what chairface said: many cheap DVD players use computer DVD-ROM components. The $50 DVD player exisits because of the availability of cheap PC parts, like DVD drives. Those cheapo portable DVD players use laptop DVD drive components. It's the same thing. Having said that, some drives are better than others.
posted by GuyZero at 5:48 PM on November 9, 2006


Ambrose: I had this same conversation with a friend the other day. I had found that I could use dd to get a bit-for-bit copy of the disc, and XBMC would play said copy in .iso form (because it cracks the decryption on the fly), but if I burnt it to disc, I still basically had a coaster.

So, under some circumstances, the copy will work - due to CSS being a weak scheme, but not on a $50 dvd player.
posted by pompomtom at 5:49 PM on November 9, 2006


Here's one way to make "byte-for-byte" ripping hard: create lots of bad sectors randomly all over the disk. However, those sectors are NOT part of the actual content--when you play the DVD in a $50 player, none of those sectors are actually used.

These bad sectors will cause all sorts of low-level read errors during a byte-for-byte copy. Maybe the read will fail after too many errors. Or maybe it will slow down to a snails pace as the software retries again and again. In either case, it will not be fun.

You also seem to be asking, "Why does software react to low-level read errors that way, instead of just barging ahead?" There are two answers: in the common "it's really just a temporary error", case, retry logic is usually a good thing--it's just the crazy copy protected disk case where it's bad.

And the other answer is: there is software plenty of software out there that does in fact barge ahead, and can make copies of these disks. But I know, you don't need help ripping, so I'm sorry I mentioned it.
posted by IvyMike at 12:03 AM on November 10, 2006


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