How Does My CD Player Understand Where to Pause?

CD still spins. But the laser doesn’t move. They are two separate things.

edit :: The laser is what reads the CD.

@johnpowell I understand that, but when the song stops, it is obvious that the bumps and pits in the CD that represent the certain part of the song is still going around. How does the laser remember the exact spot?

One line on the CD is like a fraction of a second. It isn’t precise, but it is so close you won’t notice.

The music on a CD is digitized as PCM data, and that data is organized into tracks, sectors, and frames. Info here. If you know anything about how disk drives for computers work, it’s pretty much the same thing. When you put the player on pause, the player’s microcontroller remembers the track, sector, and frame you were on when you hit the pause button. When you hit play again, the controller synchronizes the laser to the track, sector, and frame address in its memory and resumes play. It’s fast because the disk is still spinning and the laser arm is still in the same lateral position, so there is little latency due to the physical medium. @johnpowell, the error involved is actually no more than 1/75th of a second, which is the length of an audio frame in real time.

If the stereo in your car has a CD player, you’ve probably noticed that, when you turn it on, it will return to the same point in the program you were listening to when you last turned the radio off. It uses the same mechanism. The only difference between that and pause is, the disc has to spin up, and the laser has to move transversely to the correct physical location on the disc, so it takes a little longer to sync.

Unlike a vinyl record, a CD is like a computer disk containing a collection of digital files. The CD player knows what file is currently loaded and where it is in that file when you pause the song.