Question

Describe the working of pick-up assembly of CD player with the help of neat sketch.

Answer 1

Diagram:

Explanation: The pick-up assemble consist of – * A low power laser diode to illuminate the CD tracks. * Lens and prism arrangement to direct the laser beam to the CD surface and to direct the reflected laser beam towards photodiode array. * A photodiode array to obtain data, focus and tracking signal from the reflected laser beam. * Focus and tracking coils to focus the beam to the CD surface and to move the assembly to proper track across the disc surface. * Some optical units do not contain the tracking coil, for example, the single-beam radial tracking assembly, this is explained in latter sections. * Optical arrangement in a single-beam radial tracking pick-up assembly : * In the optical pickup unit, the laser diode emits laser beam from a small point into an elliptical or conical distribution. This beam is passed through various prism and lens to form a very small diameter light beam on the disc surface at the ce77nter of the track. * The objective lens is controlled by the tracking and focusing coil to keep the beam focused on the CD and to keep the condensed beam at the center of the track. * This laser beam is reflected back by the flat area and the pits on the disc surface. This reflected beam is applied to a group of photodiodes through objectives lens, collamination lens and some prism arrangement.