This is a basic glossary of terms found in the audio/video arena which should provide a foundation of knowledge for the further study of audio/video concepts.

A-D Conversion

Analog to digital conversion, also known as modulation, involves special chips to convert analog signals to digital strings, or vice-versa. A-D conversion is necessary to send computer data through regular telephone lines, to convert analog audio and video to digital video, to have computerized telecommunications, to display data on analog displays and so on.

Birefringence

In CDs and other optical discs, it means double refractive ability. It is caused mainly by improper cooling of the substrate during the injection-molding process. In optical applications, substrate birefringence is unwanted, since it interferes with the read function. Users, however, can not determine that it is birefringence that is causing read errors or poor performance of their CD-ROM applications--it is detected by special equipment and tests.

Blue Book

Released by Philips-Sony (Dec 1955), the Blue Book was also known as the CD-Extra format (in the Mixed-Mode family). It provides for including data and audio in the same disc--using CD- ROM-XA sector structure for graphics and data. CD-Plus was a somewhat similar product that is no longer mentioned. Microsoft released an Enhanced CD Sampler in CD-Extra format (Music Industry Conference, Mar96). The Recording Industry Association of America endorsed the Enhanced CD format. The Blue Book fixes the 'track one' problem (awful screech when CD-Audio players play the data track), and this format can be read by current CD-Audio and CD-ROM drives.

Blue Laser

The development of a blue-light emitting diode (based on gallium nitride) in Japan (1993), opened the way for the production of short-wavelength (@16nm) blue laser optical devices. Philips, Sony and Toshiba demonstrated the blue laser in 1996. Obviously, since the blue laser will make smaller pits and use narrower tracks--disc capacities should increase even more. (Current red lasers are @ 600+ nanometers.)

Caddy

A 'caddy' is a special plastic case that holds and protects the CD during operation--especially when the drive is mounted on its side. Caddies are not used for shipping. For WORM and Erasable media, they are called cartridges--probably because they do not allow extraction of the disc itself. DVD-RAM (phase change) will also use a cartridge.

CD

The Compact Disc was developed by Philips and Sony, and was first implemented commercially for storing digital audio data (CD-Digital Audio). The physical specifications for the 12cm disc, since known as CD, were issued in the now famous Red Book. The CD is made up of a polycarbonate substrate, a thin reflective metallic layer (the mirror-like is aluminum), and a lacquer coating. The encoded data track is a continuous spiral track of about 1.6 to 2.2 microns wide, and the pits are about 0.6 microns wide. Essentially, any other size or type of disc is not a CD.

CD-Digital Audio

Philips and Sony developed the necessary technology for storing digital audio signals on a Compact Disc, and introduced the CD-Digital Audio (1982). This new product was based on the now famous Red Book (1981)--which specified the physical structures for the track and sectors in the disc. CD-Digital Audio was implemented to hold about 60 minutes of audio data, in up to 99 tracks (songs) at a sampling rate of 44.1 KHz and a sample size of 16 bits, to produce high quality stereo sound. The success of CD-Digital Audio has been key for the growth and success of CD-ROM and other CD implementations.

CIRC

Cross-Interleaved Reed-Solomon Code is used in compact discs for the first two levels of error detection and correction. CIRC in CD-Audio, implemented at the frame level, provides an integrity of one erroneous byte in a gigabyte (two CDs). In CD- ROM, which has an additional and more sophisticated third level 'layered' error detection and correction in CD-ROM claims an integrity of one byte in 2,000 CD-ROMs.

Codec

Derived from COder-DECoder, a Codec is a software program that implements algorithms that are central to compression- decompression packages, especially those that deal with digitized streams produced from analog video source. Most of the reliable codecs maximize their power by using specific compression decompression boards (such as "MPEG boards"), especially since pure software compression-decompression packages have hardware requirements beyond what comes in standard PCs. Well-known codecs include, Intel's Indeo, MPEG, Cinepak, PLV, etc.

Constant Angular Velocity (CAV) and Constant Linear Velocity (CLV)

Magnetic and optical storage drives can rotate with constant angular velocity (CAV), or constant linear velocity (CLV). CAV, used by magnetic drives (and record players), is measured in RPM, and means that the read head sweeps the same angle, for the same amount of time, at all radii. CLV, used in CD-ROM, allows the head to read the same length of track at all times and radii, at a rate of 1.3 m/sec--also called 'reference speed, or scan rate.' CLV requires that the disc spin slower as the head moves to the outer edge of the disc; a CD-ROM, for example, spins from 539 RPM at the inner edge, to 210 RPM at the outer edge. DVDs also use CLV, at a reference speed of 4.0 m/sec.

