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From http://www.wordiq.com/definition/Laserdisc

Technical information
Video was stored on LD as an analog signal, while audio could be stored in either analog or digital format and in a variety of surround sound formats. Like on a CD, the surface of the disc is an aluminium foil covered by pits and lands, but whereas on a audio CD (or DVD) the pits and lands will signify binary codes, on an LD the pits are created using frequency modulation of an analog signal.

NTSC discs could carry two analog audio tracks, plus two uncompressed PCM digital audio tracks, which were generally CD quality. PAL discs could carry one pair, either analog or digital. Dolby Digital (also called AC-3) and DTS, which are now common on DVD titles, first became available on Laserdisc. Star Wars: Episode I (1999) was released on Laserdisc in Japan, and holds the distinguishing record of being both the first home video release to include 6.1 channel Dolby Digital EX Surround, and the only Laserdisc to ever use the EX codec. Many later discs have no analog audio track, instead offering the choice of a CD-quality PCM audio track (carried directly via the player's optical output) or Dolby Digital. Players could generally convert PCM tracks to analog using an internal digital to analog converter. DTS equipped discs would still be equipped with an analog channel, as DTS uses the space the PCM tracks would normally occupy.