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Technical Glossary
Glossery of Video & Broadcast Terms
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.
ADC (A-D, A/D, A-to-D): Analog to Digital Conversion. Also referred to as digitization or quantization. The conversion of an analog signal into the digital data representation of that signal-normally for subsequent use in a digital machine. For TV, samples of audio and video are taken, the accuracy of the process depending on both the sampling frequency and the resolution of the analog amplitude information-how many bits are used to describe the analog levels. For TV pictures eight or 10-bits are normally used; for sound, 16 or 20-bits are common, and 24-bits are being introduced. The ITU-R 601 standard defines the sampling of video components based on 13.5 MHz, and AES/EBU defines sampling of 44.1 and 48 kHz for audio.
For pictures, the samples are called pixels, each containing data for brightness and color.
See also: Binary, Bit.
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.
ASCII: American Standard Code for Information Interchange. A standard code for transmitting data, consisting of 128 letters, numerals, symbols, and special codes each of which is represented by a unique binary number.
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.
Asynchronous: Lacking synchronization. In video, a signal is asynchronous when its timing differs from that of the system reference signal. A foreign video signal is asynchronous before it is treated by a local frame synchronizer.
ATM: Asynchronous Transfer Mode. A data transmission scheme using self-routing packets of 53 bytes, 48 of which are available for user data. Typically 25 and 155 Mbps, with 622 Mbps coming soon-the latter of which could be used to carry non-compressed ITU-R 601 video as a data file.
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: A signaling technique in which the signal is transmitted in its original form and not changed by modulation. Local Area Networks as a whole, fall into two categories: baseband and broadband. Baseband networks are simpler and cheaper; the entire bandwidth of the LAN cable is used to transmit a single digital signal. In broadband networks, the capacity of the cable is divided into channels, which can transmit many simultaneous signals. Broadband networks may transmit a mixture of digital and analog signals, as will be the case in hybrid fiber/coax interactive cable television networks.
Baud: A unit of signaling speed equal to the number of signal events per second. Baud is equivalent to bits per second in cases where each signal event represents exactly one bit. Often the term baud rate is used informally to mean baud, referring to the specified maximum rate of data transmission along an interconnection. Typically, the baud settings of two devices must match if the devices are to communicate with one another.
Binary: A base-2 numbering system using the digits 0 and 1 (as opposed to 10 digits [0 - 9] in the decimal system). In computer systems, the binary digits are represented by two different voltages or currents, one corresponding to 0 and the other corresponding to 1. All computer programs are executed in binary form.
Binary representation requires a greater number of digits than the base 10 decimal system more commonly used. For example, the base 10 number 254 is 11111110 in binary.
The result of a binary multiplication contains the sum of digits of the original numbers. So:
10101111 x 11010100 = 1001000011101100
(In decimal 175 x 212 = 37,100)
(From right to left, the digits represent 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768)
Each digit is known as a bit. This example multiplies two 8-bit numbers to produce a 16-bit result-a very common process in digital television equipment.
See also: Bit, Byte, Digital.
Byte: A group of data bits that are processed together. Typically, a byte consists of 8, 16, 24 or 32 bits.
1 Byte = 8 bits = 256 discrete values (brightness, color, etc.)
1 kilobyte = 210 bytes = 1,024 bytes: (not 1000 bytes)
1 Megabyte = 220 bytes = 1,048,576 bytes: (not 1 million bytes)
1 Gigabyte = 230 bytes = 1,073,741,824 bytes: (not one billion bytes)
1 Terabyte = 240 bytes = 1,099,511,627,776 bytes: (not one trillion bytes)
A full frame of digital television, sampled according to ITU-R 601, requires just under 1 Mbyte of storage (701 kbytes for 525 lines, 829 kbytes for 625 lines). HDTV frames are 4-to-5 times as large and digital film frames may be that much larger again.
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.
Cliff effect: An RF characteristic that causes DTV reception to change dramatically with a small change in power. At the fringes of reception, current analog TV pictures degrade by becoming "snowy." With DTV, relatively small changes in received power in weak signal areas will cause the DTV picture to change from perfect to nothing and hence the name: cliff effect.
Clock frequency: The master frequency of periodic pulses that are used to synchronize the operation of equipment.
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.
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 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.
dB (decibel): A measure of voltage, current, or power gain equal to 1/10 of a bel. Given by the equations 20 log Vout/Vin, 20 log Iout/In, or 10 log Pout/Pin.
see also:Bel.
Digital: Circuitry in which data carrying signals are restricted to either of two voltage levels, corresponding to logic 1 or 0. A circuit that has two stable states: high or low, on or off.
