How is so much information stored on such a thin layer of material?
When you press skip, how does the reader know how to go to the next track/chapter?
In: Engineering
They’re not paper. They’re plastic. Not sure where that confusion comes from.
The data is encoded as little pits in the plastic, either molded at the printer, or burned with a disk writer. The drive then reads the pits and… lack of pits as 0s and 1s, as computers do.
As for how it knows where to go, that’s because at the very “beginning” of the disk, a specific location the drive knows to look for, will be a bunch of information indexing the contents of the disk, telling the drive where to look for the rest of the data it needs to find.
From there it’s just normal computer stuff, opening and reading electronic files, and displaying you their contents.
A lot like a hard drive. Instead of having a metal disk that’s literally being written to, the DVD is laser scribed with the data
While a CD/DVD looks flat it’s got a ton of teeny-tiny little pits. A laser will bounce off the bumps in different ways depending on the pit. How the laser bounces off the pit reads as a 0 or a 1. From there you just have data encoded in various ways depending on the particular format. That data also encodes locations on the disc itself so it knows where a “skip” is to go to.
The data is stored in the plastic of the disk. The underside of the paper label is reflective like a mirror. The reader shines a laser through the plastic, which reflects off of the “mirror” and then goes into a light sensor. The plastic is printed/melted very carefully so that as the disk spins, the laser will either be reflected properly and enter the sensor, or the laser will be unfocused as it leaves the plastic and it will *not* go into the sensor. [Here is a diagram](https://volga.eng.yale.edu/sites/default/files/resize/images/CD-laserbounce-300×123.gif) which *not* accurate ([it’s more complicated than that](https://www.researchgate.net/profile/Charles-Eggleton/publication/51193522/figure/fig1/AS:214235722117126@1428089184484/a-Schematic-of-an-optical-pickup-displaying-the-laser-diode-the-tracking-coil-the.png)) but it’s enough to give you the idea.
Where the plastic is normal, it’s a “land” and the light is reflected back to the sensor. When the plastic is deformed it’s a “pit” and the light is not reflected. By arranging the pits and lands, you store data. [Here is a microscope image](https://hackaday.com/wp-content/uploads/2023/01/cd-microscope-featured.jpg) of a CD where you can see the pits (all the dots). The red line is one *track*. The tracks are like the grooves of a record – it’s the sequence of pits and lands that the reader is following to read that data.
Some people erroneously say that a land is a 1 and a pit is a 0, or vice versa. In actuality, it’s a bit more complicated. When it *changes* from a pit to a land or land to a pit, that means changing from either a 1 or 0 to 0 or 1. As in, if it is already a 1 and there’s a change, the data switches to a 0. If it’s already a 0, it switches to a 1. In this way, you create a series of 0s and 1s which is just binary data, which is how computers store data. Since the reader knows how fast the disk is spinning and it knows the position of the reader, it’s trivial math to figure out how quickly the data should be read, which is how it keeps track of how many 0s and 1s there should be.
The beginning of the disk has some simple data that the device remembers, which includes metadata like the music track length in minutes, track names, etc. It also has position data for where on the CD that track is physically located. When you skip forwards or backwards, the reader arm physically moves to that location on the disk to read that data and play the music. Or if it’s a DVD, it’s the chapter location and it plays video and audio.
To fast forward or rewind, the device has a small amount of memory. It’s not much, but it can store a few seconds, maybe a few minutes of data. It continuously updates the data in memory to store a little bit ahead and a little bit behind where you’re listening/watching, and when you fast forward or rewind it pulls from that memory.
DVDs improved capacity over CDs by having smaller pits and lands which a more accurate reader, along with a smaller wavelength of light from the laser. Light has its own size and if the light is bigger than the pits/lands, it can’t read them correctly. Smaller wavelengths require higher energy and it took time to develop LEDs capable of creating smaller wavelengths and then to make those LEDs cheaply enough to be commercially available. CDs use infrared lasers, while DVDs use a higher energy visible red light. Smaller wavelength means smaller pits/lands, which means you can fit more data tracks in the same area, so you store more data on the disk.
HD-DVDs improved storage further by having multiple layers of pits and lands. The reader has an additional mechanism that focuses the beam at a very specific depth, so it only “looks at” one layer at a time. In this way, multiple data tracks can be stacked on top of each other (IIRC, a max of four layers). Bluray improved storage even more by using much more expensive (at the time) blue light LEDs which a much smaller wavelength than even the visible red light of DVDs. The smaller wavelength blue light could fit even more data tracks in addition to having several layers.
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