Digital TV Glossary

To help you better understand WHYY's transition to digital technology, here are some terms you may be seeing in the coming months.

Advanced Television (ATV): The FCC's name for Digital Television (DTV).

Analog: The technology in use for more than 50 years to transmit conventional TV signals. Vinyl recordings and audiocassettes are also examples of analog technology.

Aspect Ratio: The ratio of screen width to screen height. For HDTV, the aspect ratio is 16:9 (16 units wide by 9 units high), much like a movie screen. Today's analog television screens are 4:3, or nearly square.

Back channel: A means of communication from users back to content providers. At the same time that content providers are transmitting interactive television (analog or digital) to users, users can connect through a back channel to a Web site - for example, to the original content provider or an advertiser. The back channel can be used to provide feedback, purchase goods and services, request more information, and so on. A simple type of back channel is an Internet connection using a modem.

Bandwidth: The amount of spectrum available to each communications licensee. For digital conversion, the "Grand Alliance" plan approved by the FCC calls for the allocation of 6 MHz (megahertz) of the broadcast spectrum for each television broadcaster. It can be used for one analog signal, or one HDTV signal and one digital, or four multicast digital signals, and/or data transmission. During the transition to DTV all broadcasts can utilize two 6 Mhz. spectrum assignments.

Channel: As in television channel; the television broadcast frequency. The over-the-air television channel assignments are set by the FCC and are currently found in the VHF or UHF band. During the transition period each television broadcast station is being permitted to operate a 2nd channel for DTV. The DTV assignment will be different than the channel currently being used by the station. WHYY'S DTV assigned channel is 55. However, WHYY transmits its "virtual ID," which designates the channel as 12.1.

During the transition period which will range from 7 to 15 years, U.S. television stations will be able to broadcast on their current channel and their new DTV frequency. At the end of the transition, the stations will broadcast only their DTV signal. WHYY's DTV service wil return to VHF channel 12 at the end of the transition.

Datacasting: The broadcast of information and other services using a digital television channel. Broadcasters can use their DTV channels to offer additional data or services. For example, a travel program might be transmitted along with data consisting of lodging, sightseeing and transportation information, which the viewer can access while watching the program. Teachers will be able to access lesson plans and learning materials while watching TV. This information might appear as a menu of choices on the TV or computer screen, which the viewer can read on the screen or print out for future reference. One of the major benefits of a digital broadcast system is the ability not only to send pictures and sound, but also to send data. Program related data enhances the television show you are currently watching. FOR instance, Real Science could include science quizzes for teachers, periodic table graphs, a molecule building game, audio clips of various animal vocalizations, or anything else you can imagine that can be transmitted digitally.

Digital Television (DTV): Refers to transmitting a broadcast signal by encoding it as 0s and 1s - the digital binary code used in computers. DTV can provide high quality programming (HDTV) or provide four, five or more channels in the same bandwidth required for one channel of the current standard television. Calculators, computers, compact discs, and the Internet are examples of digital technology.

Federal Communications Commission (FCC): Independent US government agency, directly responsible to Congress, and charged with regulating interstate and international communications by radio, television, wire, satellite and cable. The FCC's jurisdiction covers the 50 states, the District of Columbia, and U.S. Possessions.

Flat Screen Display: Television screens that flatten the size of television sets to that of a framed picture. The screens on today's television sets use bulky cathode ray tubes (CRTs). CRTs are made of a glass envelope and use a controlled beam of electrons striking light-emitting material to display the picture. Flat screen displays use plasma display systems that does not require the use of CRTs and other bulky tube equipment.

High Definition Television (HDTV): Offers approximately twice the vertical and horizontal resolution (clarity) of NTSC television. Provides crystal-clear quality widescreen pictures with compact disc-quality surround sound. The aspect ratio of HDTV pictures is 16:9, similar to a movie screen's dimensions, as opposed to the 4:3 format of today's television. HDTV duplicates movie theater display quality.

Interactive Television: A combination of television with interactive content and enhancements. Interactive television provides better, richer entertainment and information, blending traditional TV-watching with the interactivity of a personal computer. Programming can include richer graphics, one-click access to Web sites through TV crossover links, electronic mail and chats, and online commerce through a back channel. See back channel.

Interlaced: A system that skips every other line of a picture on the first pass, then fills in those lines on the second pass. When talking about resolution, this method is indicated by "i" after the number of lines, as in "480i" or "1080i". When you watch analog television, you're looking at an interlaced video display. Because early television tubes couldn't draw the whole screen before the top began to fade, TV engineers implemented interlacing. This system might result in a little flicker, but it avoids having the bottom of the screen perpetually brighter than the top. On digital TVs, as on computer monitors, the whole screen is "progressive" (drawn line by line).

