Optical Disc Technology

Introduction

Optical disc technology is a growing force in home video which became popular with the introduction of the audio Compact Disc. Today, various formats exist, which besides sound can also carry data, graphics and full motion video parts. First we will take a look at the basics of the optical disc technology, and then at some of the existing formats of optical discs and accompanying hardware. The types of optical discs discussed in this handbook are Laserdisc (LD) and CD-i, and to a lesser degree CD-ROM and Photo CD.

Digital Technology

Optical disc technology makes use of digital signal processing, contrary to the analog audio and video carriers, such as the gramophone record and the magnetic videotape. But why digital?

In analog transmission, any imperfection during the registration, storage or reproduction phases of recording will decrease the quality of the audio and/or video signal. For example, a dirty record causes noise, an irregular revolution or winding speed causes problems, a worn needle or a dirty head causes distortion. These imperfections do not occur in digital registration. (For more detailed and in-depth information, please refer to the Audio Handbook, in which the basic principles of the Compact Disc Technology are explained.)

From analog to Digital

In optical disc technology, analog signals are converted into digital signals (with the exception of the video information on laserdiscs, as will be explained later). During this process, the analog signal (audio and/or video) is measured in parts and converted into a series of values, called sampling. One can envision a waveform representing an audio or video signal, being measured with intervals. The signal strength and polarity at these intervals can be expressed in decimal numbers (1,2,3, etc.) and express the signal strength and polarity (+ or -) from point to point. The frequency of how often the signal strength is measured in a waveform, determines the accuracy of the registration of the original waveform. These steps must therefore be very small. The measurements are in the order of tenths of millivolts.

An analog-to-digital (A/D) converter translates these decimal values into binary notation (bits). Bits are made up of '1's and '0's only, and by combining these ones and zeros in different combinations, decimal numbers can be expressed in binary notation. Some examples of binary notation (in 3 bits) are:

     Decimal     Binary
     ------------------
     1           001
     2           010 
     3           011
     4           100 
     5           101
     6           110
     7           111

Thus the analog signal becomes a digital signal which is now a series of pulses: pulses for the '1's, and non-pulses for the '0's. For optical discs these pulse series are recorded on the surface of the disc as microscopically small pits and lands, with the help of a fine laser beam. Pits stand for '0's and lands stand for '1's. In most recordings every value (44,100 per second) is converted into a string of 16 bits (instead of the three bits example). This totals over one million bits per second. A 16-bit number of '1's and '0's can indicate no less than 65,536 different values (2 values possible for each bit = 216 = 65,536 possibilities). More information is available in the Audio Handbook.

Conversion process of an analog signal to digital and back to analog.

Scanning the Disc

Like gramophone records, the information on optical discs is recorded on a spiral track. The laser starts 'reading' the disc from the inside and ends at the outside. When played back, a laser beam shines on the pits and lands. When the beam strikes a land, the beam is reflected onto a photoelectrical cell. When it strikes a pit, the photo cell will receive only a weak reflection. Thus the photoelectrical cell receives series of light pulses corresponding to the pits and lands in the disc. A D/A-converter (digital to analog converter: DAC) converts the series of pulses back to binary coding, and then to decimal values. Now the original analog signal can be rebuilt.

Cutaway view of the laser pickup. Depending on whether the laserbeam hits a pit or a land, the laserbeam is reflected and received by the photo-electrical cell.

Compact Disc player mechanism. The laser pickup reads the disc from below.

CIRC

Thanks to this optical scanning system, there is no friction between the laser beam and the disc. As a result, the discs do not wear, however often they are played. However, they must be treated carefully, as scratches, grease stains and dust might intercept or diffract the light, causing whole series of pulses to be skipped or distorted. This problem can be solved, as during the recording the Cross Interleaved Reed Solomon Code (CIRC) is added, which is an error correction system that automatically inserts any lost or damaged information, by making a number of mathematical calculations. Without this error correction system, optical disc players would not have existed, as even the slightest vibration of the floor would cause sound and image distortions.

Laserdisc

The Laserdisc was developed by Philips as an advanced form of video. Just like the CD-player, the Laserdisc player is characterized by simple principles of operation, be it directly on the LD-player or via remote control.

