Video broadcasting is the process of transmitting video images to a plurality of receivers. The broadcasting medium may be via radio waves, through wired systems (such as CATV), or through packet data systems (such as the Internet). Television involves the transmission and reception of visual images via electrical signals. Video is an electrical signal that carries TV picture information.
For many years, video (television) broadcasters had monopolized the distribution of some forms of information to the general public. This had resulted in strict regulations on the ownership, operation, and types of services broadcast companies could offer. Due to the recent competition of wide area information distribution, governments throughout the world have eased their regulation of the broadcast industry. In 1996, the United States released the Telecommunications Act of 1996 that dramatically deregulated the telecommunications industry. This allowed broadcasters to provide many new services with their existing networks.
Also in the mid 1990’s, a major shift occurred in the broadcast industry. The conversion from analog systems to digital systems provided broadcasters with the tools they needed to bundle multiple types of services onto a television channel signals. This included cable modems, digital television, and even telephone service. The ability to integrate several services into one transmission signal allows the cable television operator to offer many new services without significant investment in new cable systems. Analog CATV systems typically provide 50-100 video channels while digital CATV systems to provide hundreds of video channels, high-speed Internet access, and telephone service.
Broadcast Television
The technology that is used for television broadcast was developed in the 1940s. The success of the television marketplace is due to standardized, reliable, and relatively inexpensive television receivers and a large selection of media sources. The first television transmission standards used analog radio transmission to provide black and white video service. These initial television technologies have evolved to allow for both black and white and color television signals, along with advanced services such as stereo audio and closed caption text. This was a very important evolution as new television services (such as color television) can be on the same radio channel as black and white television services.
While analog television technology is efficient at distributing good video and audio signals, it does not easily allow the sending and receiving of digital data. Several new television broadcast exist that can deliver high quality video and audio as well as information services using digital signal transmission. New technologies allow transmission of high-definition television (HDTV). HDTV is the term used to describe a high-resolution video and high quality audio signal as compared to standard NTSC or PAL video transmission. HDTV signals can be in analog or digital form. Digital HDTV systems have the added benefits of providing data and other multimedia services.
A television system consists of a television production studio, a high power transmitter, a communications link between the studio and the transmitter, and network feeds for programming. The production studio controls and mixes the sources of information including videotapes, video studio, computer created images (such as captions), and other video sources. A high power transmitter broadcasts a single television channel. The television studio is connected to the transmitter by a high bandwidth communications link that can pass video and control signals. This communications link may be a wired (coax) line or a microwave link. Many television stations receive their video source from a television network. This allows a single video source to be relayed to many television transmitters.
Cable Television
Cable television is a television distribution system that uses a network of cables to deliver multiple video and audio channels to consumers. Cable television systems can be one-way systems (only from the head-end to consumers) or two-way (both to and from the customer).
Figure 1 shows a one-way cable television system. This diagram shows that various video sources are selected in the head-end. Each of the video sources that will be distributed on the cable network are applied to an RF modulator that converts the video signals into RF signals on a specific frequency. The many RF signals are combined (added together), amplified, and sent to the cable television system distribution network.
The distribution network supplies part of the signal (signal tap) as the cable passes near each home or business location. As the distribution system progresses away from the head-end, the signal level begins decrease. Periodically, amplifiers are used to increase the composite video signal.
A two-way cable television system allows customers to receive and send information between the cable system and their set-top box. There are two options for two-way cable television systems: a hybrid system and an integrated system. Hybrid two-way systems use different technologies to transfer information in different directions and integrated systems use the cable network for both directions of communication.
Because the design of most cable systems started as a one-way cable system, hybrid systems were first used to add two-way communication capability. For hybrid systems, a different technology is used to transfer information to the user (downstream) and from the user (upstream). Early systems used the cable for the downstream and the telephone network or wireless data devices for the upstream.
As cable systems evolved to include fiber (optical) cable and two-way amplifiers, cable networks evolved to allow data transmission in both directions. On the coaxial (RF) cable, the return path was assigned to frequencies in the range below 50 MHz. This frequency range was unassigned for television operation. Fiber optic cables use separate strands for each direction as each fiber cable often has many (30+) fiber strands.
The two-way cable system requires cable modems at the user end and a coordinating modem at the head-end of the system. The cable modem is a communication device that modulates and demodulates (MoDem) data signals to and from a cable television system. A modem at the head-end coordinates the customer’s modem and interfaces data to other networks (such as the Internet).
Figure 2 shows a two-way cable television system. This diagram shows that the two-way cable television system adds a cable modem termination system (CMTS) at the head-end and a cable modem (CM) at the customer’s location. The CMTS also provides an interface to other networks such as the Internet.
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