Convergence

A term in the industry that tries to explain the pressures on optical technology, mainly CD-ROM development, to bridge the gap between computer users and television viewers. The aim is, ostensibly, to produce multimedia applications that would serve and satisfy the needs of both groups, with one hardware device. In some circles, the term currently includes conjunction of CD-ROM technology with the Internet and other network services.

Copy Protection

The movie industry's concerns about unrestricted copying or piracy of their DVD-Video content led to imposing copy protection measures in the DVD-Video specifications. The current main option is Macrovision, which implements Analog Protection System (APS) --which degrades the video stream that is being copied. There are other options as well--of varying sophistication or complexity, such as the Content Scrambling System. Moreover, the industry is working hard to make it illegal everywhere to defeat copy protection measures.

CRC

Cyclic Redundancy Check is a method for detecting errors in data transfers. A special polynomial algorithm produces and uses a coefficient and a remainder (16 or 32 bits long) to check if the transmission proceeded without problems. CRC values change even if only one bit in the file changed--which makes it extremely reliable for checking integrity of files transmitted between computers.

Dolby AC-3

This is Dolby Digital audio, using AC-3 compression (from a source PCM stream), sampled at 48 kHz, and 16 bits. This is one of the digital audio formats that are included in the DVD-Video specifications--required for DVD-Video titles for NTSC countries. It involves 1 to 5.1 channel surround-sound, and is of higher quality than the CD-Digital Audio.

DVD-Video

Book B includes the specifications for DVD-Video, which influenced the key aspects of the specifications for the DVD family. DVD-Video supports high quality full motion MPEG-2 video of 720x480 pixels/frame, at 30 frames/sec (NTSC), and 720x576 pixels/frame at 25 frames/sec (PAL). It provides for high quality audio (MPEG-2, 7-1; Dolby AC-3, 5-1; or Linear PCM), with up to 32 sub-picture elements such as captions and stills (to a TV monitor). In addition, it offers the user choices of languages, various navigation and interactive controls (including random access for interactive games), and regional coding, copy protection and other features demanded by the movie industry. All DVD players will support a basic set of options. A single-side, single-layer DVD-Video can contain a 133 minute full-featured movie title.

Injection Molding

This is a common industrial process to produce plastic products of all shapes. The mastering and replication plants require costly equipment and highly clean environments. The injection molding machines fitted with appropriate stampers, stamp or press the molten polycarbonate. Thus, the replicate (or substrate) is allowed to cool before it is moved for metallizing and given a coat of protective lacquer. Some injection molding machines produce 7 to 10 replicates per minute, while some of the newer machines claim even higher rates. There is a new method in testing, which uses photolithography, and a continuous roll of metallized polyester, to produce the CDs (by exposing, developing, cutting and bonding to the substrate). But, injection molding is here to stay for the foreseeable future, specially since current replication plants claim that they will be able to handle DVD replication--with some adjustments or new equipment.

ISO

The International Standards Organization, composed of scores of international specialized committees, with main Secretariats worldwide, is the accepted source of standards for electronic and computerized data communications and information processing within the Open Systems Interconnection (OSI) framework.

JPEG

A versatile and commonly used color graphics compression specification adopted by the Joint Photographic Experts Group. Hardware and software JPEG implementations allow setting the desired compression, from 24-bit lossless (usually 2:1) to smaller bit size lossy compression rates (up to 60:1 in some cases). This allows users to insure retention of detail and precision of the original. For everyday graphics work, recommended JPEG compression ratios range between 25 and 35:1.

Laser

Light Amplification by Stimulated Emission of Radiation was demonstrated about half a century ago, with an original ruby laser. Today, lasers abound to suit diverse technologies and applications. Some magazines have reported tests of blue, blue- green, and blue-purple lasers of higher precision, which will make possible higher density optical discs. Ultraviolet lasers produce heat and require cooling--just as do the barely visible violet Krypton gas lasers. Cooling and size requirements are key to the implementation of a laser to optical disc technologies.

