Glossary A-C

3:2 pull-down
Method used to map the 24 fps of film onto the 30 fps (60 fields) of 525-line TV, so that one film frame occupies three TV fields, the next two, etc. It means the two fields of every other TV frame come from different film frames making operations such as rotoscoping impossible, and requiring care in editing. Some sophisticated equipment can unravel the 3:2 sequence to allow frame-by-frame treatment and subsequently re-compose 3:2. The 3:2 sequence repeats every five TV frames and four film frames, the latter identified as A-D. Only film frame A is fully on a TV frame and so exists at one time code only, making it the editable point of the video sequence.

4:1:1
This is a set of sampling frequencies in the ratio 4:1:1, used to digitize the luminance and color difference components (Y, R-Y, B-Y) of a video signal. The four represents 13.5 MHz, the sampling frequency of Y, and the ones each 3.75 MHz for R-Y and B-Y.

With the color information sampled at half the rate of the 4:2:2 system, this is generally used as a more economical form of sampling for 525-line picture formats. Both luminance and color difference are still sampled on every line. But the latter has half the horizontal resolution of 4:2:2, while the vertical resolution of the color information is maintained. For 525-line pictures, this means the color is fairly equally resolved in horizontal and vertical directions.

4:2:0
A sampling system used to digitize the luminance and color difference components (Y, R-Y, B-Y) of a video signal. The four represents the 13.5 MHz sampling frequency of Y, while the R-Y and B-Y are sampled at 6.75 MHz--effectively between every other line only (one line is sampled at 4:0:0, luminance only, and the next at 4:2:2).

This is generally used as a more economical system than 4:2:2 sampling for 625-line formats so that the color signals have a reasonably even resolution in the vertical and horizontal directions for that format.

4:2:2
A commonly used term for a component digital video format.

A ratio of sampling frequencies used to digitize the luminance and color difference components (Y, R-Y, B-Y) of a video signal. It is generally used as shorthand for ITU-R 601. The term 4:2:2 describes that for every four samples of Y, there are two samples each of R-Y and B-Y, giving more chrominance bandwidth in relation to luminance compared to 4:1:1 sampling.

ITU-R 601, 4:2:2 is the standard for digital studio equipment and the terms "4:2:2" and "601" are commonly (but technically incorrectly) used synonymously. The sampling frequency of Y is 13.5 MHz and that of R-Y and B-Y is each 6.75 MHz providing a maximum color bandwidth of 3.37 MHz--enough for high-quality chromakeying. The format specifies eight bits of resolution.

The details of the format are specified in the ITU-R BT.601-2 standard document. See also: ITU-R BT.601-2.

4:2:2:4
Same as 4:2:2, but with the addition of a key channel that is sampled four times for every four samples of the luminance channel.

4:4:4
Similar to 4:2:2, except that for every four luminance samples, the color channels are also sampled four times.

4:4:4:4
Similar to 4:2:2:4, except that for every four luminance samples, the color and key channels are also sampled four times.

601
See: ITU-R BT.601-2.

AES
Audio Engineering Society that promotes standards in the professional audio industry.

AES/EBU
Informal name for a digital audio standard established jointly by the AES (Audio Engineering Society) and EBU (European Broadcasting Union) organizations. The sampling frequencies for this standard vary depending on the format being used; the sampling frequency for D1 and D2 audio tracks is 48 kHz.

Algorithm
A formula or set of steps used to simplify, modify, or predict data. Complex algorithms are used to selectively reduce the high digital audio and video data rates. These algorithms utilize physiologists´ knowledge of hearing and eyesight. For example, we can resolve fine detail in a still scene, but our eye cannot resolve the same detail in a moving scene. Using knowledge of these limitations, algorithms are formulated to selectively reduce the data rate without affecting the viewing experience. See also: Compression, MPEG.

Aliasing
Defects or distortion in a television picture. In analog video, aliasing is typically caused by interference between two frequencies such as the luminance and chrominance frequencies or the chrominance and field scanning frequencies. It appears as moirŽ or herringbone patterns, straight lines that become wavy, or rainbow colors. In digital video, aliasing is caused by insufficient sampling or poor filtering of the digital video. Defects are typically seen as jagged edges on diagonal lines and twinkling or brightening (beating) in picture detail.

Analog
1. An adjective describing any signal that varies continuously as opposed to a digital signal, which contains discrete levels. 2. A system or device that operates primarily on analog signals.

Analog-to-Digital Conversion (ADC)
Process of converting analog signals to a digital representation. DAC represents the reverse translation.

Archive
Off-line storage of video/audio onto backup tapes, floppy disks, optical disks, etc.

Artifacts
Undesirable elements or defects in a video picture. These may occur naturally in the video process and must be eliminated in order to achieve a high-quality picture. Most common in analog are cross color and cross luminance. Most common in digital are macroblocks, which resemble pixelation of the video image.

Aspect ratio
The ratio of television picture width to height. In NTSC and PAL video, the present standard is 4:3. In widescreen video, it is typically 16:9, however, 14:9 has been used as a transition.