DS3: A terrestrial and satellite format for transmitting information digitally. DS3 has a capacity of 672 voice circuits at a transmission speed of 44.736 Mbps (commonly referred to as 45 Mbps). DS3 is used for digital television distribution using mezzanine level compression-typically MPEG-2 in nature, decompressed at the local station to full bandwidth signals (such as HDTV) and then re-compressed to the ATSC's 19.39 Mbps transmission standard.
Ethernet (IEEE 802.3): A type of high-speed network for interconnecting computing devices. Ethernet can be either 10 or 100 Mbps (Fast Ethernet). Ethernet is a trademark of Xerox Corporation, Inc.
GIF (pronounced jif): Graphics interchange format. A computer graphics file format developed by CompuServe for use in compressing graphic images, now commonly used on the Internet. GIF compression is lossless, supports transperancy, but allows a maximum of only 256 colors. Images that will gain the most from GIF compression are those which have large areas (especially horizontal area) with no changes in color.
HDTV: High definition television. The 1,125-, 1,080- and 1,035-line interlace and 720 and 1,080-line progressive formats in a 16:9 aspect ratio. Officially a format is high definition if it has at least twice the horizontal and vertical resolution of the standard signal being used. There is a debate as to whether 480-line progressive is also high definition. It is the opinion of the editors that 480-line progressive is not an HDTV format, but does provide better resolution than 480-line interlace, making it an enhanced definition format.
HFC: Hybrid fiber coax. A type of network that contains both fiber-optic cables and copper coaxial cables. The fiber-optic cables carry TV signals from the head-end office to the neighborhood; the signals are then converted to electrical signals and then go to coaxial cables.
Huffman coding: This compresses data by assigning short codes to frequently-occurring sequences and longer ones to those less frequent. Assignments are held in a Huffman Table. The more likely a sequence is to occur the shorter will be the code that replaces it. It is widely used in video compression systems where it often contributes a 2:1 reduction in data.
Interlaced: Short for interlaced scanning. Also called line interlace. A system of video scanning whereby the odd- and even-numbered lines of a picture are transmitted consecutively as two separate interleaved fields. Interlace is a form of compression.
Java: A general purpose programming language developed by Sun Microsystems and best known for its widespread use on the World Wide Web. Unlike other software, programs written in Java can run on any platform type (including set-top boxes), as long as they contain a Java Virtual Machine.
Internet: java.sun.com.
see also:Windows CE.
Jitter: An undesirable random signal variation with respect to time.
JPEG: Joint Photographic Experts Group. ISO/ITU-T. JPEG is a standard for the data compression of still pictures (intrafield). In particular its work has been involved with pictures coded to the ITU-R 601 standard. JPEG uses DCT and offers data compression of between two and 100 times and three levels of processing are defined: the baseline, extended and "lossless" encoding.
see also:Motion-JPEG.
Mezzanine compression: Contribution level quality encoded high definition television signals. Typically split into two levels: High Level at approximately 140 Mbps and Low Level at approximately 39 Mbps (for high definition within the studio, 270Mbps is being considered). These levels of compression are necessary for signal routing and are easily re-encoded without additional compression artifacts (concatenation) to allow for picture manipulation after decoding. DS3 at 44.736 will be used in both terrestrial and satellite program distribution.
Modem: Modulator/demodulator. A device that transforms a typical two-level computer signal into a form suitable for transmission over a telephone line. Also does the reverse-transforms an encoded signal on a telephone line into a two-level computer signal.
MPEG: Compression standards for moving images conceived by the Motion Pictures Expert Group, an international group of industry experts set up to standardize compressed moving pictures and audio. MPEG-2 is the basis for ATSC digital television transmission.
Its work follows on from that of JPEG to add interfield compression, the extra compression potentially available through similarities between successive frames of moving pictures. Four MPEG standards were originally planned, but the accommodation of HDTV within MPEG-2 has meant that MPEG-3 is now redundant. MPEG-4 is intended for unrelated applications, however, can be used to display ATSC formats on a PC. The main interest for the television industry is in MPEG-1 and MPEG-2. A group of picture blocks, usually four, which are analyzed during MPEG coding to give an estimate of the movement between frames. This generates the motion vectors that are then used to place the macroblocks in decoded pictures.
see also:B frames, GoP, I frames, P frames.
MPEG-1 : A group of picture blocks, usually four, which are analyzed during MPEG coding to give an estimate of the movement between frames. This generates the motion vectors that are then used to place the macroblocks in decoded pictures. This was designed to work at 1.2 Mbps, the data rate of CD-ROM, so that video could be played from CDs. However the quality is not sufficient for TV broadcast.