Like the CD, the Laserdisc is scanned by a laser beam. In excess of one million shallow pits in the disc per second are read one-by-one in the same way as the CD. A Laserdisc-player can play back not only laserdiscs, but also audio CDs. The main difference between laserdisc and other optical discs is that the audio signal on Laserdiscs is recorded digitally, just like CDs, but that the video signal is recorded in an analog way. Combining digital and analog information makes efficient use of the available disc space, enabling longer playing times than when all the information is digitized. analog video can offer very good image quality, with sharpness and details better than the average videotape reproduction. Due to the analog video processing, laserdisc still depends on local television standards (for example a PAL disc cannot be read by an NTSC player nor a NTSC disc by a PAL player). However, there are players that can handle more than one system.

Laserdisc Sizes

Laserdiscs come in three sizes, each having a different application. Laserdisc players can handle all sizes.

12 cm Laserdisc single
The small, gold or silver colored Laserdisc single has a diameter of 12 cm, which is similar in size as the CD. The Laserdisc single can contain 6 minutes of video programming plus accompanying digital sound. Alongside the video information, it has space for 20 minutes of digital audio.

The video program is recorded on the outer tracks of the Laserdisc single, the audio program on the inside tracks. In this way the audio part can also be played back via any existing CD player, because the CDs are read from the inside to the outside. When a Laserdisc single is started, the laser-pickup will move first to the video part. After having played back the video part from the inner to the outer tracks, the laser will move automatically to the inside of the disc and starts playing the audio track. The Laserdisc single is an ideal carrier for video clips and music video.

Most Laserdisc players are multi-disc players which can play several sizes of laserdiscs and audio compact discs.

20 cm Laserdisc EP
For longer programs the Laserdisc EP (Extended Play) was invented. An EP laserdisc can be double-sided, has a 20 cm diameter and can contain a maximum of 20 minutes of video plus audio per side, making a total of 40 minutes on a double sided disc. When the disc has two recorded sides, the disc actually consists of two discs glued together, back-to-back. The Laserdisc-EP is especially suitable for music video clips, cartoons, documentaries and instruction films.

30 cm Laserdisc LP
For complete concerts, films, operas, etc., the 30 cm Laserdisc LP was developed. This disc can contain a maximum of 60 minutes of video plus audio on each side, making a total of two hours. Here too, the double-sided disc consists of two discs glued together. The Laserdisc LP is the format used most often.

CLV and CAV

In audio Compact Discs, the scanning starts at the center at a speed of 500 revolutions per minute (rpm), which reduces as the beam moves towards the outside edge where the speed is 200 rpm. This is the principle of the Constant Linear Velocity (CLV), which means that the laser beam reads the disc at a constant speed of 1.25 meters per second. CLV discs are also called Linear Play discs.

The Laserdisc is also scanned at a constant speed, but at a much higher speed of around 11 meters per second. If a conventional CD is inserted, the player automatically starts at the standard speed of 500 rpm. The introductory track of the disc tells the Laserdisc player which type of disc has been inserted. The number of revolutions is automatically increased to 2700 rpm when a Laserdisc single is inserted, and to 1800 rpm in the case of a Laserdisc LP.

Apart from the conventional CLV (Linear Play) discs as described above, there are also discs that have a Constant Angular Velocity (CAV discs), which means that the revolutions per minute remain constant. The advantage of this is that it allows fast and slow playback (at variable speeds, forward and backward), still playback, repeat playback and fast track (within a number of seconds) with the help of image and index numbers. As the number of revolutions remains constant during playback, the scanning speed will decrease as the laser-pickup approaches the outer tracks of the disc. CAV discs have only half the playing times of CLV discs, as they make less efficient use of the disc capacity. Contrary to the CAV disc, the conventional CLV disc does not contain any image numbers. As with audio CDs, CLV discs can indicate the playing time in hours, minutes and seconds. By indicating a start and a stop point at the counter setting, musical pieces or scenes can be stored in memory and be played back separately. CLV and CAV discs are available in both 20 cm and 30 cm format.

Laserdisc Recording

In Laserdisc technology, the video signal is recorded as analog, whereas the audio signal is recorded digitally. The video signal contains the color, luminance, black-level and synchronization information.