Lead-In--Lead-Out

These are lengths of track before the beginning and after the end of the coding. In single session applications, they serve as 'markers;' the lead-in includes the Table of Contents, and the lead-out can include code to stop the player. CD-Audio tracks (songs) implement lead-in and lead-out to help song selection. In mixed-mode applications, each track with different type of data (text, video, audio) is required to include pre-gap and post-gap spaces. DVD also uses lead-in and lead-out.

Lossy/Lossless Compression

Certain compression algorithms can produce outstanding compression ratios, but often at the cost of imperfect decompression; that is, the decompressed data is not identical to what it was before compression. Imperfect decompression (even if only a few bits per millions of bits) is called lossy--because of the loss of bits in the process. Lossless compression, on the other hand, employs algorithms that do not lose data in decompression, and although they may not produce great compression ratios, they provide integrity or reliability. When working with graphics and sound, some lossy compression is considered adequate, especially when storage and bandwidths are serious considerations.

Mastering

Mastering involves producing a glass master disc that is necessary for the mass reproduction process. Mastering takes place in a 'clean' environment, where the encoders use a high power blue argon laser beam to 'burn' pits on a large glass disc coated with a sensitive recording layer (usually photoresist). Once treated or 'developed' (chemically), the glass disc is referred to as the master or positive. Using electroforming technology, this glass master serves for the production of a metallic master (usually nickel), generally known as the 'father.' (It is also called a stamper, if it is used for reproduction of small runs.) For large mass reproduction jobs, the 'father' is used to produce intermediate 'mother' molds which are used to produced the necessary metal stampers ('sons' or production stampers) that are used in the injection molding machines. Mastering and reproduction are usually done at the same plant. For DVD, double-layer, and double-sided products will require sophisticated replication, bonding, lacquering and drying procedures.

Micron

One millionth of a meter, or a thousand of a millimeter. For example , the CD-ROM track pitch is 1.6 microns wide, and the pits are about 0.6 microns wide. In DVD, the track pitch is 0.74 microns and the pits are 0.4 microns wide.

M-O Technology

Magneto-optical technology is the most used recording technology in the Rewritable (a.k.a. Erasable) line of optical products. The substrate is covered with a complex stack of thin films or layers--one of them the recording layer (of iron, cobalt and terbium), in which the pits are recorded. Two such discs are glued together to make the 5.25in, double-sided M-O disc. M-O discs can be rewritten millions of times, because the technology employs a magnetic field to realign (polarize) the molecular structure of the pit to its original unwritten state. This process exploits the Curie and Kerr effects, and does not cause degradation in the coding layer. The major drawback some see in M-O is that the process takes multiple passes to seek the area, erase, write, and verify--which, according to detractors, make it a slow performer. Under the Orange Book, Part 1 (M-O), magneto- optical technology is employed on the 12cm CD, formatted following the ISO 9660 specifications. Unlike the optical M-O discs (proprietary formats in various sizes), the standard CD-MO product has given rise to new types of drives. The multi- function drives, for example, are able to read and write the CD- MO ('Rewritable CD') and read a standard CD-ROMs as well.

MPEG

A Codec adopted by ISO's Motion Pictures Expert Group for compression and playback of full-motion video and audio streams-- often referred to as 'MPEG video.' MPEG-1 is now an open standard (ISO/IEC 11172, 1991)--which establishes the structure for a standard MPEG file, and specifies a transfer rate of 1.5Mb/sec, with a resolution of 352x240 at 30 fps. MPEG-2 accepts transfer rates up to 15Mb/sec, with a high resolution of 720x480 at 30 fps, and it also requires a 2MB buffer. Today, most of the demands of multimedia and full-motion video are met by various MPEG add-on boards. Incidentally, CD-I uses MPEG-1, and Video CD was promoted as the first MPEG-1 optical disc for multiple platforms. MPEG add-on boards use special chip sets for compression and decompression--but there are various software-only MPEG decoding programs. High-end hardware solutions claim compression ratios up to 50 to 1. But, since MPEG is lossy, such high compression rates often signify lower quality playback at 30 fps. MPEG-2 (ISO 13818-1, 1994) offers higher quality and speeds than MPEG-1. In certain circles, it is considered a temporary solution in the wait for a software solution (which will require CPUs to provide code streams above 10 Mbits/sec). DVD, however, uses MPEG-2 primarily, and all DVD-players will include hardware to handle MPEG-2 contents.