MPEG-2: This has been designed to cover a wide range of requirements from "VHS quality" all the way to HDTV through a series of algorithm "profiles" and image resolution "levels." With data rates of between 1.2 and 15 Mbps, there is intense interest in the use of MPEG-2 for the digital transmission of television-including HDTV-applications for which the system was conceived. Coding the video is very complex, especially as it is required to keep the decoding at the reception end as simple and inexpensive as possible. MPEG-2 is the compression used by the ATSC and DVB standards. MPEG can offer better quality pictures at high compression ratios than pure JPEG compression, but with the complexity of decoding and especially coding and the 12-long group of pictures (GoP), it is not an ideal compression system for editing. If any P or B frames are used then even a cut will require the re-use of complex, and not perfect, MPEG coding. However, MPEG Splicers are beginning to appear to alleviate this difficulty.
Of the five profiles and four levels creating a grid of 20 possible combinations, 11 have already been implemented. The variations these define are so wide that it would not be practical to build a universal coder or decoder. Interest is now focused on the Main profile, Main level, sometimes written as MP@ML, which covers broadcast television formats up to 720 pixels x 576 lines at 30 frames per second. These figures are quoted as maximums so 720 x 486 at 30 frames are included, as are 720 x 576 at 25 frames. As the coding is intended for transmission the economy of 4:2:0 sampling is used.
A recent addition to MPEG-2 is the studio profile. Designed for studio work its sampling is 4:2:2. The studio profile is written as 422P@ML. To improve the picture quality, higher bit rates are used. The first applications for this appear to be in electronic news gathering (ENG), and with some video servers.
see also:B frames, Compression, GoP, I frames, JPEG, P frames.
Nyquist frequency (Nyquist rate) : The lowest sampling frequency that can be used for analog-to-digital conversion of a signal without resulting in significant aliasing. Normally, this frequency is twice the rate of the highest frequency contained in the signal being sampled.
OC3: Optical Carrier Level 3. A 155 Mbps ATM SONET signal stream that can carry three DS3 signals.
Progressive: Short for progressive scanning. A system of video scanning whereby lines of a picture are transmitted consecutively, such as in the computer world.
QAM: Quadrature amplitude modulation. A downstream digital modulation technique that conforms to the International Telecommunications Union (ITU) standard ITU-T J. 83 Annex B which calls for 64 and 256 quadrature amplitude modulation (QAM) with concatenated trellis coded modulation, plus enhancements such as variable interleaving depth for low latency in delay sensitive applications such as data and voice. Using 64 QAM, a cable channel that today carries one analog video channel could carry 27 Mbps of information, or enough for multiple video programs. Using 256 QAM, the standard 6 MHz cable channel would carry 40 Mbps.
see also:The Engineering & Transmission chapter.
QuickTime: Apple Computer's system-level software architecture supporting time-based media, giving a seamless integration of video, sound, and animation. For Macintosh and Windows computers.
RGB: The abbreviation for the red, green and blue signals, the primary colors of light (and television). Cameras and telecines have red, blue and green receptors, the TV screen has red, green and blue phosphors illuminated by red, green and blue guns. Much of the picture monitoring in a production center is in RGB. RGB is digitized with 4:4:4 sampling which occupies 50 percent more data than 4:2:2.
Ringing: An oscillatory transient on a signal occurring as a result of bandwidth restrictions and/or phase distortions. A type of ringing causes ghosting in the video picture.
RS-232: A standard, single-ended (unbalanced) interconnection scheme for serial data communications.
SCSI: Small computer systems interface. A very widely used high data rate general purpose parallel interface. A maximum of eight devices can be connected to one bus, for example a controller, and up to seven disks or devices of different sorts-Winchester disks, optical disks, tape drives, etc.-and may be shared between several computers.
SCSI specifies a cabling standard (50-way), a protocol for sending and receiving commands and their format. It is intended as a device-independent interface so the host computer needs no details about the peripherals it controls. But with two versions (single ended and balanced), two types of connectors and numerous variations in the level of implementation of the interface, SCSI devices cannot "plug and play" on a computer with which they have not been tested. Also, with total bus cabling for the popular single ended configuration limited to 18 feet (6 meters), all devices must be close.
Serial: One bit at a time, along a single transmission path.
Serial digital: Digital information that is transmitted in serial form. Often used informally to refer to serial digital television signals.
SMPTE 292M: The SMPTE standard for bit-serial digital interface for high-definition television systems.
Widescreen: Term given to picture displays that have a wider aspect ratio than normal. For example TV's normal aspect ratio is 4:3 and widescreen is 16:9. Although this is the aspect ratio used by HDTV, widescreen is also used with normal definition systems.
WYSIWYG: What you see is what you get-usually, but not always. Referring to the accuracy of a screen display to show how the final result will look. For example a word processor screen showing the final layout and typeface that will appear from the printer.
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