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?
Best Answer
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.