# DVD copying - why not bit-for-bit? **URL:** \ **Category:** Factual Questions\ **Created:** [December 8, 2002, 1:53pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300 "2002-12-08T13:53:31Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![amarone](https://avatars.discourse-cdn.com/v4/letter/a/e0b2c6/32.png) [@amarone](https://boards.straightdope.com/u/amarone)\ **Post date:** [December 8, 2002, 1:53pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/1 "2002-12-08T13:53:31Z")
I have no intention of copying DVD movies, but the subject came up in discussion with friends. One person, who works in a related industry, says that DVD movies cannot be copied because of encryption. A bit of googling tends to support this, but does find lots of people who have cracked the encryption. My question is: why do you need to crack the encryption to do the copying? Why can you not just do an exact bit-for-bit copy of the DVD? The result would be identical to the original, so should work whether you know the encryption method or not. Do DVD drives themselves prevent bit-level access? ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [December 8, 2002, 2:02pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/2 "2002-12-08T14:02:38Z")
In order to obtain a licence from the patent holders, manufacturers of DVD drives are supposed to sign an agreement that their drives will not allow such access (amongst other things). ---
**Author:** ![Dreaming\_of\_Maria\_Callas](https://avatars.discourse-cdn.com/v4/letter/d/f14d63/32.png) [@Dreaming\_of\_Maria\_Callas](https://boards.straightdope.com/u/Dreaming_of_Maria_Callas)\ **Post date:** [December 8, 2002, 3:19pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/3 "2002-12-08T15:19:02Z")
> [@](#): > > \*Originally posted by Desmostylus \* > \*\*In order to obtain a licence from the patent holders, manufacturers of DVD drives are supposed to sign an agreement that their drives will not allow such access (amongst other things). \*\* But that is usually based on settings in the firmware, and people who professionally make illegal copies of DVDs know how to patch firmware to let them do whatever they want. UnuMondo ---
**Author:** ![sigSEGV](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@sigSEGV](https://boards.straightdope.com/u/sigSEGV)\ **Post date:** [December 8, 2002, 5:18pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/4 "2002-12-08T17:18:20Z")
You can make a bit for bit copy of DVDs, no problem. The decryption only becomes involved when you try to play it. ---
**Author:** ![amarone](https://avatars.discourse-cdn.com/v4/letter/a/e0b2c6/32.png) [@amarone](https://boards.straightdope.com/u/amarone)\ **Post date:** [December 8, 2002, 5:21pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/5 "2002-12-08T17:21:27Z")
> [@](#): > > \*Originally posted by sigSEGV \* > \*\*You can make a bit for bit copy of DVDs, no problem. The decryption only becomes involved when you try to play it. \*\* But if it is a bit-for-bit copy, the decryption should work just as it did for the original DVD. ---
**Author:** ![sigSEGV](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@sigSEGV](https://boards.straightdope.com/u/sigSEGV)\ **Post date:** [December 8, 2002, 5:24pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/6 "2002-12-08T17:24:54Z")
I would post a link to the relevant technical details, but would probably get in trouble. :rolleyes: Lets just say, go to Google, search for DeCSS and click on the 7th link down. Scroll to the section titled DeCSS. ---
**Author:** ![sigSEGV](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@sigSEGV](https://boards.straightdope.com/u/sigSEGV)\ **Post date:** [December 8, 2002, 5:26pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/7 "2002-12-08T17:26:42Z")
Yes, you are correct. > [@](#): > > \*Originally posted by amarone \* > \*\*But if it is a bit-for-bit copy, the decryption should work just as it did for the original DVD. \*\* ---
**Author:** ![amarone](https://avatars.discourse-cdn.com/v4/letter/a/e0b2c6/32.png) [@amarone](https://boards.straightdope.com/u/amarone)\ **Post date:** [December 8, 2002, 5:31pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/8 "2002-12-08T17:31:29Z")
> [@](#): > > \*Originally posted by sigSEGV \* > \*\*I would post a link to the relevant technical details, but would probably get in trouble. :rolleyes: Lets just say, go to Google, search for DeCSS and click on the 7th link down. Scroll to the section titled DeCSS. \*\* I read those types of articles before doing the OP. I still don’t understand why cracking the encryption has any relevance IF you can do a bit-for-bit copy. Let’s try an analogy. I don’t speak German. You could send me a document in German and I could reproduce it in many ways to create a copy that a German speaker could understand perfectly. I could scan it, photocopy it, retype it letter for letter. All of these would work and I would achieve a perfect copy without needing to understand or translate the German in any way. The typing letter by letter is analgous to a bit-by-bit copy of a DVD (or CD or hard drive). If you can do that, then wouldn’t the encryption be irrelevant - it will still be used by the decoder when playing, but that’s OK. My point is that decryption would not be needed for the copying process. ---