In order to record the video signal on the Laserdisc, it is first put on a high carrier frequency via frequency modulation (FM). The frequency modulation makes sure that the signal's strength, or amplitude, remains constant under all circumstances. To this modulated analog signal the accompanying digitized audio signal is added. For this purpose, Pulse Width Modulation (PWM) is applied, in which the continuous bit flow of the audio signal is transferred bit-by-bit at very high speed. As a result of adding the digital audio signal to the high-frequency video signal, the usually very stable FM-signal starts varying along with the audio signal. In other words: the PWM audio signal is superimposed on the FM video signal. This new signal is accurately topped, and thus a combined audio/video signal is created which can be used to burn an A/V hole pattern on a disc via a high-frequency laser. This master disc can then be used to produce the actual Laserdiscs.

When played back, the hole pattern is converted back to the combined A/V signal via laser beam and photo diodes. This signal is then sent to an audio and video demodulator. The demodulator for audio responds to the slowly varying audio signal and decodes the information with the help of the usual compact disc circuits. This part also contains a CIRC error correction circuit. Finally, D/A-converters generate the actual analog audio signal.

The demodulator for the video information demodulates the high-frequency video signal, and thus generates the video signal. In the meantime, special circuits check the synchronization pulses, automatically making any necessary corrections. Then the video signal is demodulated into the separate RGB components that, when connected to the RGB-connection of a TV set, lead to the highest possible image quality.

Combining the FM modulated analog video signal (A) with the digital audio signal (B) gives a special waveform (C), which after topping (D), is being used to burn the pits (E) into the master disc.

CD-i

Compact Disc Interactive was designed to make high-quality interactive multimedia available to the general public. Invented by Philips, the CD-i standard has been defined as world standard. Any CD-i disc will play on any CD-i player anywhere in the world, regardless of who published the disc, who made the player, or what the local television standard may be, unless it is encoded for a specific market.

With CD-i, all the information is digitized, whether it is audio (music, narration, sound effects), video (animation, still video, full-motion video, visual effects) or data (graphics and text). This makes it possible to interleave all kinds of information and gives it great flexibility. Another impressive feature of CD-i is the ability to adapt the amount of storage capacity to the application.

The CD-i specification defines two physical formats: Form I and Form II. Form I, with error detection and correction code, is similar to CD-Read Only Memory (CD-ROM) formats, and is applicable principally to the requirements of text, computer data, and highly compressed visual data. Form II, without error detection or correction codes, is tuned to the requirements of real-time audio and visual data. Thus, the CD-i System is capable of reading CD-ROM-compatible discs and fully integrated CD-i discs, as well as CD-Digital Audio discs.

CD-i Use

The CD-i player can be connected easily to virtually any television and stereo system, and is operated via remote control. The standard 5-inch CD-i disc is loaded in the same way as an audio CD. Discs are easy to use and require no special instruction or knowledge. Users can control the action on the television screen and activate various selections highlighted by pointing to and clicking on command areas on the screen (by means of a remote control). CD-i is called interactive because the user can interrupt the running sequence of a program to recall a certain choice, go back to a previous step, ask for more detailed information, or perhaps request to have the explanation in another language - simply by the press of a button.

CD-i Compatibility

Developed as a world standard, the CD-i player will play any CD-i disc, regardless of make or manufacturer, or country of origin. In addition, the CD-i player is fully compatible with:

     - CD-Digital Audio discs 
     - CD-Graphics
     - Photo CD discs
     - CD ROM-XA "bridge" discs

CD-i Design: Balancing Quantity and Quality

The CD-i disc can hold a maximum of 650 million characters of data. This may seem a lot in comparison with the average office PC's hard disc, but audio-visual information requires a lot of storage capacity and this can be used up quite quickly. The designer must make choices based on quantity and quality trade-offs from the range of audio and video information.

Audio Possibilities for CD-i

Four basic levels of sound quality are defined in the CD-i specification:

• CD-Digital Audio 1 channel stereo (16-bit pulse-code modulation of existing standard): Offers more than one hour of stereo sound of digital audio CD quality.
• Hi-Fi Music Mode (Level A) 2 channel stereo or 4 channel mono: Equivalent to LP music quality, offering the user more than two hours of high-quality and noise-free stereo sound.
• Mid-Fi Music Mode (Level B). 4 channel stereo or 8 channel mono: equivalent to FM stereo broadcast quality and takes up only 25% of the disc space occupied by CD sound.
• Speech Mode (Level C). 8 channel stereo or 16 channel mono: equivalent to AM broadcast quality, suitable for speech and background audio, and occupies only 12.5% of the disc.

