How do CD & DVD players work?

Factual Questions
1

Some questions:

  1. Is there a FAT-type directory of where the scenes are? If not, how does it find specific scenes?

  2. At what RPM does the disk rotate?

  3. Is the data burnt in in concentric circles, or a spiral?

  4. Is there any known lifespan for CD or DVD disks?

  5. Do the disk cleaning/rejuvinating kits really help?

  6. How far from the disk is the lens (much further, I expect than the head of a computer disk drive)? Difficult to peer into that little slot, unlike a VCR.

2

Here’s an article on How Stuff Works about DVDs. It might not be in depth enough, but I’m sure it’ll answer at least some of your questions. They’ve got some other good stuff on that site. You should check it out.

3
  1. Yes, there are information files (*.IFO) on the DVD that help the player navigate. You can use a program like IfoEdit to examine them.

The video “tracks” on a DVD are divided into a hierarchy, like this: Title > PGC (program chain) > program > PTT (part of title) > cell. Each title is like a track, each PTT is a chapter, and the cells are the small blocks where the video is actually kept. When you select a chapter, whether it’s by hitting the skip button, using your player’s chapter search feature, or selecting a scene from a menu, your player looks up the appropriate PTT in the IFO file to find out which cell to start playing next.

  1. A 1X CD spins at around 500 RPM, and a 1X DVD spins at around 1400 RPM. Note that the speed can vary a lot: faster drives will obviously spin the disc faster, and drives in CLV mode (constant linear velocity) will spin faster when the laser is near the center and slower when it’s near the edge. This delivers data at a constant rate. CAV mode (constant angular velocity) spins the disc at the same speed no matter where the laser is, so data will be delivered faster at the edge than at the center.

The “X” rating on a drive is often just a theoretical maximum, and different writing strategies will cause burners to move faster or slower as they complete the disc. For example, Z-CLV mode divides the disc into zones of progressively higher speeds, so the disc starts out spinning quickly, slows down as it nears the end of the zone, then speeds up again at the start of the next zone.

  1. A spiral. For CDs, the spiral always starts at the center and moves to the outside. DVDs can have two data layers, and while the outer (first) layer always spirals from the center to the edge, the second layer can go either way on pressed discs.

For PTP DVDs (parallel track path), both layers start at the center and spiral outward.

For OTP DVDs (opposite track path), the second data layer starts at the point where the first layer ends–near the edge of the disc–and spirals inward. This reduces the pause between layers when you’re watching a movie, because your DVD player doesn’t have to move the laser back to the center of the disc to start reading the inner layer. However, it means the second layer can’t contain more data than the first. All recordable dual-layer discs use OTP.

  1. Yes, if your disc is scratched or dirty.
4

The linked article gives a good overview of DVDs, and at its end there is a link to the DVD FAQ, but to answer the OP’s first question, thereis no FAT-like directory of scenes on a DVD.

DVD discs use a filesystem, to be true, but the files of a DVD disc do not correspond directly to movie scenes. When a DVD is created, video and audio files are combined (‘multiplexed’) with menus and subtitles and other information and are spit out into a completely-new set of files.

5

If you really want a good geeky understanding of audio CD formats, check out this guy’s lecture notes

It’s fairly thorough, going into details such as “EFM encoding” and the forty zillion layers of interleaving, parity bits, subchannels, and the like.

It’s pretty impressive that they were able to create consumer electronics that did all of that fancy stuff at the beginning of the 80s.

6

Thanks, guys, for the fascinating info and the interesting links.