MFDigital CD DVD Information Library
Thursday, June 10, 2004
Compact Disc Introduction
The Compact Disc was originally developed from the larger Laser disc for music recordings and uses a similar 'pit' structure. The main difference (apart from the size of disc) is that the CD uses a digital technique where the pits indicate whether a data bit is '0' or '1'.
Whereas Laserdiscs can be either CAV (Constant Angular Velocity) or CLV (Constant Linear Velocity), all CDs use CLV. This means that the pit sizes do not vary from inside to outside of the disc but the angular velocity does vary.
The read head is a small laser diode emitting infra red 'light' which is focused to a beam small enough to read the sub-micron pits. The laser diode is mounted on a swivel arm which can be moved radially to follow the pits up and down to keep them in focus.
A semi-reflective mirror allows the reflected light to pass back to a photo detector. When the laser beam falls on a pit very little is reflected. The changing light pattern detected is then converted into a series of zeros and ones which are then decoded into the original audio or computer data signal.