Laserdisc - Design

Design

The standard home video Laserdisc was 11.81 in (30 cm) in diameter and made up of two single-sided aluminum discs layered in plastic. Although appearing similar to compact discs or DVDs, Laserdiscs used analog video stored in the composite domain with analog sound and/or some form of digital audio. However, despite its analog nature, the Laserdisc at its most fundamental level was still recorded as a series of pits and lands much like CDs, DVDs, and even Blu-rays are today. The spiral track of a Laserdisc is 42 miles (67 km) long. Early Laserdiscs featured in 1978 were entirely analog but the format evolved to incorporate digital stereo sound in CD format (sometimes with a TOSlink or coax output to feed an external DAC), and later multi-channel formats such as Dolby Digital and DTS.

Since digital encoding and compression schemes were either unavailable or impractical in 1978, three encoding formats based on the rotation speed were used:

CAV

Constant angular velocity or Standard Play discs supported several unique features such as freeze frame, variable slow motion and reverse. CAV discs were spun at a constant rotational speed (1800 rpm for 525 line and 1500 rpm for 625 line discs) during playback, with one video frame read per revolution. In this mode, 54,000 individual frames (30 minutes of audio/video) could be stored on a single side of a CAV disc. Another unique attribute to CAV was to reduce the visibility of crosstalk from adjacent tracks, since on CAV discs any crosstalk at a specific point in a frame is simply from the same point in the next or previous frame. CAV was used less frequently than CLV, and reserved for special editions of feature films to highlight bonus material and special effects. One of the most intriguing advantages of this format was the ability to reference every frame of a film directly by number, a feature of particular interest to film buffs, students and others intrigued by the study of errors in staging, continuity and so on.

CLV

Constant linear velocity or Extended Play discs do not have the "trick play" features of CAV, offering only simple playback on all but the high-end Laserdisc players incorporating a digital frame store. These high-end Laserdisc players could add features not normally available to CLV discs such as variable forward and reverse, and a VCR-like "pause". By gradually slowing down their rotational speed (1,800–600 rpm) CLV encoded discs could store 60 minutes of audio/video per side, or two hours per disc. For films with a run–time less than 120 minutes, this meant they could fit on one disc, lowering the cost of the title and eliminating the distracting exercise of "getting up to change the disc", at least for those who owned a dual-sided player. The vast majority of titles were only available in CLV (a few titles were released partly CLV, partly CAV. For example, a 140-minute movie could fit on two CLV sides and one CAV side, thus allowing for the CAV-only features during the climax of the film).

CAA

In the early 1980s, due to problems with crosstalk distortion on CLV extended play Laserdiscs, Pioneer Video introduced constant angular acceleration (CAA) formatting for extended play discs. Constant Angular Acceleration is very similar to Constant Linear Velocity, save for the fact that CAA varies the angular rotation of the disc in controlled steps instead of gradually slowing down in a steady linear pace as a CLV disc is read. With the exception of 3M/Imation, all Laserdisc manufacturers adopted the CAA encoding scheme, although the term was rarely (if ever) used on any consumer packaging. CAA encoding noticeably improved picture quality and greatly reduced crosstalk and other tracking problems.

As Pioneer introduced Digital Audio to Laserdisc in 1985, they further refined the CAA format. CAA55 was introduced in 1985 with a total playback capacity per side of 55 minutes 5 seconds, reducing the video capacity to resolve bandwidth issues with the inclusion of Digital Audio. Several titles released between 1985 and 1987 were analog audio only due to the length of the title and the desire to keep the film on one disc (e.g., Back to the Future). By 1987, Pioneer had overcome the technical challenges and was able to once again encode in CAA60, allowing a total of 60 minutes 5 seconds. Pioneer further refined CAA, offering CAA45, encoding 45 minutes of material, but filling the entire playback surface of the side. Used on only a handful of titles, CAA65 offered 65 minutes 5 seconds of playback time per side. The final variant of CAA is CAA70, which could accommodate 70 minutes of playback time per side. There are not any known uses of this format on the consumer market.