**Author:** ![amarone](https://avatars.discourse-cdn.com/v4/letter/a/e0b2c6/32.png) [@amarone](https://boards.straightdope.com/u/amarone)\ **Post date:** [December 8, 2002, 5:32pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/9 "2002-12-08T17:32:38Z")
Oops - typed my response in after seeing your first one and before your second had appeared. ---
**Author:** ![sigSEGV](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@sigSEGV](https://boards.straightdope.com/u/sigSEGV)\ **Post date:** [December 8, 2002, 5:37pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/10 "2002-12-08T17:37:13Z")
It doesn’t have any relevance. When the DVD standard came out, it was stupidly assumed that a DVD reader that allowed bit for bit copying and a DVD burner were way off, therefore, no one would pirate. When the DeCSS software came out, all the sudden everybody assumed it was possible to copy DVDs, when it had been all along. > [@](#): > > \*Originally posted by amarone \* > \*\*I read those types of articles before doing the OP. I still don’t understand why cracking the encryption has any relevance IF you can do a bit-for-bit copy. > > Let’s try an analogy. I don’t speak German. You could send me a document in German and I could reproduce it in many ways to create a copy that a German speaker could understand perfectly. I could scan it, photocopy it, retype it letter for letter. All of these would work and I would achieve a perfect copy without needing to understand or translate the German in any way. > > The typing letter by letter is analgous to a bit-by-bit copy of a DVD (or CD or hard drive). If you can do that, then wouldn’t the encryption be irrelevant - it will still be used by the decoder when playing, but that’s OK. My point is that decryption would not be needed for the copying process. \*\* ---
**Author:** ![Kwyjibo](https://avatars.discourse-cdn.com/v4/letter/k/59ef9b/32.png) [@Kwyjibo](https://boards.straightdope.com/u/Kwyjibo)\ **Post date:** [December 8, 2002, 5:43pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/11 "2002-12-08T17:43:10Z")
> [@](#): > > \*Originally posted by Desmostylus \* > \*\*In order to obtain a licence from the patent holders, manufacturers of DVD drives are supposed to sign an agreement that their drives will not allow such access (amongst other things). \*\* I believe this function is called MacroVision. When I first used a DVD player, it was at work and we were watching the Matrix. I wanted to see what happened why you tried to copy it as I had heard it some how knows and scrambles the signal. MacroVision kicked in and it did just that. ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [December 8, 2002, 6:08pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/12 "2002-12-08T18:08:08Z")
I believe Macrovision was for VCRs. \>\> Do DVD drives themselves prevent bit-level access? The answer is much more complex than you may realise at first. Say you have a file and want to store it. The first thing you (or rather your computer) do is to break it up in blocks of (say) 512 bytes, but now each block has its own header, CRC, and other overhead. Now you want to record this on a CDrom but it is further encoded at a lower level with more error correcting codes etc. In other words, if you record the same file on a CDR there are going to be multiple differences but the error correction at read time, done by the drive, will be able to reconstruct the original block and return it. So, only the drive itself has access to the lowest level and will return the next upper level which in turn will be decoded and the file reconstructed. At any rate, you are right that if I copy a file bit for bit at the file level, I should get the same result. The problem is if the drive which I am asking to do the job will not cooperate because there is some bit somewhere telling it not to. I remember a couple years ago, I believe IBM and MS were cooperating on a new kind of hard disks which would not allow copying of copyrighted materials. I never heard agin. I guess not many people were interested in buying these drives. ---
**Author:** ![Kwyjibo](https://avatars.discourse-cdn.com/v4/letter/k/59ef9b/32.png) [@Kwyjibo](https://boards.straightdope.com/u/Kwyjibo)\ **Post date:** [December 8, 2002, 6:18pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/13 "2002-12-08T18:18:55Z")
> [@](#): > > \*Originally posted by sailor \* > **I believe Macrovision was for VCRs.** Possibly. But I do know that my DVD player had MacroVision on it. ---