Multi-read

In DVD, multiread applies specifically to the capability of DVD players to read DVD-R and DVD-RAM products, as well as CD-R and CD-RW. In similar vein, CD-ROM and CD-R players can not read CD-RW discs either. The problems involve low reflectivity of CD-RW on the one hand, and the higher laser wavelengths in DVD on the other. Therefore, read-heads have to be given multiread capability, or the player has to use two heads. The industry (mainly OSTA) has promised multi-read specifications to make DVD players fully compatible with all CD and rewritable products.

Nanometer

A thousandth of a micron. Laser wavelengths are usually measured in nanometers.

 

NTSC

The National Television Standards Committee supports the NTSC signal and display technology used in the TV industries of North America, Japan, and a few other countries. It specifies 525 lines/screen, and 29-30 frames/sec.

Optical Discs

Technically, optical discs are those that are 'written' (encoded) and read using a laser optical device. 

Optical Recording Technologies

Although often referred to as encoding, optical recording technologies are varied and quite sophisticated--the main ones are summarized below. For CD-Audio and CD-ROM, which are mass-replicated products, a glass disk, coated with photoresist, undergoes recording, development, and a special process to produce the metallized glass master--which is then used to produce the stampers for the reproduction equipment. On the desktop, Write-Once (W-O) and Rewritable drives record the optical discs in real time. CD-Recordable drives encode either in Track-at-Once (TAO), or Disc-at-Once (DAO) mode in the same CD-Recordable media. All those 'one-off' discs have a recording layer prepared for the specific recording technology to be applied. W-O uses Ablative, Phase Transition, Bubble Formation, Alloy Formation, and Texture Change recording technologies. Ablative technology, which is the most common, uses a recording layer with tellurium alloy (low melting point) that allows formation of holes when the high power laser beam is applied--thus forming holes or 'pits.' In similar fashion, the other technologies produce some sort of 'pit' by a phase, color, or texture change. Rewritable discs implement Magneto-Optical (M-O), Dye Polymer, or Phase Change technology. M-O is the most common, and it uses a magnetic film (of rare earths) for the recording layer, an appropriate magnetic field, and a high power laser beam to record or 'rewrite'--applying the Curie and Kerr principles about changes in structure when heat is applied, and the realignment of particles (polarization) when a magnetic field is present. Dye polymer and phase change also use special recording films or layers, on which the write laser produces the pits. The pits in these technologies are, however, erasable--they can be reverted to their original state. In phase-change, for example, a pulse from the laser changes a spot from crystalline to amorphous (which has different reflectivity), and another pulse of the laser heats it up and changes the spot back to crystalline (erasing, as it were, the previous pit).

Optical Technology

Technically, optical technology refers to all processes that involve light, lenses and other devices dealing with transmission of light.

Orange Book

The Recordable Compact Disc Standard was published by Philips, in 1990, reportedly in a binder with Orange Covers. The Orange Book defined two new 12cm CD products: the Magneto-Optical and the Write-Once. Recently, Part 3 was released, which covers Rewritable (Phase Change) products. Part 1, Magneto-Optical (CD-MO), defines tracks that can be erased and rewritten--reason why this format is more appropriately known as Rewritable. M-O drives implement magneto- optical recording technology, on 12cm CDs that are rated to allow millions of rewrites. These drives are however slower than other optical drives, because they use two heads--one to erase and the other to write, in a double-pass process. Some CD-MO products include a small premastered Read-Only area that usually contains system and other information--but which can also be read by a regular CD-ROM drive. The remainder space is the Recordable User Area, and the user can reuse this area at will. Part 2, Write-Once (CD-WO), defines tracks that can be written to, but not erased and rewritten--in the tradition of WORM (write-once read-many) discs. A Write-Once drive records appropriate 12cm CDs--which involve special recording layers, pregrooved tracks and, generally, a gold reflective layer. The initial tracks include a Program Calibration Area, are followed by a Lead-In area (where the Table of Contents will be written), and by the Program Area--for the user data. The recording session is finished with the Lead Out. A CD-WO 'Hybrid' disc involves an area where Read-Only files can be placed, and the rest of the disc is the W-O area. Part 3, Rewritable (CD-RW). Some brands designate it as Rewritable PD (for Phase Change). Developed by Philips and Sony (Oct 96), these specifications implement Phase Change technology and the Universal Disc Format(UDF) promoted by OSTA, to produce a CD that can be rewritten in one pass. Currently, CD-RW can not be read by CD-ROM and CD-R drives, because CD-RW media has much lower coefficients of reflectivity (15-25 compared to 65-70%). But, while drives with multiple heads are considered one solution, the industry is working towards a 'single-head multiread drive.'

