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Lets assume I have a CD/DVD burner, and I'd like to burn the disc in the "wrong" direction, i. e., I want the DVD drive motor to turn the other way around.

Can this be done (with a normal drive) by reversing the polarity of the motor driving the disc? Or would that break the motor / the mechanics of a standard DVD drive because these are not designed to turn the other wa around?

If I can get the drive to turn the other way, can I burn a disc? I've read that blank discs have something like a "blank" data track which contains information about the disc? Would that even work when the disc rotates in the "wrong" direction? Can I get a standard DVD drive to ignore that track and just force write even if it doesn't find that track?

And finally, if I actually managed to burn a disc that way, could that disc even be read in that drive? Or in other drives modified in the same way?

Florian Bach
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  • The motor is brushless, so you can change the way that the stator electromagnets are driven to change the direction. – jonk May 24 '17 at 17:17
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    You might have to write custom drivers to get the computer to actually write to the disk. – Hearth May 24 '17 at 17:18
  • I believe the spiral is preformed on the disc. If I'm right about that, the drive is going to get very confused and won't find any tracks. – Oskar Skog May 24 '17 at 17:19
  • @Felthry: I think the firmware on the drive itself may need changes too. I doubt there is a SATA command for read/write backwards. – Oskar Skog May 24 '17 at 17:20
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    @OskarSkog I don't think there is a spiral, at all. It's just rings and stepping. (I worked on the optics and polymer design of the first writable CD in the 1980's.) – jonk May 24 '17 at 17:20
  • @jonk: So they needlessly pretend it's a spiral instead of allowing the disc to be accessed by sectors and tracks? – Oskar Skog May 24 '17 at 17:22
  • @OskarSkog I can't speak to the mindset of other people. – jonk May 24 '17 at 17:25
  • @OskarSkog Good point re: firmware. But if you're modifying the motor, you might be able to do something to the mechanism that moves the laser head too. Also, I could swear I heard about some video game console that spun its discs backwards for copy-protection.... – Hearth May 24 '17 at 17:25
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    @jonk, no, you probably cannot simply change the stator wires, as CD/DVD brushless motors use hall sensors. You'd need to verify that the placement of those is symmetric with respect to the coils, and swap those as well. – Chris Stratton May 24 '17 at 18:13
  • @ChrisStratton I didn't say "simply change the stator wires." But I take your point. I do think it is achievable given that it's a brushless motor and given the ones I've seen after tearing them apart, myself. The rest is just "details." This question is rather abstract anyway. So I felt that anwering "yes it is possible" was sufficient enough. You make a good point, regardless. – jonk May 24 '17 at 18:21
  • Haha, nice thought, like this kind of brain wave. Do you want to create your own readable discs? Maybe it is an idea to reverse all data on de disk. Create an iso, revert the data of the files (not of the whole disk) and write it to a disc. Another solution is to create reversed discs somehow, you only have to change the direction of the spindle motor. There is some data on every disc, also on empty ones, this must be readable. Lens position will not change when in opposite spindle direction, it still moves from the lens from the inside to the outside. – Codebeat May 24 '17 at 18:58
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    @jonk DVDs are written as a spiral continuous track: https://en.wikipedia.org/wiki/DVD and https://en.wikipedia.org/wiki/Opposite_Track_Path – Jack Creasey May 24 '17 at 19:08
  • I have no idea. But it would be fun to know if you figure it out. – Entrepreneur May 24 '17 at 19:11
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    @JackCreasey Thanks for the correction!! My work was focused on the optics (quad detector and required astigmatism for focus), and on the polymer chemistry (and dyes.) I appreciate knowing better and your time offered in correcting my incorrect impressions. My earlier comments weren't helpful and hope Oskar will accept my apologies. (I will remove those earlier comments if it's felt they aren't helpful.) – jonk May 24 '17 at 19:47
  • @Felthry Video game consoles with disks that "spin backwards" is a recurrent myth. I've heard that claim for Dreamcast, Game Cube, Wii… it's false in every case. –  May 24 '17 at 19:48
  • @duskwuff That must be why I couldn't remember which one it was. I'd heard it for the gamecube and dreamcast, but I knew it was false in both of those cases (really, it's quite easy to just open the disc cover and see which way it's spinning...) – Hearth May 24 '17 at 20:10

1 Answers1

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TL;DR: Just reversing the direction of the motor does not help - the disc dictates the direction.

If one manages to let the motor run backwards, writing a disc might be possible on the first view.

Reading could be a bit more difficult: The optical receiver actually consists of four sensors: two perpendicular to the track which are used to keep the optical system aligned with the track, and two parallel to the track to read the data. By comparing the signal of this two, one can detect the transition from pits to lands and v.v.
I guess that the electronics / firmware would be confused when the "rear" sensor sees a change before the "front" sensor. (In reality, reading out the signal is a bit more complicated due to interference effects)

But even if we ignore this, there's still the CD/DVD itself. There is indeed a single, spiral-shaped track over the entire disk, and this makes perfectly sense: Both media were developed for music / video, which are use cases where continuous reading is much more important than seeking. Jump to the next title? Just place the head roughly there, a few seconds until the music starts does not matter. And following an endless track is much easier than jumping to the next circle after each revolution.

If CD-Rs were completely blank, one might be able to burn them spinning backwards, but they are not blank. They have a so-called pre-groove to guide the head when writing, which also contains the ATIP (DVDs: ADIP). This are some information in the very beginning of the track about the medium like type, capacity and powersetting, as well as timing information over the entire disk. Think about: The rainbow colors come from the track, which forms an optical lattice. But they are also visible on an empty medium. This means, though it's empty, there already must be some structure on it.

Since the orientation of the spiral-shaped track on the medium is predefined, the only way to read/write a backwards running disc would be from outside to inside. And this means heavy modifications to the firmware.

However, dual-layer DVDs are written from inside to outside on the first layer, and from outside to inside on the second. The reason is, at the end of the track of the first layer, it would cost many seconds for the head to move inside, and the disc to speed up to the velocity needed there. With this trick, the velocity stays the same, and the optics just has to focus to the second layer and catch the track there. One sometimes notices this as short glitch in the middle of a movie.

sweber
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