John Griffiths, of Nelson, asks :-

How does a DVD keep on the track on the disc? It isn't like an old record where the needle follows a grove?

Andy Edgar, a physicist at Victoria University of Wellington, responded.

In a DVD (“Digital Video or Versatile Disc”), the music or data is recorded in a spiral track, though of a different nature to that in a gramophone record. The read-out is electro-optical rather than electro-mechanical.

A single track contains any mixture of digitally encoded music, video or data - there is only one kind of track. The tracks have a sequence of “pits”, flat-bottomed shallow depressions, which are produced from a master disc by mechanical pressing. The transparent polycarbonate disc is coated with highly reflecting aluminium on the pressed side. A light beam from a solid-state laser is focused onto the tracks. When viewed from the laser side, the pits are plateaus. The reflected light from the disc surface is focused onto a set of four tiny light detectors arranged in a square.

How does the laser stay on the track? The two detectors which lie along a disc radius are used for tracking correction. When tracking is perfect, the light spot on the detectors is an image of that on the disc and so evenly straddles all of them, and the two tracking detectors in particular. But if the laser is off to one side of a track, and light is scattered away by a plateau edge, then one of the tracking detectors has a bigger signal than the other, with the relative strength depending on whether the spot on the track is towards the centre or outside of the disc with respect to the track. By taking the difference of these two signals, a tracking correction signal can be generated which is then applied to tracking correction motors. A positive difference means move the laser in to the centre, a negative difference move out, no difference means on track.