Glossary
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## Glossary

active video

  The portion of the video signal having the chrominance or luminance information; also, all video lines not in the vertical blanking signal that contain the chrominance or luminance information. See also chrominance, composite video, horizontal blanking interval, luminance, and video waveform.

aliasing

  A rendering technique that assigns to pixels the color of the primitive being rendered, regardless of whether that primitive covers all of the pixel's area or only a portion. This results in jagged edges, or “jaggies”. In video systems, aliasing results when an image is sampled that contains frequency components above the Nyquist limit for the sampling rate. See also Nyquist limit.

antialiasing

  A rendering technique that assigns pixel colors based on the fraction of the pixel's area that's covered by the primitive being rendered. Antialiased rendering reduces or eliminates the jaggies that result from aliased rendering.

artifact

  In video systems, an unnatural or artificial effect that occurs when the system reproduces an image; examples are aliasing, pixellation, and contouring.

aspect ratio

  The ratio of the width to the height of an image. For example, the standard aspect ratio for television is 4:3. Maintaining the original aspect ratio of an image prevents it from being distorted.

back porch

  The portion of the horizontal pedestal that follows the horizontal synchronizing pulse. In a composite signal, the color burst is located on the back porch, but is absent on a YUV or GBR signal. See also blanking level and video waveform.

bit map

  A region of memory that contains the pixels representing an image. The pixels are arranged in the sequence in which they are normally scanned to display the image.

bit plane

  A rectangular array of bits mapped one-to-one with pixels. The framebuffer is a stack of bit planes.

black burst

  Active video signal that has only black in it. It is the black portion of the video signal, containing color burst. See also color burst.

black level

  In the active video portion of the video waveform, the voltage level that defines black. See also horizontal blanking interval and video waveform.

blanking level

  The signal level at the beginning and end of the horizontal and vertical blanking intervals, typically representing zero output (0 IRE). See also video waveform and IRE units.

breezeway

  In the horizontal blanking part of the video signal, the portion between the end of the horizontal sync pulse and the beginning of the color burst. See also horizontal blanking and video waveform.

buffer

  See framebuffer.

chroma

  See chrominance.

chrominance

  In an image reproduction system, a separate signal that contains the color information. Black, white, and all shades of gray have no chrominance and contain only the luminance (brightness) portion of the signal. However, all colors have both chrominance and luminance.

  Chrominance is derived from the I and Q signals in the NTSC television system and the U and V signals in the PAL television system. See also luminance.

color burst

  The segment of the horizontal blanking portion of the video signal that is used as a reference for decoding color information in the active video part of the signal. The color burst is required for synchronizing the phase of the 3.58 MHz oscillator in the television receiver for correct hues in the chrominance signal.

  In composite video, the image color is determined by the phase relationship of the color subcarrier to the color burst. The color burst sync is 8 to 11 cycles of 3.58 MHz color subcarrier transmitted on the back porch of every horizontal pulse. The hue of the color sync phase is yellow-green.

  It is also called “burst” and “burst flag”. See also color subcarrier and video waveform.

color space

  A space defined by three color components, such as R, G, and B.

color subcarrier

  A portion of the active portion of a composite video signal that carries color information, referenced to the color burst. The color subcarrier's amplitude determines saturation; its phase angle determines hue. Hue and saturation are derived with respect to the color burst. The color subcarrier's frequency is defined as 3.58 MHz in NTSC and 4.43 MHz in PAL. See also color burst.

component video

  A color encoding method for the three color signals—R, G, and B; Y, I, and Q; or Y, U, and V—that make up a color image. See also RGB, YIQ, and YUV.

composite video

  A color encoding method or a video signal that contains all of the color, brightness, and synchronizing information in one signal. The chief composite television standard signals are NTSC, PAL, and SECAM. See also NTSC, PAL, and SECAM.

field

  One of two (or more) equal parts of information into which a frame is divided in interlace scanning; or a vertical scan of a frame carrying only its odd-numbered or its even-numbered lines. The odd field and even field make up the complete frame. See also frame and interlace.

field averaging

  A filter that corrects flicker by averaging pixel values across successive fields. See also flicker.

field parallel

  Typical video format data (Red, Green, and Blue) output in parallel through three wires.

field-sequential

  Video format data (Red, Green, and Blue) output sequentially through a single wire. This format is used, for example, in head mounted displays.

flicker

  The effect caused by a one-pixel-deep line in a high-resolution graphics frame that is output to a low-resolution monitor, because the line is in only one of the alternating fields that make up the frame. This effect can be filtered out by field averaging. See also field and frame.

frame

  The result of a complete scanning of one image. In television, the odd field (all the odd lines of the frame) and the even field (all the even lines of the frame) make up the frame. In motion video, the image is scanned repeatedly, making a series of frames.

