Where is a song on a CD located and how does the reader know where each track is. How does it know where to skip to. Why do “machined” CDs look different from self burnt CDs?
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The innermost part of the cd is the TOC (table of contents), that’s where all the track info are stored. The reader knows the exact position of each track by reading the TOC when you load a disc.
Machined CDs look different than CD-R because pits and lands are *actually* pits and lands engrooved on the CD plastic layer, whereas in CD-R this is simulated via a special resin coating that changes its reflectance when burnt by the laser at certain temperatures.
A cd is a precision mirror that is spun at a constant speed around its center. This allows for a laser that focuses on the surface to very accurately bounce it’s ray of the mirror surface along a circle. The circle is called a track, and the tracks can be reached by moving the laser closer or farther from the center. The light from the reading will bounce off the mirror into a sensor that detects light.
Any slight deviation of the mirror surface will bounce the light away from the sensor. Music is then stored on the cd by creating dents in the mirror to represent sound.
Audio is recorded digitally by a computer via a microphone. Sound travels through the air as a series of pressure waves. The pressure in the air moves the thin membrane of the microphone, which in turn moves metal relative to a coil of copper wire. This will in turn move the free electrons in the wire to form a slight current that is analogous to the pressure. High pressure means a high positive current, low pressure means high negative current and no pressure (relative to atmosphere) means no current.
The electric current is then “sampled” into digital data. A clock will trigger one sample to be collected 44100 times per second. Sampling means that the current is converted to a 16bit number. 16 switches with increasing sensitivity will “trigger” from the current, effectively capturing the pressure level as an integer number in the range -32768 to +32767.
When burning a CD another stronger “burning” laser is used. Each sample will result in the laser etching a hole in the mirror 2 x 16 x 44100 times per second of music(2for stereo, 16 for the bits per sample, 44100 for the number of samples per second). The burning cd media looks different because the mirror has coatings that allow for it to be effectively etched by a laser.
Store bought CDs are mass produced. A master CD is used as a template and used to “stamp” the holes in the mirror all at once (similar to a vinyl).
Information about where tracks are stored on a cd are recorded as numbers before the audio in the outermost track. There exist different standards for how this data is stored on a CD. These are referred to as “rainbow books”. A classic audio only cd is “orange book”. The data cd is called ISO-9660 or “teal book”.
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