Laserdisc

LaserDisc (LD, or LaserVision) was the DVD format's immediate predecessor, introduced to test markets in 1978 and its final titles were released in 2000. The LaserDisc system stores high quality digital and analog audio and analog video on a 12-inch (or 8-inch, used mostly for music videos) optical disc. Physically similar to an oversized CD, virtually every LaserDisc player manufacturered since 1986 will also play conventional CDs. One 12-inch LaserDisc is capable of storing approximately sixty minutes of video material on each side of a CLV (Constant Linear Velocity) disc, or 30 minutes on each side of a CAV (Constant Angular Velocity) disc. CAV discs could be played with freeze-frame and slow-motion, while CLV discs could not- pausing would result in a blank screen, but later players included digital capabilities allowing freeze-frame and frame advance on these discs. Because of these playback limitations, movies over two hours in length are usually recorded on two or more discs. Many players include an automatic side change function, which allows up to two hours of continuous viewing (albeit with a slight pause of up to fifteen seconds as the side is changed).
The first laserdiscs had only analog audio, though it was superior to that on any home videotape format at the time.  In 1981, CX noise reduction was added- this was a noise reduction system developed by CBS intended for use on records, but only a few albums were released with CX.  Its main use was on laserdiscs and the shorter-lived CED videodisc format.  PCM digital sound, equal to audio CD, was added in 1986 along with the capability of players to play audio CDs.  The first laserdisc released with a digital soundtrack was "Wings", but not all releases included digital sound until about 1990.  All discs with digital sound also included standard analog tracks for compatibility with older players, but these tracks were often used for supplementary audio such as commentaries rendering them useless on older players without digital sound.  Both Dolby Digital and DTS were first used on LaserDisc.  Dolby Digital AC-3 was first used on "Clear and Present Danger", and its signal took up the space occupied by the right analog channel.  This left the 2-channel PCM digital tracks (which were uncompressed; a criticism of AC3 was that it was compressed even though it added the capability of 6 discrete channels), and a mono analog track- if played on an older player without AC3 capability, noise would be heard in the right channel. Dolby Digital LaserDiscs require a LaserDisc player with an 'AC-3 RF Digital Out' and a receiver with an AC-3 RF Digital In or an external RF Demodulator to convert the RF signal into a standard digital output. DTS discs may be played on any LaserDisc player with an S/PDIF digital output (TosLink or coaxial) and compatible receiver that can decode DTS.  DTS soundtracks took the place of the PCM digital soundtrack and worked the same way as DTS audio CDs.  The 2-channel analog track remained for use on non-DTS systems, although a few titles also included supplementary audio so that the main audio could not be heard without DTS.