Multicasting: Broadcasting several programs at once via DTV on a single channel. Eventually, a WHYY-DT viewer might be able to receive Sesame Street or Nova at the same time, and choose the program preferred.

Nonlinear editing: Nonlinear distinguishes editing operation from the "linear" methods used with tape. Nonlinear refers to not having to edit material in the sequence of the final program and does not involve copying to make edits. It allows any part of the edit to be accessed and modified without having to reedit or recopy the material that is already edited and follows that point. Nonlinear editing is also nondestructive, as it uses computer hard disks rather than videotape.

NTSC: National Television Systems Committee, the group that set the analog television standard 50 years ago. The abbreviation is used to refer to the current US standard.

Off-line (editing): A decision-making process using low-cost equipment usually to produce a rough cut "edit decision list," which can then be used to make the high quality final version of the program.

Pixel: A shortened version of "picture element," the smallest element in a television picture. The total number of pixels limits the detail that can be seen on a television. A typical television set has less than half a million pixels. The pixel count for HDTV is nearly two million.

Progressive Scanning: The ability of most computer monitors and high resolution sets to display all the lines of a picture at the same time. When talking about resolution, this method is indicated by indicated by "p" after the number of lines, as in "720p".

Resolution: A measure of the finest detail that can be seen, or resolved, in a reproduced image. While influenced by the number of pixels in an image (for HDTV approximately 2,000 x 1,000; current broadcast NTSC TV 720 x 487), note that the pixel numbers do not define ultimate resolution but merely the resolution of that part of the equipment. The quality of lenses, display tubes, film process and film scanners, etc., used to produce the image on the screen must all be taken into account.

Server (file): A storage system that provides data files to all connected users of a local network. Typically the file server is a computer with large disk storage which is able to record or send files as requested by the other connected (client) computers. The file server often appears as another disk on client systems. The data files are typically around a few kilobytes in size and are expected to be delivered within moments of request

Server (video): A storage system that provides audio and video storage for a network of clients. While there are some analog systems based on optical disks, most used in professional and broadcast applications are based on digital disk storage. Aside from those used for video on demand (VOD), video servers are applied in three areas of television operation: transmission, post production and news. Compared to general purpose file servers, video severs must handle far more data, files are larger and must be continuously delivered. Store sizes are very large, typically up to 500 gigabytes or more. Operation depends entirely on connected devices, edit suites, automation systems, secondary servers, etc., so the effectiveness of the necessary remote control and video networking is vital to success.

Set-top box (STB): VCR-sized electrical converter box that will sit atop television sets and convert analog and digital signals to be seen on analog and digital televisions. Very similar to set-top boxes currently used by customers of Comcast, DirecTV and other satellite and cable providers. Other set top boxes also received broadcast signals directly off-air without a subscription fee using an outdoor TV antenna. May contain a hard disk to increase functionality.

Simulcast: To broadcast the same program over two different transmission systems at the same time. Currently, some AM and FM radio stations simulcast the same program for part of the day. The FCC plans to have mandatory simulcast of DTV programming over the current television system. This requirement, which is intended to protect the public interest, should reduce the possibility of two types or quality levels in programming.

Standard definition Television (SDTV): A digital television system that is similar to current standards in picture resolution and aspect ratio. The picture and sound will be clearer than NTSC. Offers the ability to transmit four or more standard-quality programs (equivalent to NTSC) instead of HDTV programs, using the same channel. SDTV also incorporates stereo sound plus a wide range of data services.

Stream: 1. To transmit multimedia files that begin playing upon arrival of the first packets, without needing to wait for all the data to arrive. 2. To send data in such a way as to simulate real-time delivery of multimedia.

Streaming media: Multimedia content - such as video, audio, text, or animation - that is displayed by a client as it is received from the Internet, broadcast network, or local storage.

WebTV: WebTV Networks, Inc. is a manufacturer of set-top boxes used for viewing interactive television and regular television. These receivers let users access the Internet, including use of electronic mail and online chats. WebTV set-top boxes like the WebTV Plus Receiver connect to a standard television and a phone line. The WebTV Plus Receiver supports TV Crossover Links and WebPIP. WebPIP lets users simultaneously view Web pages and TV programming on the same screen, without a special picture-in-picture TV.

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. Widescreen Definition Programs: Term used by PBS to denote programs produced in the widescreen aspect of 16:9, but not broadcast in HDTV.