User:Jeh/wip/LaserDisc

Technical details

Comparison of several forms of disk storage showing tracks (not to scale); green denotes start and red denotes end. Some CD-R(W) and DVD-R(W)/DVD+R(W) recorders operate in ZCLV, CAA or CAV modes

Overview

The standard home video LaserDisc is 30 cm (12 in) in diameter and made up of two single-sided aluminum discs layered in plastic. LaserDiscs store analog video in composite format. The video bandwidth is approximately equivalent to the 1-inch C-Type VTR format, commonly quoted as "400 lines of horizontal resolution".

LaserDiscs carry audio in either two analog tracks, two digital tracks, or, for NTSC discs, both. (PAL discs cannot carry both analog and digital audio.) Older NTSC LDs made before 1984 (such as the original DiscoVision discs) only had two analog audio tracks. More recent discs added a digital audio bitstream. Originally, this was always in a similar or identical format to the "Red Book" CD audio format, providing two digital audio channels at CD-like quality.

On many movie releases both the stereo audio programs (analog and digital) were encoded for Dolby Surround. A common practice on discs marketed to movie enthusiasts was to provide the normal movie soundtrack on the digital channels and a "director's commentary" or similar material on the analog. Later, two different forms of digital discrete surround sound, AC-3 and DTS, were supported.

As on a typical vinyl record, there is only one spiral "track" per side of a LaserDisc. The track can be up to 42 miles (67 km) long. The LaserDisc pickup head follows the track smoothly from beginning to end of the program. This article will follow typical convention in using terms such as "the inner tracks" to refer to the portions of the recording that are closer to the center of the disc, as if those were discrete tracks in the manner of the tracks on a magnetic disk drive.

Because LaserDiscs are normally handled by their edges, the outer edges are more likely than the inner surfaces to suffer handling damage. Program material therefore begins with the innermost tracks and progresses toward the outer edge. If the disc is not of the maximum possible program length, the track will end before reaching the edge.

Encoding

Although appearing similar to compact discs or DVDs, even to the extent of using "pits" (depressions) and "lands" (the spaces between the depressions) in the aluminum surface, the information on a LaserDisc is recorded as an analog signal that combines three FM carriers and, optionally, a digital audio bitstream.

For an NTSC disc, three analog signals (the two analog audio channels, plus composite video complete with chroma subcarrier and inter-line and inter-field synchronization signals) each frequency-modulate a carrier (of center frequency 2.3 MHz, 2.8 MHz, and 8.5 MHz, respectively). These three FM signals are summed together. If digital audio is to be added, the digital bitstream is added to the sum. The digital bitstream's bandwidth is less than 1.4 MHz, so it fits "under" the 2.3 MHz left-channel sound FM carrier.

For a PAL disc, either the analog audio channels or the digital audio bitstream is omitted.

The result is clipped to form a signal that approximates a square wave. This signal has a long-term duty cycle of 50%, but the various frequency-modulated carriers and the digital audio bitstream cause short-term variations of the duty cycle. This signal is then recorded as pits and lands in the recording layer, with the lengths of the pits and lands showing the durations of the "high" and "low" parts of the square wave. (Thus is unlike optical digital media, where the pit-to-land and land-to-pit transitions represent transitions from 0 to 1 or 1 to 0.)

Upon playback the three FM carriers and the digital bitstream are separated by bandpass filters. The analog audio channels and composite video signal are then recovered by standard FM demodulators.

Video framing

Originally, the video frames on a LaserDisc could be recorded in any one of three different ways:

Constant Angular Velocity LaserDisc showing the NTSC field setup and individual scanlines. Each rotation has two such regions.

CAV

Constant angular velocity or Standard Play discs rotated at a constant speed of (approximately 1798 rpm for 525-line and exactly 1500 rpm for 625-line discs, i.e. 29.97 or 25 revolutions per second, respectively). Thus the rotation rate matches the video frame rate; one video frame is stored or played back for each revolution.