ATSC
Advanced Television Systems Committee. Formed to establish technical standards for advanced television systems, including digital high definition television (HDTV). ATSC Formats are 18 voluntary video formats, known as Table 3.

The U.S. digital television transmission standard using MPEG-2 compression and the audio surround-sound compressed with Dolby Digital (AC-3). So that a wide variety of source material, including that from computers, can be best accommodated, two line standards are included--each operating at 24, 30, and 60 Hz.

The Consumer Electronics Manufacturers Association (CEMA) has said that all receivers will be capable of operating with all of the formats.

All pixels are square and pixel sampling rates vary, but all are around 75 MHz. There is a Transport Layer that packages video, audio and auxiliary data and allows their mix to be dynamically varied--opening the door to new services and forms of programming (e.g., many channels of stereo audio, distribution of computer software, or very high resolution images). The data is compressed to 19.39 Mbits per second and delivered using a 6 MHz bandwidth channel. HD and SD assignments are per ATSC announcement on February 20, 1998.

Note that 1,088 lines are actually coded in order to satisfy the MPEG-2 requirement that the coded vertical size be a multiple of 16 (progressive scan) or 32 (interlaced scan).

See also: HD0, HD1, HD2, MPEG-2, HDTV.

AVI
Audio video interleaving. The Microsoft Video for Windows file format for combining video and audio into a single block in time such as a 1/30th second video frame. In this file format, blocks of audio data are woven into a stream of video frames. ASF is intended to supersede AVI.

AVO
Audiovisual object. In MPEG-4, audiovisual objects (also AV objects) are the individual media objects of a scene--such as video objects, images, and 3D objects. AVOs have a time dimension and a local coordinate system for manipulating the AVO are positioned in a scene by transforming the object´s local coordinate system into a common, global scene coordinate system.

B frames
Bi-directional predictive frames used in the MPEG-2 signal. These are composed by assessing the difference between the previous and the next frames in a television picture sequence. As they contain only predictive information, they do not make up a complete picture and so have the advantage of taking up much less data than the I frames. However, to see that original picture requires a whole sequence of MPEG-2 frames to be decoded.

See also: I frames, P frames, MPEG.

Bandwidth
1. The complete range of frequencies over which a circuit or electronic system can function with minimal signal loss, typically less than 3 dB. 2. The information-carrying capability of a particular television channel. In PAL systems, the bandwidth limits the maximum visible frequency to 5.5 MHz, in NTSC, 4.2 MHz. The ITU-R 601 luminance channel sampling frequency of 13.5 MHz was chosen to permit faithful digital representation of the PAL and NTSC luminance bandwidths without aliasing. In transmission, the United States analog and digital television channel bandwidth is 6 MHz.

Baseband
The basic direct output signal in an intermediate frequency based obtained directly from a television camera, videoconference television receiver, or video tape recorder. Baseband signals can be viewed only on studio monitors. To display the baseband signal on a conventional television set a "modulator" is required to convert the baseband signal to one of the VHF or UHF television channels which the television set can be tuned to receive. Bit rate reduction: See: Compression.

Broadband
1. A response that is the same over a wide range of frequencies. 2. Capable of handling frequencies greater than those required for high-grade voice communications (higher than 3 to 4 kilohertz).

The term applied to networks having bandwidths significantly greater than that found in telephony networks. Broadband systems are capable of carrying a large number of moving images or a vast quantity of data simultaneously. Broadband techniques usually depend on coaxial or optical cable for transmissions. They utilize multiplexing to permit the simultaneous operation of multiple channels or services on a single cable. Frequency division multiplexing or cell relay techniques can both be used in broadband transmission.

Broadcast FTP Protocol (BFTP)
A one-way IP multicast based resource transfer protocol, the unidirectional Broadcast File Transfer Protocol (BFTP) is a simple, robust, one-way resource transfer protocol that is designed to efficiently deliver data in a one-way broadcast-only environment. This transfer protocol is appropriate for IP multicast over television vertical blanking interval (IPVBI), in IP multicast carried in MPEG-2, like with the DVB multiprotocol encapsulation, or in other unidirectional transport systems. It delivers constant bitrate (CBR) services or opportunistic services, depending on the characteristics and features of the transport stream multiplexor or VBI insertion device.

Bit Rate
The speed of a digital transmission, measured in bits per second.

bps
Bits per second, a unit of measurement of the speed of data transmission and thus of bandwidth.

Broadcast
A packet delivery system where a copy of given packet is given to all hosts attached to the network. Example: Ethernet.

Brouter
Concatenation of "bridge" and "router". Used to refer to devices which perform both bridging and routing functions.

Byte
A group of eight bits; usually the smallest addressable unit of information in a data memory storage unit.

Cable modem
A data modem that uses the bandwidth of a given cable system, which promise speeds of up to 80 times faster than an ISDN line or six times faster than a dedicated T1 line (the type of connection most large corporations use). Because cable modems provide Internet access over cable TV networks (which rely primarily on fiber optic or coaxial cable), they are much faster than modems that use phone lines. Bandwidths are typically up to 30 Mbps in the downstream direction.

