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Particle's Laserdisc Resource
Laserdisc, more properly capitalized as "LaserDisc" and frequently abbreviated as "LD", is a pretty awesome video format. Launched as a commercial platform in 1978, its optical storage technique was ahead of its time. The discs themselves look like Godzilla-sized compact discs, but the video is encoded as a composite analog signal instead of as digital data. The laser pickup enables the recovery of an FM carrier from the variations in reflectivity caused by running it over a track of pits and lands, and this carrier ultimately contains the video signal. This is an interesting deviation from the read technique used by later digital disc formats which directly treat the pits and lands as distinct, binary bits.

Laserdisc is essentially a specialized form of broadcast-on-a-disc, sort of like how cable is a specialized form of broadcast-in-a-wire. The effect is largely the same; there are no concerns about external interference of recpetion caused by obstructions in the path of a physical broadcast nor multiple devices reducing the signal strength like in cable systems. Instead, a laserdisc player is able to recover a very clean analog video signal which provides exceptional clarity when compared to the other video storage systems of the time.

Interestingly, laserdisc was the first of three disc-based home video formats that all launched within five years of one another. The first competitor was Capacitance Electronic Disc (CED) which launched commercially in 1981 after nearly two decades of sitting in developmental limbo. The second competitor was Video High Density (VHD) which launched commercially in 1983. All three formats had similar goals and capabilities even though the technology behind each was somewhat different.

Whereas laserdisc is an optical format, CED and VHD are both variable capacitance formats which require physical contact to read. CED is a 30 cm diameter format like laserdisc and has physical grooves even though they're only 657 nanometers wide. These grooves are so narrow that they're only about as wide as the wavelength of crimson red light. The discs are read at 450 RPM (NTSC) or 375 RPM (PAL). Each revolution yields eight or six fields (four or three frames) respectively. VHD by contrast is a 25 cm diameter format and has no physical grooves. Tracking is instead performed electronically through what I can only assume is some form of voodoo magic. The result is less physical wear and enhanced performance with media that is worn or damaged since physical grooves aren't required to keep the read head on track. The discs are read at 900 RPM (NTSC) or 750 RPM (PAL) with each revolution yielding four fields (two frames) regardless of the video standard.

By contrast, laserdisc features a 30 cm disc spinning at 1800 RPM (NTSC) which yields two fields (one frame) in constant angular velocity (CAV) mode, resulting in 30 minutes of playtime. Constant linear velocity (CLV) mode proved to be more popular due to its increased capacity of 60 minutes, however, and varies from 1800 RPM down to 600 RPM depending on where on the disc the laser read head is. CAV mode discs spin at a constant rotation rate where the pit density varies to produce the same amount of data per track. CLV mode spins at a variable rotation rate with the pit density remaining constant. The effect is readily apparent--CLV mode encodes twice as much video data per side.

  LD CED VHD
Launched 1978 1981 1983
Markets North America
Japan
North America
Japan
Japan (Only)
Diameter 30 cm 30 cm 25 cm
RPM (NTSC) 1800 RPM 450 RPM 900 RPM
RPM (PAL) 1500 RPM 375 RPM 750 RPM
Modes CAV, CLV, CAA CAV CAV
Track Density (NTSC, CAV) 1 frame 4 frames 2 frames
Runtime/Side 30m NTSC CAV
36m PAL CAV
60m NTSC CLV
64m PAL CLV
63m NTSC CAV
75m PAL CAV
60m NTSC CAV
Resolution 425 lines 240 lines 250 lines

One non-apparent side effect of CLV is slightly decreased video quality due to crosstalk between tracks. This is presumably caused by tracking imperfections. While interference from surrounding tracks on a CAV disc would result in a pixel that is subtly influenced by a very similar pixel in that same position from the frame before or after, an adjacent track on a CLV disc results in a pixel that is influenced by a completely different pixel in the image from a surrounding frame. As a result, laserdisc videophiles often prefer to purchase the CAV editions of their favorite titles when available. Some of the better-produced mainstream movie releases consist of two discs.  The first disc contains one or two CLV sides (60~120 minutes) while the second disc contains either one CAV side (up to 30 more minutes) or one CLV side and one CAV side if more runtime is required.  The climax of the movie in such cases gets the benefit of the increased picture quality that CAV provides.  Releases made early in the life cycle of laserdisc were often CAV, but CLV became dominant for nearly all releases fairly quickly. With fewer discs, CLV titles were cheaper to produce and distribute as well as being less obnoxious for the end-user due to fewer disc flips and disc swaps during the playback of a title. The initial cost of mastering was higher for CLV than CAV, however.

It's worth noting that the CLV encoding scheme mostly fell out of favor by the late 1980s. In its place, a third encoding scheme called constant angular acceleration (CAA) was employed by most laserdisc manufacturers. CAA is a sort of hybrid between CAV and CLV with pit density changing in steps every so many tracks across the disc instead of continuously. This had the effect of reducing playback artifacts that were introduced by tracking imperfections and timing skew such as herringbone patterns.  In practice, CAA was never really marketed as a new encoding scheme and as such CLV continued to be used to describe it long after true CLV had mostly disappeared.

Related Links:
The Laserdisc FAQ
VHD DiscWorld
CED Magic

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