PAL

Phase Alternation Line, a television standard, is used by European, Asian and some Latin American Countries. It specifies 768 pixels/line, 576 lines/screen and 25 frames/sec.

PCM

Pulse Code Modulation is used to sample analog audio into digital code (generally, 8000 samples/sec), and to structure the analog signal that is produced by the digital-analog converter of the CD-Audio player. PCM makes it possible to hear the various instruments, their different ranges and depth of sound, etc.

Phase-Change Technology

This rewritable technology employs a recording layer that shifts phase, from amorphous to crystalline. A pulse from the 'write' laser beam changes a spot to the amorphous state. To 'erase', a laser beam of higher power heats up the area and, essentially, melts it--which then cools to the crystalline state, and is therefore ready to be written again. Since phase-change made possible 'single pass' erasing and rewriting, vendors claim that phase-change drives provide faster operation than M-O drives, and that its slight advantage in storage capacity will play a bigger role when discs of over one Gigabyte become common. Some industry magazines claim that it has already reached about 30 percent of the rewritable market, challenging the popular M-O drives. In 1995, Panasonic introduced a Phase-Change multifunction drive. In 1966, phase change drives, drives in CD-Rewritable format, compliant with Orange Book, part 3, were introduced by Panasonic, Pioneer, Pinnacle Micro, and others.

Photo-CD

This product, was introduced by Kodak and Philips, in 1992. The Photo CD is a hybrid disc that uses the CD-ROM XA Form 1 sector structure to store up to 100 35mm photographs in one disc, in one or more sessions. The photographs are scanned into digital files (18 MBytes--compressed to about 4.5 MBytes, each), in five different resolutions. The Kodak Photo CD player displays on a TV monitor, but a multi-session CD-ROM XA drive, with appropriate software, can display on a PC monitor. When issued as a Bridge Disc, it can be played by Photo CD and CD-I drives connected to a TV set. Photo-CD allows multisession recording, with one TOC per session (Orange Book, Part II, Hybrid disc). CD-ROM XA players need an appropriate interface (or a software patch) to display multi-session contents. Kodak also licensed a consumer product that takes film rolls (or color photos), and processes them into a Photo CD. 

Pits

During optical encoding, pulses of a high power laser beam 'burn' microscopic 'pits' on the recording layer. The untouched spaces between such pits are called 'lands.' During the read process, the laser light focuses on the spinning spiral track, and since the pits reflect light less intensely, the read head detects the changes in reflectivity, and those changes are processed as 1s to produce a binary data stream. In CD-ROM, the track pitch is 1.6 microns, and the pits are .83 microns wide. In DVD, the track pitch is 0.74 microns, and the pits are 0.4 microns wide.

Pixel

A Picture Element, used mainly in graphics and video circles, is the smallest unit of display that can be given color and intensity values or codes. The larger the number of bits per pixel, the higher the range of colors that can be displayed.

Red Book

Philips and Sony, developers of the CD technology, and of the 12cm CD, published their specifications for CD-Audio in 1980-- reportedly in a binder with red covers. The Red Book addressed the physical specifications for the CD; the tracks, the sector and block layout, coding and sampling of digital audio files, and other specifications. The Red Book was key for the high quality sound of CD-Audio, which became a standard and key for the worldwide CD-ROM industry. The International Electrotechnical Commission published the Red Book as their Doc IEC 908 (1987).

Reed-Solomon Codes

These are error detection and error correction codes, based on mathematical algorithms and binary structural logic. The Red Book implemented the basic two levels of error detection and correction using Cross Interleaved Reed Solomon Codes (CIRC). The Yellow Book specified a third level of 'layered' error detection and error correction codes, to attain the level of integrity that computer data require. DVD implements the Reed- Solomon Product Code (RSPC), which is said to be about ten times effective. 

Reed-Solomon Product Code

The RSPC is a compression algorithm that expands the Reed-Solomon Cyclic Redundance compression algorithm by generating a product (row x colums) as a final code. This product code algorithm is said to be ten times more robust than the CIRC algorithms used in CD-ROM.

