DIGITAL TELEVISION

Digital television is the application of digital technology to television to the process of producing and transmitting television programming. Television was developed as an "analog" medium, but the replacement of analog technology with digital technology throughout the television production and transmission process promises to increase the capabilities of the medium.

The term "digital" refers to a type of electronic signal in which the information is stored in a sequence of binary numbers ("on" or "off", representing one and zero) rather than in a continuously varying signal (known as an analog signal). Almost all naturally occurring communication media, including sound and light waves, are analog signals. Because analog signals are composed of waves, they are extremely susceptible to interference, as the waves of external signals can interact with a specific signal, altering the shape of the wave. Digital signals are much less susceptible to interference because a slightly altered sequence of "on" and "off" signals can still be read as the original sequence of ones and zeroes.

The primary attributes of a digital signal are the sampling frequency and the bit rate. In order to convert an analog signal to a digital one, the signal must be "sampled" by measuring the height of the analog signal at discrete points in time. The "sampling frequency" is a measure of how many samples are taken to represent the analog wave. A higher sampling frequency indicates more samples, providing a more faithful reproduction of the analog signal. But doubling the sample rate means doubling the amount of data needed to represent the original analog signal. Bit rate refers to the number of different "bits" (zero/one values) used to represent each sample. A higher bit rate results in a greater number of values for the signal, and, hence, a higher resolution. (Each additional bit doubles the number of values for each signal, so that an eight bit signal has twice the resolution of a seven-bit signal.) Most digital audio signals use eight or sixteen bits of information for each sample.

Digital signals have a number of advantages over analog signals. The primary advantage is that digital signals allow for perfect copies (and perfect copies of copies, etc.). Digital signals may also be manipulated by computers, allowing for elaborate modifications of digital video and audio signals. The primary drawback of digital signals vs. analog ones are that it takes a great deal more space to store a digital signal than an analog one and that extra equipment is needed to covert analog video and audio signals to digital signals and later convert the digital signals back to analog.

Digital technology was first applied to television to create special video effects that were not possible using analog technology. The analog images were digitized, and mathematical algorithms processed the resulting data, allowing a picture to be blown up, shrunk, twisted, etc. The next innovation was the creation of digital video recorders, which stored television signals as a sequence of binary numbers. Digital video recording is extremely complicated because the sequence of numbers used to represent a single picture required much more storage space than the corresponding analog signal. However, copies of digital signals are exactly the same as the original, enabling higher-quality pictures during the editing process, especially when many signals have to be "layered" together to create a single picture or sequence.

The television production process is gradually moving from a system that interconnects a variety of digital sources with analog equipment to the use of an all-digital environment. Along the way, analog and digital tape formats will be replaced by new digital recording devices similar to computer disk drives, allowing random access to any portion of a recording.

Digital technology has also been applied to the process of transmitting television signals. The bandwidth required for high-definition television required development of a means of transmitting up to five times the video information of a traditional television signal in the same bandwidth. The solution was the application of digital compression technology. Digital compression is the process by which digital signals are simplified by removing redundancy. (For example, each of the thirty individual pictures used to create one second of video is quite similar to the previous picture. Instead of transmitting the entire picture again, some compression algorithms transmit only the parts of the picture that change from one picture to the next.) There are two general types of digital compression: "Lossless" compression in which the decompressed signal is exactly the same as the uncompressed signal and "lossy" compression, in which the decompressed signal contains less information (or less detail) than the original uncompressed signal.

The flexibility of digital signals has led many engineers to develop uses for digital broadcasting other than high-definition television. The use of digital compression will allow the transmission of at least four, and perhaps eight or more, standard-definition channels of programming in the same bandwidth required for a single analog channel. Furthermore, the fact that digital signals are less susceptible to interference will eventually allow more television stations on the air in a given market. (Interference problems with analog signals requires wide spacing of television stations on the same or adjacent channels, resulting in use of only a few channels in most cities to protect stations in nearby cities.)

The primary drawback of digital broadcasting is that it will require viewers to either buy new receivers or obtain adapters to convert digital signals to analog form for viewing on a traditional television receiver. Ultimately, the use of television by consumers should be revolutionized as they begin buying digital receivers and video recorders and enjoy the quality and flexibility provided by digital technology.

-August Grant

FURTHER READING

Alten, S. Audio In Media. Belmont, California: Wadsworth, 1981, (4th Edition, 1994).

Chyt, H. S. "Digital Audio." In, Grant, August, Editor. Communication Technology Update. Austin, Texas: Technology Futures, 1992; Newton, Massachusetts: Focal, 5th Edition, 1996.

Straubhaar, Joseph, And Robert Larose. Communication Media In The Information Society. Belmont, California: Wadsworth, 1996.