Grades 6–8 · Computing
How does a CD recover music from tiny marks?
A pressed audio CD has a long spiral track of tiny pits and flat regions beneath a reflective layer. The track stores an encoded digital description of sound. It is not a tiny sound wave waiting to shake a needle.
In the diagramAn optical measurement becomes an audio signalenlarged pit-and-land tracklaserdetectordecode + correctsamples → audioencoded symbolsEdges and timing matter; one pit is not one sound sample.
A player focuses laser light through the clear plastic toward the track. Light reflected from the pit-and-land structure combines differently as the track passes the spot. A detector measures changes in the returning light, while control systems keep the spot focused and following the track.
Electronics recover timing and encoded symbols from those changes. Pit edges and the intervals between them matter: reading is more involved than treating every pit as a one and every flat as a zero. Extra information helps correct some reading errors.
The recovered numbers describe successive sound samples. A digital-to-analog converter and filtering turn the sequence into an electrical audio signal, which an amplifier can send to speakers. A scratch matters because it can interfere with reading; it does not physically remove a strip of already audible music.
About this explanation
Starting knowledge: Understand reflected light, digital records and the difference between stored data and sound.
Sources
- Georgia State University, HyperPhysics — Audio Compact Disc Player: Laser Tracking and Detection