For recording, first a master disc is prepared. The laser beam acts as a stylus, which is modulated by digital audio signal. For this the audio signal to be recorded is obtained from the microphone, amplified and sampled @ 44.1 KHz with digital quantization of 16 bits.
Thus 7 mega bits (44.1 x 1000 x 16) per channel per second are used to modulate the laser. This modulated laser is made incident on the CD. When laser beam is ON, the output is 1 and when the beam is OFF, the output is 0. It forms pits on the photo resist material of the CD.
For replay a disc, a focused laser beam of suitable wave length, is made incident on the CD tracks through a mirror, A half polished mirror is used, as it allows the laser to pass through it, but does not allow it to return. The reflection of laser beam from pits represents 0 and from space represents 1. This reflected beam is again passed through a focusing system and then through a photo diode is fed to a digital to analog converter. The analog signal so obtained is fed to a loudspeaker for reproduction.