What does a CD do?

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Like gramophone records, the information on optical discs is recorded on a spiral track. However, with a CD the laser starts reading the disc from the inside ring (table of contents) and ends up on the outside. When play back starts, a laser beam shines on the ridges and lands on the data membrane layer. If you look at the image on the right you can see the data layer moving in gray.

During playback, the number of revolutions of the disc decreases from 500 to 200 rpm (revolutions per minute) to maintain a constant scanning speed. The disc data is converted into electrical pulses (the bit stream) by reflections of the laser beam from a photoelectric cell.

When the laser beam strikes "land", the beam is reflected onto a photoelectric cell. When it strikes a "ridge", the photocell will receive only a weak reflection. Thus the photoelectrical cell receives series of light pulses corresponding to the ridges and lands in the disc. These light pulses are the foundation of binary 'digital' data. A simple substitution for the weak signal "0" and the in-focus signal "1" results in a pure digital playback without alteration, every time, without failure or degradation.
In music playback, a D/A-Converter (digital to analogue converter; DAC) converts the series of pulses (binary coding) from a decimal code to a waveform, which can then be processed for amplification. The longer the decimal code, the better the sound. Current standard CD audio is 44,100 pulses per second and 16 bit (decimal places) in digital Word length. Thus a 24 bit system sounds all that much better, in fact DVD audio is set to allow 24 bit AND pulse at 97,000 times per second!