Each channel is equivalent to a maximum of 70 minutes of continuous playing time. In the fourth audio level, up to 16 parallel channels are available, which makes the CD-i disc uniquely suited for multilingual applications.

Video Possibilities for CD-i

The CD-i standard encompasses both video resolution and coding requirements. There are two levels to which the resolution can be defined: Normal and High. The normal resolution of 384 x 280 pixels is equivalent to a domestic television set. High level, at 768 x 560 pixels, is the equivalent of color monitors and future digital television receivers. Pictures are generally non-interlaced, although interlacing can be used.

Natural pictures, such as photographs and movie clips, occupy about 325 kilobits (325.000 bits, 325 KB) per picture without interlacing, and 650 KB with interlacing. To decrease throughput times and still maintain natural picture quality, all natural pictures on the CD-i disc are compressed to 108 KB, with no visible difference between the compressed and non-compressed pictures.

Video information can be stored in various ways:

• Four levels for images ranging from natural photographs to graphics, animation, and complex graphics mixes. The number of full-screen images will depend on the chosen image quality. A minimum of 6000 full-color, full-screen photograph images can be stored on one disc at the high end, and many more at the lower end.
• Up to 72 minutes of full-screen, full-motion, super-VHS quality video with Level B stereo (Mid-fi mode) can be stored on one disc. This is called FMV (Full-Motion Video).
• Four different visual planes, and a whole set of different special effects, such as wipes, dissolves, etc.

Full-Motion Video

In 1990, the extension of the CD-i standard with Full-Motion Video (FMV) was announced, at that time with only mono (B) sound. In the meantime, the development in compression technologies has made it possible to improve picture quality and to add, instead of mono sound, high-quality stereo sound. Philips, Sony, and Matsushita decided to adhere to the MPEG (Moving Picture Experts Group) standard as the extension for full-motion video on CD-i.

The MPEG audio and video standard, as adopted for FMV, allows CD-i title designers to use video sequences in several picture formats (4:3 and 16:9) and in combination with the variety of audio qualities. It is this combination, MPEG/FMV with the functionality of CD-i the various video planes, the various audio levels, and the real-time operating system, which makes it possible to produce entirely new pieces of highly attractive software.

In order to run FMV on the CD-i player, a special digital video cartridge is required. Some CD-i players already have one installed, while others can be upgraded.

Synchronization

CD-i is distinguished by the system's ability to integrate audio, video, and computer data. Synchronization is necessary to ensure that the information read from the disc is directed correctly into the audio, video, and computer processors, and that the corresponding pictures and text are viewed simultaneously, together with any accompanying audio.

Synchronizing is not a matter of simply maintaining a constant distance between picture and sound, particularly if multiple language audio tracks are used for a common picture sequence. The information for all channels must be mixed together in the data stream, and the time required for processing the different elements cannot be known until the processing actually occurs. To manage these variable sequence timings, CD-i employs a physical interleaving process combined with data-driven interrupts.

The ability to merge or interleave all of these and play them back perfectly synchronized at run-time is made possible by CD-i's unique operating system, called CD-RTOS. (Compact Disc Run Time Operating System)

Other Compact Disc Technologies

Besides CD-i, there are more formats of Compact Disc technology in use:

Official logos for CD-DA, CD-G and Photo CD.

The Digital Audio Compact Disc (CD-DA)
The 12-cm/5-inch Compact Disc Digital Audio, commonly known to the consumer as the CD, has been a phenomenal success ever since the inventors, Philips together with Sony and Matsushita, agreed the world standard and introduced it in 1983. The silver-colored disc delivers high-quality digital stereo sound, and can also carry limited amounts of simple text and graphics (e.g., notes, lyrics, and information about the music and artists on the disc). With a suitably equipped player (such as CD-i), a CD+Graphics disc can supply pictures to an ordinary television set and stereo through a hi-fi TV set.

