Glossary of Audiovisual Terms
The audiovisual lexicon is as diverse as its technologies and applications, and is expanding and evolving just as fast. The following is a primer of terms you'll likely encounter when purchasing projectors, monitors and other presentation products.
Aspect Ratio: The ratio of width to height of a frame or screen. Television screens usually have aspect ratios of 4:3, while plasma and LCD monitors often have aspect ratios of 16:9, making them more compatible with widescreen movies and HDTV programs.
Back-Room Projector: A projector equipped with a lens designed to be used much farther away from the screen than is usual - often in a projection booth or at the back of an auditorium.
Bandwidth: In video, the range of frequencies passed along a single channel.
Cathode-Ray Tube (CRT): An analog display device using a vacuum tube that generates images on a layer of phosphors driven by an electron beam, or gun, inside the tube. It is commonly referred to as a picture tube and is often used in front-room projector models.
Digital Light Processing (DLP): A video projection technology from Texas Instruments that creates images by reflecting light off hundreds of thousands of tiny rotating mirrors (each representing one pixel) mounted on a microprocessor. Three-chip DLP projectors use separate mirror arrays for red, green and blue. Single-chip projectors use a color-filter wheel that alternates each color in front of the mirrors.
Digital Micromirror Device (DMD): The name of the micromirror microprocessor from Texas Instruments that powers a DLP projector. (See also DLP.)
Digital Television (DTV): The umbrella term for the digital transmission formats designated by the Advanced Television Systems Committee (ATSC) to replace the traditional National Television Standards Committee (NTSC) system by 2006.
Direct-Drive Image Light Amplifier (D-ILA): A video projection technology from JVC that uses a reflective LCD to create images as a light source is reflected off the LCD and sent through the lens to the screen. (See also LCD.)
Halogen Lamp: A projection lamp used in most low- and medium-priced models that lasts about 1,000-2,000 hours and delivers a consistent output throughout its life. Images from halogen lamps look very white compared to those from incandescent lamps but less white than those from metal halide lamps.
High-Definition Television (HDTV): A high-resolution broadcast and display format commonly understood to include a widescreen 16:9 image with twice the horizontal and vertical resolution of traditional NTSC video, as well as 5.1 channels of Dolby Digital audio.
High-Gain Screen: A projection screen employing one of several techniques designed to reflect an unusually bright image back to the audience.
Horizontal Scan Rate: The speed in kilohertz at which the horizontal lines of the projected image are created. The higher the scan rate, the greater the resolution at a given vertical frame rate, provided that the scan rate of the video source is within the scan-rate range of the projector.
Inverted Image: A feature enabling many video projectors to flip an image from top to bottom to compensate for mounting a projector upside down on the ceiling.
Letterbox: A format to display widescreen films as originally intended in which the image is shrunk to fit the width of the available screen - leaving blank space on the top and bottom of the screen.
Liquid Crystal Display (LCD): A video display process based on liquid crystals sandwiched between two polarizing transparent layers of glass. The crystals, charged by transistors, open or close to block or let light pass. In a projection format, light from an internal lamp passes through the LCD panel onto the screen. Each crystal represents one pixel.
Long-Throw Lens: A lens designed for projection from a long distance, typically 50-100 feet, such as might be found in a projection booth in the rear of an auditorium.
Lumen: A measurement unit for the amount of light emitted by a projector. The higher the lumen rating, the brighter the image.
Metal Halide Lamp: A very bright projection lamp used in many medium-end and all high-end portable models that emits a lot of light relative to the power it consumes. Metal halide lamps usually last between 400-600 hours and slowly lose brightness as they age.
NTSC: The traditional color video standard established in 1953 by the National Television Standards Committee in the United States, and subsequently adopted by Canada and many other countries. It consists of a 525-line image in which frames are displayed at 30 per second.
Overhead Projector: A display device designed to project images from transparencies onto a screen. It typically consists of a light, a transmitting or reflective platform, and an adjustable-lens assembly.
Pixel: Short for picture element, a pixel is the smallest data component in a video display. It is represented as a point with specified color and intensity.
Plasma: A flat-panel display technology that uses phosphor stripes painted in ribs - one stripe per color - which are then charged to create an image on a flat surface. Most plasma displays range in size from 40-61 inches, measured diagonally.
Reverse Image: A feature on many video projectors that enables them to flip an image horizontally. This is necessary for rear projection.
Short-Throw Lens: A projection lens designed to display the largest possible image from a short distance, such as in a rear-projection system where space behind the screen is limited.
SVGA: Super video graphics array is the computer-video graphics standard with resolutions as high as 2,000 x 1,620 pixels and 24-bit color (16 million colors).
VGA: Video graphics array is the basic computer-video graphics standard, offering a resolution of 640 x 480 pixels (16 colors) or 320 x 200 pixels (256 colors).
Viewing Angle: The widest, or maximum, angle at which viewers can clearly see an image projected onto a screen.
Zoom Lens: A projection lens with a variable focal length, making it possible to enlarge or shrink the image on a screen by adjusting the lens instead of moving the projector or the screen.
This glossary of terms was compiled by the International Communications Industries Association, with reference assistance from Kayye Consulting. ICIA is the premier trade association for the professional AV communications industry and founder of InfoComm, the annual tradeshow for AV communications professionals. InfoComm '04 will be held in Atlanta from June 5-11. For more information, visit www.infocomm.org.
This article originally appeared in the 06/01/2004 issue of THE Journal.