I’m just wondering physically how is it possible? Why is it that if I wanna pick one song from the CD the player ‘knows’ where the song I want is?
In: Technology
The backside of the label is made of a reflective coating, so that when light is shone through the plastic of the disc it is reflected back out. A laser shines upwards through the disc.
In the plastic of the disc there are microscopic dimples or *pits* that change how the laser light is reflected. Specifically, they cause the waves of light to overlap in a way that causes destructive interference, which means no light reaches the detector next to the laser emitter.
So either there’s a pit, or a land. Light enters the detector, or it doesn’t. The 1s and 0s of binary are encoded in the pits and lands. It is *not* that a pit or land is a 1 and the other is 0. Instead, a 1 is when the laser detects a *change* from land to pit or vice versa.
The laser follows tracks, like the grooves in a record. A chip precisely controls the position of the laser, so the player knows exactly where on the disc the reader is. That means it also knows precisely how fast the disc is turning. It uses that to calculate the clock rate, or how fast 1s and 0s should be happening.
DVDs use a more precisely controlled reader and more sensitive reader so the pits can be smaller and closer together, which means more data can be packed onto the disc. The format of the data is also different.
Blue ray players use a blue laser, which has a smaller wavelength. That means it can read smaller pits, so they can be even smaller than in DVDs and pack in more data. Blue ray players and HD-DVDs both have multiple layers, too. The laser has a lens to change the focus to ignore or pick up different layers.
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