**Author:** ![Kwyjibo](https://avatars.discourse-cdn.com/v4/letter/k/59ef9b/32.png) [@Kwyjibo](https://boards.straightdope.com/u/Kwyjibo)\ **Post date:** [December 8, 2002, 6:20pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/14 "2002-12-08T18:20:01Z")
(Add a 😉 to that. I just realised that might have sounded snooty.😉 ) ---
**Author:** ![neutron\_star](https://avatars.discourse-cdn.com/v4/letter/n/51bf81/32.png) [@neutron\_star](https://boards.straightdope.com/u/neutron_star)\ **Post date:** [December 8, 2002, 6:22pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/15 "2002-12-08T18:22:43Z")
> [@](#): > > I believe Macrovision was for VCRs. According to [this](http://reoc.keyservice.co.uk/a3/macrovision), Macrovision and CSS are one and the same. From that link: > [@](#): > > Macrovison is a property of the DVD and not the player and not all DVD’s contain macrovision but a large percentage do. A little more on [this](http://www.apexmodchip.com/macrovision.html) page: > [@](#): > > A standard DVD player contains a special Macrovision-enabled digital-analog conversion chip that is activated when a DVD is played. The activated chip applies copy protection to the analog output and causes copies made on most VCRs to be substantially degraded. ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [December 8, 2002, 6:31pm UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/16 "2002-12-08T18:31:11Z")
\>\> Macrovison is a property of the DVD and not the player and not all DVD’s contain macrovision but a large percentage do. \>\> A standard DVD player contains a special Macrovision-enabled digital-analog conversion chip that is activated when a DVD is played. The activated chip applies copy protection to the analog output and causes copies made on most VCRs to be substantially degraded. The two above statements can be interpreted to contradict each other. The main thing is that “A standard DVD player contains a special Macrovision-enabled digital-analog conversion chip that is activated when a DVD is played”. In other words, the player is cooperating in preventing the copying and in that sense the player “has” macrovision. Then certain DVDs may or may not have macrovision enabled. ---
**Author:** ![handy](https://avatars.discourse-cdn.com/v4/letter/h/b5a626/32.png) [@handy](https://boards.straightdope.com/u/handy)\ **Post date:** [December 9, 2002, 12:25am UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/17 "2002-12-09T00:25:07Z")
Ah, when I put a copy protected dvd in the drive the Nero software says the dvd is copyrighted. But then it does that when I make my own dvds too. Odd. Dvds are basically just a VIDEO\_TS & a AUDIO\_TS folder with their associated files. Look at one yourself. ---
**Author:** ![happyheathen](https://avatars.discourse-cdn.com/v4/letter/h/b5e925/32.png) [@happyheathen](https://boards.straightdope.com/u/happyheathen)\ **Post date:** [December 9, 2002, 1:11am UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/18 "2002-12-09T01:11:56Z")
I have equipment which will copy all known video formats. The CR does not like discussion of the details, so I won’t go there. I will say this: I seriously doubt that bit-copy will defeat the DVD encryption - I have an old bit-copy CD burner, and it can be beaten by modern CD’s. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [December 9, 2002, 1:31am UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/19 "2002-12-09T01:31:47Z")
**happyheathen:** Bit copy and encryption are different issues. A pirate can make a bit-copy of a DVD. The resulting copy is still encrypted, but that’s not a problem for the pirate. All DVD players contain the necessary decryption algorithms. The point of the OP was whether DVD drives will allow bit-copying, and the answer is that they won’t. The pirate has to hack the drive’s firmware first. ---
**Author:** ![Servo](https://avatars.discourse-cdn.com/v4/letter/s/49beb7/32.png) [@Servo](https://boards.straightdope.com/u/Servo)\ **Post date:** [December 9, 2002, 1:57am UTC](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300/20 "2002-12-09T01:57:23Z")
I don’t know if this has been answered or not, but the whole point of breaking the encryption was not to be able to copy the disc, but being able to _view_ it. If I remember the whole DeCSS thing correctly, someone wanted a DVD player for Linux but there were none available. So in order to write a program that could play back the video and audio stored on a DVD he needed to break the encryption. It wasn’t about copying, it was about viewing. [Next page](https://boards.straightdope.com/t/dvd-copying-why-not-bit-for-bit/141300.md?page=2)