framebuffer

  The individual pixel output of the raster subsystem is written to the framebuffer. Here the information associated with each pixel is stored in a form easily read by the display subsystem. Information associated with each pixel includes image planes, depth planes, stencil planes, overlay and/or underlay planes, and window clipping planes. The complexity of the graphics subsystem determines which of these planes are present in the system. See also pixel.

frequency

  Signal cycles per second.

front porch

  The portion of the video signal between the end of active video and the falling edge of sync. See also back porch, horizontal blanking interval, and video waveform.

gamma correction

  A function applied to colors stored in the framebuffer to correct for the nonlinear response of the eye (and sometimes of the monitor) to linear changes in color-intensity values.

genlocking

  Synchronizing with another video signal serving as a master timing source. The master timing source can be a composite video signal, a video signal with no active video (only sync information), or, for video studio, a device called house sync. When no master sync is available, VideoFramer, for example, can be set to “free run” (or standalone) mode, so that it becomes the master timing device to which other devices sync. See also line lock.

horizontal blanking interval

  Also known as the horizontal retrace interval, the period when a scanning process is moving from the end of one horizontal line to the start of the next line. This portion of the signal is used to carry information other than video information. See also video waveform.

horizontal drive

  The portion of the horizontal blanking part of the video signal composed of the sync pulse together with the front porch and breezeway; that is, horizontal blanking minus the color burst. See also video waveform.

horizontal sync

  The lowest portion of the horizontal blanking part of the video signal; it provides a pulse for synchronizing video input with output. Also known as hsync. See also horizontal blanking interval and video waveform.

hue

  The designation of a color in the spectrum, such as cyan, blue, or magenta. It is sometimes called tint on NTSC television receivers. The varying phase angles in the 3.58 MHz (NTSC) or 4.43 MHz (PAL) C signal indicate the different hues in the picture information.

hue-saturation-intensity

  A tristimulus color system based on the parameters of hue, saturation, and intensity luminance). Also referred to as HSI or HSV.

image processing

  Manipulating an image by changing its color, brightness, shape, or size.

interlace

  A technique that uses more than one vertical scan to reproduce a complete image. In television, the 2:1 interlace used yields two vertical scans (fields) per frame: the first field consists of the odd lines of the frame; the other, the even lines. See also field and frame.

IRE units

  A scale for measuring analog video signal levels, normally starting at the bottom of the horizontal sync pulse and extending to the top of peak white. Blanking level is 0 IRE units and peak white level is 100 IRE units (700 mV). An IRE unit equals 7.14 mV (+100 IRE to -40 IRE = 1 V). IRE stands for Institute of Radio Engineers, a forerunner of the IEEE.

leading edge of sync

  The portion of the video waveform after active video, between the sync threshold and the sync pulse. See also video waveform.

level

  Signal amplitude.

line

  The result of a single pass of the sensor from left to right across the image.

line lock

  Input timing derived from the horizontal sync signal and implying that the system clock (the clock being used to sample the incoming video) is an integer multiple of the horizontal frequency and that it is locked in phase with the horizontal sync signal. See also video waveform.

linear matrix transformation

  The process of combining a group of signals through addition or subtraction; for example, RGB signals into luminance and chrominance signals.

luma

  See luminance.

luminance

  The perceived brightness of a surface. Typically refers to a weighted average of red, green, and blue color values that gives the perceived brightness of the combination. For video systems, luminance is the video signal that describes the amount of light in each pixel. See also chrominance and Y signal.

multiburst

  A test pattern consisting of sets of vertical lines with closer and closer spacing; used for testing horizontal resolution of a video system.

NTSC

  A color television standard or timing format encoding all of the color, brightness, and synchronizing information in one signal. Used in North America, most of South America, and most of the Far East, this standard is named after the National Television Systems Committee, the standardizing body that created this system in the U.S. in 1953. NTSC uses a total of 525 horizontal lines per frame, with two fields per frame of 262.5 lines each. Each field refreshes at 60 Hz (actually 59.94 Hz).

Nyquist limit

  The highest frequency of input signal that can be correctly sampled without aliasing. The Nyquist limit is equal to half of the sampling frequency.

overscan

  To scan a little beyond the display raster area of the monitor so that the edges of the raster are not visible. Television is overscanned; computer displays are underscanned.