Because of the simple, constant relationship between disc rotational position and video timing, CAV discs supported several unique features such as freeze frame (accomplished simply by playing the same frame over and over), variable slow motion, and reverse play. In this mode, 54,000 individual frames (30 minutes of audio/video) could be stored on a single side of a disc. CAV reduced the visibility of crosstalk from adjacent tracks, since on CAV discs any crosstalk at a specific point in a frame is from the same point in the next or previous frame, which often carried similar picture data. CAV was used less frequently than CLV, and often 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. Some players included 10-key numeric pads on their remote controls to facilitate entry of a frame number. Such discs were also used to carry collections of still pictures, which could be similarly retrieved by number, the index being provided on an accompanying paper insert.

CLV

Constant linear velocity or Extended Play discs rotate at a varying speed (1,800–600 rpm),: faster when playing the inner tracks, slower for the outer, resulting in the disc surface passing at a constant velocity under the read head.

The innermost tracks of such a disc would carry one video frame per revolution as on a CAV disc, but at the outer edge of the disc, each revolution provided three frames. CAV discs do not easily provide the "trick play" features of CAV, initially offering only simple playback. But by taking advantage of much more linear track length on outer tracks, CLV discs could store 60 minutes of audio/video per side, or two hours per disc. Most LaserDisc 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. Eventually, high-end LaserDisc players incorporating a digital frame store appeared. These could provide functions not normally available to CLV discs such as variable forward and reverse, and a VCR-like "pause".

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 (for example, 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 per side. 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. There are a handful of titles pressed by Technidisc that used CAA50. 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.

Audio

As described previously, early LaserDiscs carried only analog sound. The digital bitstream was added later. Initially it supported only a PCM (CD-like) stereo audio track, sometimes encoded with Dolby Surround. Most LaserDisc players that could play discs with PCM audio could also play audio CDs, and like audio CD players, usually included a TOSlink or coax output to feed an external digital-to-analog converter or a preamp or receiver with digital audio input.

NTSC discs could carry two analog audio tracks, plus two uncompressed PCM digital audio tracks, which were (EFM, CIRC, 16-bit and 44.056 kHz sample rate): essentially the same format as on "Red Book" Compact discs, except for the slight difference in sample rate.

PAL discs could carry one pair of audio tracks, either analog or digital. The digital tracks on a PAL disc were 16-bit 44.1 kHz, exactly as on a CD. In the UK, the term "LaserVision" is used to refer to discs with analog sound, while "LaserDisc" is used for those with digital audio.

The two FM audio channels were centered at 2.3 and 2.8 MHz on NTSC formatted discs and each channel had a 100 kHz FM deviation. The FM audio carrier frequencies were chosen to minimize their visibility in the video image, so that even with a poorly mastered disc, audio carrier beats in the video will be at least ‑35 dB down, and thus, invisible.

Due to the frequencies chosen, the 2.8 MHz audio carrier (Right Channel) and the lower edge of the chroma signal are very close together and if filters and levels are not carefully set during mastering, there can be interference between the two. High audio levels combined with high chroma levels can cause mutual interference, leading to beats becoming visible in highly saturated areas of the image. To help deal with this, Pioneer decided to implement the CX Noise Reduction System on the analog tracks. The CX system reduced the dynamic range and peak levels of the audio signals stored on the disc, relaxing filtering requirements and greatly reducing or eliminating visible beats. The CX system gives a total NR effect of 20 dB, but in the interest of better compatibility for non-decoded playback, Pioneer reduced this to only 14 dB of noise reduction (the RCA CED system used the "original" 20 dB CX system). This also relaxed calibration tolerances in players and helped reduce audible pumping if the CX decoder was not calibrated correctly.

Later LaserDiscs offered one of two digital multi-channel formats: Dolby Digital (also called AC-3) and DTS. LaserDisc was the first medium in which these formats (now common on DVD titles) were available to the consumer. Some titles were released in both forms.

Unlike DVDs, which carry Dolby Digital audio in digital form, LaserDiscs store Dolby Digital in a frequency modulated form, replacing the right channel analog audio. This is done by modulating that channel's 2.8 MHz carrier with the AC-3 bitstream, using quadrature phase shift keying (QPSK). The right channel analog audio output of any player while playing an AC-3 encoded disc therefore consists only of apparent white noise (hiss).

