Cylindrical Lens for Positioning

A clever use of a cylindrical lens generates a correction signal to position the main focusing lens for the detector in a compact disc player. The combination of a symmetric lens and a cylindrical lens produces a circular beam at only one distance past the cylindrical lens. A segmented photodiode arrangement can detect whether the beam is circular and generate an error voltage to reposition the main lens so that it is. The error voltage drives a coil which can rapidly reposition the lens in response to changes in distance to the CD as it rotates.

Index

CD concepts

Sound reproduction concepts

Reference
Rossing
Physics Teacher, Dec. 87
  HyperPhysics***** Sound ***** Optics Go Back





Cylindrical lensPhotodiodeDifference amplifier
 Index

CD concepts

Sound reproduction concepts

Reference
Rossing
Physics Teacher, Dec. 87
  HyperPhysics***** Sound ***** Optics Go Back





CD Storage Capacity

A compact disc can store more than 6 billion bits of binary data. This is equivalent to 782 megabytes, and at 2000 characters per page this is equivalent to about 275,000 pages of text (Rossing). Because the analog-to-digital conversion for making CD's samples sound waveforms at 44.1 kHz, the amount of data involved in the recording of high-fidelity sound is very large. The 12 cm diameter compact disc can hold 74 minutes of digital audio which has a frequencies over the full audible range of 20-20,000 Hz. The signal-to-noise ratio and the dynamic range can exceed 90 decibels.

Index

CD concepts

Sound reproduction concepts

Reference
Rossing
Physics Teacher, Dec. 87
  HyperPhysics***** Sound ***** Optics Go Back