PAL

  A color television standard or timing format developed in West Germany and used by most countries in Europe (including the United Kingdom but excluding France) as well as Australia and parts of the Far East. PAL uses a total of 625 horizontal lines per frame, with two fields per frame of 312.5 lines per frame. Each field refreshes at 50 Hz. PAL encodes color differently from NTSC. PAL stands for Phase Alternation Line or Phase Alternated by Line. PAL attempts to correct some of the color inaccuracies in NTSC. See also NTSC and SECAM.

pedestal

  See setup and video waveform.

pixel

  Picture element. Either the smallest addressable spatial element of the computer screen, or the smallest reproducible element in analog video. A pixel can have red, blue, and green color values, an alpha component, and other information associated with it. (Pixels are referred to as having a color component even if they're gray-scale or monochrome.) The bits at location (x, y) of all the bit planes in the framebuffer constitute the single pixel (x, y). In OpenGL window coordinates, each pixel corresponds to a 1.0x1.0 screen area. The coordinates of the lower left corner of the pixel named x,y are (x, y), and of the upper right corner are (x+1, y+1). See also alpha value and component video.

pixel map

  A two-dimensional piece of memory, any number of bits deep. See also bit map.

raster

  The scanning pattern for television display; a series of horizontal lines, usually left to right, top to bottom. In NTSC and PAL systems, the first and last lines are half lines.

raster operation, raster op

  A logical or arithmetic operation on a pixel value.

resolution

  Number of horizontal lines in a television display standard; the higher the number, the greater a system's ability to reproduce fine detail.

RGB

  Red, green, blue—the basic component set used by graphics systems and some video cameras in which a separate signal is used for each primary color.

sample

  To read the value of a signal at evenly spaced points in time; to convert representational data to sampled data (that is, synthesizing and rendering).

sampling rate

  The number of times per second (measured in kHz, where 1 kHz = 1000 times per second) the system reads the file when outputting audio. The greater the sampling rate, the larger the file and the better the quality of the audio output.

saturation

  Color intensity; zero saturation is white (no color) and maximum saturation is the deepest or most intense color possible for that hue. In signal terms, saturation is determined by the ratio between luminance level and chrominance amplitude. See also hue.

scaling

  Changing the size of an image.

scan

  To convert an image to an electrical signal by moving a sensing point across the image, usually left to right, top to bottom.

SECAM

  Sequentiel Couleur avec Memoire, the color television system developed in France and used there as well as in eastern Europe, the Near East and Middle East, and parts of Africa and the Caribbean.

setup

  Also called pedestal. The difference between the blackest level displayed on the receiver and the blanking level. A black level that is elevated to 7.5 IRE instead of being left at 0.0 IRE is the same as the lowest level for active video. Because the video level is known, this part of the signal is used for black-level clamping circuit operation. Setup is typically used in the NTSC video format and is typically not used in the PAL video format; it was originally introduced to simplify the design of early television receivers, which had trouble distinguishing between video black levels and horizontal blanking. See also video waveform.

subcarrier

  A portion of a video signal that carries a specific signal, such as color. See also color subcarrier.

subpixel

  A unit derived from a pixel by using a filter for sizing and positioning.

sync information

  The part of the television video signal that ensures that the display scanning is synchronized with the broadcast scanning. See also video waveform.

sync tip

  The lowest part of the horizontal blanking interval, used for synchronization. See also video waveform.

threshold

  In a digital circuit, the signal level that is specified as the division point between levels used to represent different digital values. For example, the sync threshold is the level at which the leading edge of sync begins. See also video waveform.

time-base errors

  Analog artifacts caused by nonuniform motion of videotape or of the tape head drum. Time-base errors usually cause horizontal display problems, such as horizontal jitter.

time-delay equalization

  Frame-by-frame alignment of all video inputs to one sync pulse, so that all frames start at the same time. This alignment is necessary because cable length differences cause unequal delays. See time-base errors.

transducer

  A microphone, video camera, or other device that can convert sounds or images to electrical signals.

underscan

  To scan a television screen so that the edges of the raster are visible. See also overscan.

vertical blanking interval

  The blanking portion at the beginning of each field. It contains the equalizing pulses, the vertical sync pulses, and vertical interval test signals (VITS). Also the period when a scanning process is moving from the lowest horizontal line back to the top horizontal line.

video level

  Video signal amplitude.

video signal

  The signal from a video device, such as a camera, VCR, or other scanning image sensor.

video waveform

  The main components of the video waveform are the active video portion and the horizontal blanking portion. Certain video waveforms carry information during the horizontal blanking interval.

white level

  In the active video portion of the video waveform, the 1.0-Volt (100 IRE) level. See also video waveform.

YIQ

  See also YUV and PAL. The NTSC color system uses YIQ as its color space. Y stands for the image's black and white portion (luminance component). The color portion is I and Q. They behave as a color wash laid over the black and white components.

YUV

  See also, YIQ and PAL. The PAL system uses the YUV color space, which also may be used in the NTSC color system. The luminance (black and white) component is Y, and the color wash components U and V, overlay the Y component.

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