During playback, the 2.8 MHz carrier is recovered from the combined signal via bandpass filters, just as if it was carrying analog audio for the right channel. However, LaserDisc players do not include the circuitry necessary to extract the AC-3 bitstream from this carrier; they are therefore not capable of outputting the AC-3 bitstream from their digital audio jack and so cannot drive an AC-3 decoder directly. Instead, AC-3 compatible players have a connector labeled "AC-3 RF out"; the "RF" refers to the 2.8 MHz carrier. This is usually connected to an external AC-3 demodulator which recovers the AC-3 bitstream and outputs it on a coaxial jack. This in turn is connected to the usual digital audio input on an AC-3 capable receiver or other equipment.

Another type of demodulator, marketed by Onkyo and others, included the AC-3 decoder and converted the RF AC-3 signal to 6-channel analog audio.

In the mid to late 1990s many higher-end AV receivers included the RF demodulator circuit and provided an "AC-3 RF in" jack, eliminating the need for the external demodulator. By the late 1990s, with LaserDisc player and disc sales declining due to DVD's growing popularity, the AV receiver manufacturers removed the demodulator circuit as DVD players do not require it.

On a DTS-encoded LaserDisc the DTS bitstream simply replaces the PCM digital stereo audio bitstream and is output through the player's digital audio jack, which is connected directly to a DTS-compatible receiver or decoder's digital audio input. Such discs also carried the usual stereo audio on their analog tracks. Thus, installations that did not have a DTS-capable DAC or receiver were limited to playing the analog stereo audio from such discs.

Whether multichannel or stereo, the sound quality of the digital tracks was far better than VHS or Beta videotape, but the quality of the analog audio varied greatly from disc to disc and, sometimes, from player to player. Many early and lower-end LD players had poor analog audio components, and many early discs had poorly mastered analog audio tracks, making digital soundtracks in any form most desirable to serious enthusiasts. Early DiscoVision and LaserDisc titles lacked the digital audio option, but many of those movies received digital sound in later re-issues by Universal. As time went on the quality of the analog audio tracks generally got far better as well. Many discs that had originally carried old analog stereo tracks received new Dolby Stereo and Dolby Surround tracks instead, often in addition to digital tracks, helping boost sound quality. Later analog discs also applied CX Noise Reduction, which improved the signal-noise ratio of their audio.

Star Wars: Episode I (1999) which was released on LaserDisc in Japan, is among the first home video releases ever to include 6.1 channel Dolby Digital EX Surround.

Both AC-3 and DTS surround audio were clumsily implemented on LaserDiscs, leading to some interesting player- and disc-dependent issues. A disc that included AC-3 audio forfeited the right analog audio channel to the modulated AC-3 RF stream. If the player did not have an AC-3 output available, or the owner did not have the required AC-3 demodulator as well as an AC-3 capable receiver, the next most attractive playback option would be the digital Dolby Surround (two channels with "Dolby Surround" analog encoding, as on many studio released videotapes). If either the player did not support digital audio tracks (common in older players), or the disc did not include digital audio tracks at all (uncommon for a disc which is mastered with an AC-3 track, but technically possible), the only remaining option was to listen to the left analog audio track.

However, many older analog-only players not only failed to output AC-3 streams correctly, but were not even aware of their potential existence. Such a player will happily play the analog audio tracks verbatim, resulting in garbage (static) output in the right channel.

On a DTS disc, digital PCM audio was not available, so if a DTS decoder was also not available, the only option is to fall back to the analog Dolby Surround or stereo audio tracks. In some cases, the analog audio tracks were further made unavailable through replacement with supplementary audio such as isolated scores or audio commentary. This effectively reduced playback of a DTS disc on a non-DTS equipped system to mono audio, or in a handful of cases, no film soundtrack at all.

Only one 5.1 surround sound option existed on a given LaserDisc (either Dolby Digital or DTS), so if surround sound is desired, the disc had to be matched to the capabilities of the playback equipment (LD Player and Receiver/Decoder) by the purchaser.

A fully capable LaserDisc playback system included a newer LaserDisc player that is capable of playing digital tracks, had a digital optical output for digital PCM and DTS audio, would be aware of AC-3 audio tracks, and had an AC-3 coaxial output; an external or internal AC-3 RF demodulator and AC-3 decoder; and a DTS decoder. Many 1990s A/V receivers combined the AC-3 decoder and DTS decoder logic, but an integrated AC-3 demodulator was rare both in LaserDisc players and in later A/V receivers.