Cache
Local or temporary storage.

CCIR
ComitŽ Consultatif International des Radiocommunications (International Radio Consultative Committee), an international standards committee no longer in operation and replaced by the International Telecommunications Union (ITU).

CCIR-601
See: ITU-R BT.601-2.

CIF
Common Intermediate Format, an international standard for video display formats developed by TSS. The QCIF format, which employs half the CIF spatial resolution in both horizontal and vertical directions, is the mandatory H.261 format. QCIF is used for most desktop videoconferencing applications where head and shoulder pictures are sent from desk to desk. QCIF displays 176 pixels grouped in 144 non-interlaced luminance lines.

Cinepak
A high-quality medium bandwidth compression that is not real-time but can play back in software. Its 24-bit format produces high-quality video at 320 x 240 resolution and 15 frames per second at a 150 Kbps data rate. Commonly a CD-ROM solution developed a number of years ago and not a competitor to more modern techniques.

Codec
Coder-decoder. A device that converts analog video and audio signals into a digital format for transmission over telecommunications facilities and also converts received digital signals back into analog format.

Color depth
The number of bits used to represent the color of a pixel and thus how many colors can be displayed. Color depth is typically 8-, 16-, or 24-bit. 8-bit would give 256 colors. A high color pixel requires at lest 24-bit color (1.1164 billion colors).

Color space
The color range between specified references. Typically references are quoted in television: RGB, Y, R-Y, B-Y, YIQ, YUV and Hue Saturation and Luminance (HSL). In print, Cyan, Magenta, Yellow and Black (CMYK) are used. Moving pictures between these is possible but requires careful attention to the accuracy of processing involved. Operating across the media--print, film and TV, as well as between computers and TV equipment--will require conversions in color space.

Color space conversion
The translation of color value form one color space to another. Since different media types, like video and computer graphics, use different color spaces, color space is often performed on the fly by graphics hardware.

Combiner
In digital picture manipulators, a device that controls the way in which two or more channels work together. Under software control, it determines the priority of the channels (which picture appears in front and which in back) and the types of transitions that can take place between them.

Component (video)
The normal interpretation of a component video signal is one in which the luminance and chrominance remain as separate components, such as analog components in MII and Betacam VTRs, digital components Y, B-Y, R-Y(Y, Cr, Cb) in ITU-R 601. RGB is also a component signal. Component video signals retain maximum luminance and chrominance bandwidth.

Component digital
A digital representation of a component analog signal set, most often Y, B-Y, R-Y. The encoding parameters are specified by ITU-R BT.601-2 (CCIR 601). The parallel interface is specified by ITU-R BT.656 (CCIR 656) and SMPTE 125M.

Composite (video)
Luminance and chrominance are combined along with the timing reference "sync" information using one of the coding standards--NTSC, PAL or SECAM--to make composite video. The process, which is an analog form of video compression, restricts the bandwidths (image detail) of components. In the composite result color is literally added to the monochrome (luminance) information using a visually acceptable technique. As our eyes have far more luminance resolving power than for color, the color sharpness (bandwidth) of the coded single is reduced to far below that of the luminance. This provides a good solution for transmission but it becomes difficult, if not impossible, to accurately reverse the process (decode) into pure luminance and chrominance which limits its use in post production.

Composite digital
A digitally encoded video signal, such as NTSC or PAL video, that includes horizontal and vertical synchronizing information.

Compress
A digital picture manipulator effect where the picture is squeezed (made proportionally smaller).

Compressed serial digital interface (CSDI)
A way of compressing digital video for use on SDI-based equipment proposed by Panasonic. Now incorporated into Serial digital transport interface. See: Serial digital transport interface.

Compression Reduction of the size of digital data files by removing redundant information (lossless) or removing non-critical data (lossy). Pictures are analyzed looking for redundancy and repetition and so discard unnecessary data. The techniques were primarily developed for digital transmission but have been adopted as a means of handling digital video in computers and reducing the storage demands for digital VTRs. Compression can be at either a set rate or a variable rate. Also known as Bit Rate Reduction (BRR)

Compression artifacts
Compacting of a digital signal, particularly when a high compression ratio is used, may result in small errors when the signal is decompressed. These errors are known as "artifacts," or unwanted defects. The artifacts may resemble noise (or edge "busyness") or may cause parts of the picture, particularly fast moving portions, to be displayed with the movement distorted or missing.

Compressionist
One who controls the compression process to produce results better than would be normally expected from an automated system.

Compression ratio
The ratio of the data in the non-compressed digital video signal to the compressed version. Modern compression techniques start with the ITU-R 601 component digital television signal so the amount of data of the non-compressed video is well defined--76 Gbytes/hour for the 525/60 standard and 75 Gbytes/hour for 625/50.

The compression ratio should not be used as the only method to assess the quality of a compressed signal. For a given technique greater compression can be expected to result in worse quality but different techniques give widely differing quality of results for the same compression ratio. The only sure method of judgment is to make a very close inspection of the resulting pictures.