CD-ROM
Compact Disc Read Only Memory is a derivative of the CD-DA and is the same size. Computer manufacturers saw the capacity of 650 million characters of memory (equivalent to 250,000 A4 pages of text) as a major opportunity, and got together to draw up a standard, and introduced it in 1985. It was mainly intended to be a carrier of text, and the format is unable to interleave audio and visual data. Theoretically, any CD-ROM disc should play on any CD-ROM player, but in practice, because of incompatible computer systems and software, this is not the case.

CD-ROM-XA
Compact Disc Read Only Memory eXtended Architecture is designed to compensate for the deficiencies of CD-ROM as a multimedia platform by borrowing parts of the CD-i standard.

This means that it is a link between CD-ROM and fully interactive CD-i. An XA card inside the PC supports interleaving of text, graphics, audio, and pictures. However, it requires a special CD-ROM drive. Using CD-ROM-XA means that it is possible to have XA "Bridge Discs" which will play on the PC at the office and the CD-i player at home, abolishing the need for a publisher to develop separate PC and CD-i versions of the same title.

Photo CD
Photo CD is a high-quality photographic storage medium developed by Philips and Kodak. One Photo CD can store up to 100 digitally encoded 35-mm photographs at resolutions of up to four times greater than High-Definition Television (HDTV).

Customers take their rolls of film to the photo processors for developing, and receive standard prints as they do with normal photo processing. On request, the photo processors use a work station to read processed 35-mm negatives and slides, and then write digitized pictures onto Photo CDs. Customers receive a disc containing all the exposures on the roll. The photo images can be accessed via a CD-i player or Photo CD player, and displayed on normal television sets. The Photo CD player offers random access viewing, zooming, panning, editing, and more. A Photo CD can also be played on a CD-ROM-XA-equipped PC. Via the PC it can be imported into desk-top publishing and other computer-based design and presentation systems for editing purposes.

The photo images on the Photo CD can be processed and printed like normal negatives. Very high-quality prints can be made directly from the Photo CDs. Color, sharpness, and granularity are similar to those of prints made on photographic paper from original negatives, and will only be limited by the resolution of the display.

Connecting the LD/CD-i

Although a SCART-RGB bus is the best connection to the TV set, not every TV is equipped with such a connection. Therefore, the outgoing signal is also converted to a CVBS signal which can be transmitted via both a CVBS-SCART bus and a video (cinch) bus. The latter is not ideal, as the color and brightness signals will then be transferred as composite signals, resulting in a slightly lower color quality, as there will always be some sort of cross color. The audio signal can be transferred via separate audio busses, but just like RGB, the CVBS-connection offers high sound quality, as the sound is not influenced by the video information, being transmitted separately. For sound, both Laserdisc and CD-i players have digital audio output, so they can be connected to amplifiers with digital input.

Laserdisc players can be connected to TV sets that only have an antenna connection. Like a video recorder, the Laserdisc player has a built-in sender (an oscillator), which generates a signal with a carrier frequency that corresponds to the frequency of the assigned A/V channel. If a video recorder is already connected to the TV by antenna (HF), the Laserdisc player can be installed simply by connecting all three devices in a loop. The TV antenna should be connected to the antenna input of the Laserdisc player (ANT-IN). Then the TV-OUT connection of the Laserdisc player must be connected to the antenna input of the video recorder, via a coax antenna connection cable. Finally, the output of the video recorder will be connected to the antenna input of the TV via a similar coax antenna connection cable.

Connecting TV, VCR and LD/CD-i player.

As this connection takes place via the antenna looping amplifiers installed in the Laserdisc player and the video recorder, both devices should always be plugged into the power supply, putting them automatically in Stand-By mode. It might occur that if the VCR is already tuned to the A/V channel and the laserdisc player is also tuned to the same channel, interference problems on the TV screen can be the result. To avoid such interference, the small tuning screw at the back of the Laserdisc player called '+ CHANNEL -' must be turned just a little bit, so that the signal is transmitted via another channel. As soon as the images of both the video recorder and the Laserdisc player are displayed without distortion, the tuning is satisfactory.

If the TV set is supplied with two SCART busses (connections), both the video recorder and the Laserdisc or CD-i player can be connected independently, which, of course, would be the best solution. When applying a TV set with one SCART bus and a video recorder with two SCART busses, it is possible to loop them via these two busses. This is also possible with the VIDEO IN and OUT busses of the video recorder.