Reflectivity

A measurable property of a surface. In optical technology, baseline reflectivity refers to the reflectivity of the 'lands'-- the clear spaces between the pits in the data track. The pits have lower than baseline reflectivity. In optical discs, the changes in reflectivity are detected and decoded, and then converted to magnetic coding. The differences in reflectivity between rewritable and mass reproduced and/or one-off discs can be substantial (15-25% compared to 65-70%). In fact, CD-RW media (Orange Book, III) can not be read by CD-ROM players (without additional hardware fixes). In the same vein, DVD players have wavelength problems with CD-R, because CD-R media does not reflect at all the DVD laser beam. MultiRead devices overcome this problem.

Regional Codes

Also referred to as Country Codes, or Zone Locks, these are optional codes that the movie industry imposed on DVD-Video specifications. Players will be coded by region, and these players will not play DVDs coded for a different region. It is expected that only first release DVD-Video titles will have those codes. DVDs with no regional codes will be played by any DVD player. The six regional codes are: 1: North America, 2: Japan, Europe, Middle East, South Africa, 3: Southeast Asia (plus Hong Kong), 4: Australia, New Zealand, Central & South America, 5: Northwest Asia, North Africa, and 6: China.

Resolution

In general, display devices support standard resolutions (EGA, VGA, XVGA, etc.), which specify the number of pixels of the width and height of the screen (i.e. 640x480). Video resolution, however, usually includes a third element: the quality or depth of the pixel (in bits/pixel). For example, 640x480x24 means a VGA resolution (640x480) by 24 bits/pixel--which means each pixel has over 16 million color possibilites.

Rotation

CD and DVD use clockwise rotation. All single layer discs are read from the inside out. In double-layer discs, DVD reads both layers from the same side, and there are two ways of doing so. Opposite track reading is when the top or outside layer (layer 0) is read radially from the inside out, and the inside layer (layer 1) is read radially from the outside in--after transfering at the transition area. That is why the tracks are 'running' in opposite directions. But, this implementation is used to provide for reading continuity--which is important for video applications. Parallel track reading is when the layers can be read non-sequentially during a session--tracks 'running in the same direction.' For text and data applications, the parallel track layout is preferred, because it allows random access to data anywhere in the tracks.

Sampling

Sampling is part of analog to digital conversion. Essentially, the analog signal is sampled at an specific rate and quantized--which means a numerical value is matched to each sample, and that value is converted to binary code. Although the frequency of sampling is important for continuity, the size of the sample (in bits) is important for depth of quality. CD-Audio involves sampling rate of 44.1 KHz, and sample size of 16 bits.

Substrate

This is the core of CDs and DVDs. In the injection molding machines, it starts as molten, clear polycarbonate. After pressing and cooling, the core disc or substrate is metallized, given a lacquer protective coating, and labelled. CD-Recordable media has the same substrate, but different recording layers on it. In optical 5.25in W-O and M-O media, the substrate is often glass. DVD Recordable and RAM (rewritable) will apparently use polycarbonate substrate. There have been tests on other materials, one of them, polyolyphine. But, despite better qualities shown by other materials, it seems that polycarbonate is still the cost-effective option.

Track

Optical technology uses Constant Linear Velocity rotation, which involves a spiral track of coding that begins near the center of the disc. In CD-ROM, the track has a pitch of about 1.6 microns and, in a 63-minute disc, it is about 3 miles long. DVD specifies a track of 0.74 microns. The pits in a CD-ROM track are 0.83 microns wide, and in a DVD they are 0.4 microns wide. At another level, in mixed-mode or multimedia applications, we say that data types are in 'separate' tracks--but they are placed sequentially in the same physical track, and accessed using appropriate interleaving.

VESA

The industry group, Video Electronic Standards Association, produced a non-proprietary response to IBM's Microchannel architecture. They did so with the VESA Local Bus architecture--known as the VL Bus. It is a 32-bit bus, with a maximum bandwidth of 132 MBytes per second. It was designed to aid high speed video devices. The VL Bus Interface (additional circuitry and chips) extends the CPU bus, and thus can interact directly with the CPU and memory. However, In the 486 platform, this 32-bit interface was usually limited to only three VL Bus peripherals--and only two of them could be add-on boards. The power in the Pentium architecture, and the PCI bus, have reduced the need for the VESA LB.

Wavelength

Laser wavelengths (usually in nanometers) define precision; the shorter the wavelength, the more precise the laser. The infrared laser used in CD-ROM has a wavelength or 780 nm. The red lasers in DVD have wavelengths of 635 and 650 nm (double layer disc).