cccccThe 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, respectively) 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 these signals are composed of waves, they are  extremely susceptible to interference, as the waves of external, extraneous 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” can still be read as the original sequence of ones and zeros.

The primary attributes of a digital signal are the sampling frequency and the bit rate. In order to convert an analog signal to digital, 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 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 16 bits of information for each sample.

Digital signals have a number of advantages over analog signals. The primary advantage is that they allow for perfect copies ( and perfect copies of copies and so on). Digital signals may also be manipulated by computers, allowing for elaborate and modifications of both video and audio signals. The primary drawbacks of digital versus analog signals are that it takes a great deal more space to store digital than it does analog signals and that extreme equipment is needed to convert analog video and audio signals to digital and later convert the digital signals back to analog.

Digital technology was first applied to television to create special video effects that were impossible using analog technology. The analog images were digitized, and mathematical algorithms processed the resulting data, allowing a picture to be blown up, shrunk, twisted, and so on. 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 requires 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 been “layered” together to create a signal picture of 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 have been 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 30 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, “lossy” compression, and 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 market. (Interference problems with analog signals require wide spacing of television stations transmitting on the same or adjacent channels, resulting in the use of only a few channels in most cities to protect stations in nearby cities.)

The flexibility of digital signal transmission has led to the development of 18 varieties of digital television transmission in the United States, ranging from standard-definition images that are comparable to traditional, analog television to high-definition images that provide Clarity comparable to 35-millimeter film. In choosing among the 18 formats, U.S. broadcasters must consider trade-offs between the number of different signals they can transmit simultaneously and the quality of each individual signal. The more signals transmitted, the less information–and lower quality–is available for each channel.

From the viewer's perspective, 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. Many cable television subscribers have digital converter boxes that perform such a conversion for digital signals transmitted through a cable television system. Ultimately, the use of television by consumers may be revolutionized as they begin buying digital receivers and digital video recorders and enjoy the quality and flexibility provided by digital technology.