I was exhausted after learning quite a few years ago what goes on inside of a CD player. Dvd and Blu-ray are just higher density and higher speed CDs.
Sticking with the Blu-ray, the tracks are 12 millionths of an inch apart. Do you think that hole in the middle is within 12 millionths of being in the exact center of the tracks? How flat is the disk? Nowhere near within millionths of an inch.
Therefore, the laser assembly has to follow a track “on the fly”. It can tell when it’s on the center of a track by the nature of the signal being read. It can tell, as it flys over the track, whether to move a few millionths of an inch side to side, and also up and down, to stay centered on the track and at the proper elevation above it (the laser doesn’t touch the disk).
Then there’s the matter of the rotational speed of the disk. You can be sure that a fixed-speed motor stay at exactly the correct speed so the data comes off at the rate needed play a video or audio, so the data coming off the disk is fed into a buffer. The video decoding part of the player pulls the data out of the buffer at a precisely controlled speed. If the buffer moves toward empty because it’s not being filled fast enough, the spindle motor is speeded up to increase the buffer-fill rate, and the motor is slowed if the buffer is being filled too fast.
How does that little laser jump back and forth and up and down so fast and cheaply and precisely? It is mounted in rubber with little magnets to pull the laser as needed to keep it on the center of the track.
In addition to the video or audio data the tracks contain there’s also data telling which track is being read (e.g. track 10,472).
Portable CD players can be bounced around with no interruption to the music because the player can tell if it was jostled to another track, and it can bring itself back to the proper track. Several tracks worth of data are buffered (this is another buffer) so that while the laser is being repositioned on the correct track, the video or music keeps coming with no interruption.
Then there’s the matter of multiple layers. Each layer except the bottom one is semi-transparent. The laser, which has a little lens like a microscope, can focus on one layer with the others sufficiently out of focus so they don’t interfere with the data coming off of the desired layer. The tracks also have data telling which layer is being read.
And these gadgets can be bought for a few tens of dollars! They are both high precision and not high precision. They rely on a lot of software and cheap mechanical parts and cheap compute power.
The way video data is recorded and decoded is another major drama that I’ll not go into now but will if a lot of readers just must know.
Interesting. Continue.