A television is a telecommunication medium that allows transmission and reception of both picture and audio messages. The picture messages can be colored or black and white. In this context, transmission simply refers to the propagating and sending of signals over a medium. The medium can be a coaxial cable, fiber optic cable, air, among others. The reception is simply the process of receiving the transmitted signal and converting it back into a form that can be easily interpreted by the target recipient. The video and audio signals are transmitted as either analog electrical signals (for analog television systems) or digitized packet data (for digital television systems). Other transmission configurations apply both aspects of analog and digital television in a single system (Fischer 1961, p.17).
This report explains the processes of television transmission and reception. It starts with a brief explanation of the historical background of television signal transmission and reception. Thereafter, it discusses the technical processes involved in the relaying of television messages from the broadcasting studios to the reception at the consumer television sets. The process starts with encoding of the raw data into an appropriate signal, modulating it and then the signal is transmitted over a medium to the receivers’ miles away. It also explains how the video and audio signals are generated, and transmitted together and received by a single antenna without interference. Lastly, the report explains how the signals are decoded and output as either motion pictures or sound. With the adoption of digital transmission, we have two kinds of televisions- those with in-built digital receivers and those that will need digital receiver set-up boxes. (Collins 2001, p.2).
Evolution of Television signal Transmission and Reception Techniques
The creation of the Television was never an individual effort but rather a contribution by several different researchers. The first demonstrations of a TV were done by two independent researchers- Baird (in London) and Jenkins (in Washington).This was later followed by the introduction of the picture tubes, and adoption of modulating standards. By mid 1930s, regular television broadcasts had been introduced. After the Second World War, the inter-carrier system linking audio and video signals was introduced. This aimed at easing tuning especially when dealing with the UHF transmissions and receptions. Over the years, the distinction between data and television transmission has disappeared. (Fischer 1961, p.1).
Over the years, there have been outstanding changes in the technologies used to realize television transmission and reception systems. For example, the introduction of color TV which did away with the monochrome receivers, and the adoption of cable TV systems to eliminate the line-of- sight limitations associated with UHF and VHF transmissions and receptions. The use of satellite for broadcasting has enable world-wide transmission of television signals to even the most remote parts of the world. The use of solid state charge coupled devices (CCD) and Liquid Display Systems (LCD) became the game changer for the reception dynamics. Currently, we have LCD flat screen television sets that give a better viewing angle, better picture resolution and are “cable ready” for digital signal reception as compared to the previously used CRT screens. Another very important aspect of TV evolution has been data compression. Without compression, the channel capacities available on cable, terrestrial and satellite would be insufficient for the television signals. HDTV transmission will also be unachievable (Fischer 1961, p.2).
The Video signal
The video signal- also referred to as the picture signal is the time varying electrical signal voltage obtained from the TV camera tube circuit. This usually represents the distribution of brightness information in the image. The video signal normally has a wide range of frequencies resulting in a bandwidth of between 25Hz to 5MHz. Before transmission, the weak video signals are amplified using RC-coupled amplifiers with extended frequency response. In order to ensure synchronization of the scanning rates at the receiver and transmitter, synchronizing pulses are added to the video signal. This synchronization ensures that the scanning beam retrace is not visible on the screen at the receiver end. The composite video signal is also referred to as a composite color, video and sync (CCVS) signal (Fischer 1961, p.16).
Transmission and Reception of the Video and Sound signals
A television message is made up of two separate signals, that is; a video and an audio signal. The video signal from a Camera tube is never transmitted in its ‘raw’ form. It is first amplified as explained above, and then mixed with the sync and blanking pulses to form the composite video signal that is transmitted to the receiver end through a transmission medium (Fischer 1961, p.17). This transmission medium can be a coaxial cable, fiber optic cable or space. In order to be transmitted through space, the composite video signal is used to amplitude modulate radio frequency (RF) carrier that is then radiated into space using an antenna (Fischer 1961, p.18). To ensure maintenance of the power of the video signal transmitted over long distances, there are signal repeaters designed to amplify the signals (Fischer 1961, p.20).
For proper modulation, the carrier’s frequency should always be greater than the modulating signals’ frequency. To achieve this, the transmission of the video signal is done using the Very high frequencies (VHF) and Ultra High Frequencies (UHF) all of which give carrier frequencies greater than the modulating 5MHz signals. However, it should be noted that the VHF and UHF have a disadvantage of being limited to line-of-sight transmissions, hence the need for many repeaters for long distance terrestrial transmission. With the global shift from analog to digital transmission, the use of vestigial sideband-amplitude modulation (VSB-AM) is for video signal and frequency modulation for sound signals is still in use though the analog transmission paths are losing significance (Fischer 1961, p.18).
During transmission, the amplified composite video signal is fed to the AM transmitter which transmits fully only one sideband while the other sideband is partially transmitted (“vestigial sideband”). On the other hand, the audio signal is amplified and then fed to the FM carrier transmitter. The powers from the separate video AM transmitter and audio FM transmitters are combined (superimposed on each other) in a diplexer before they are fed to a common antenna for radiation into space as a single complex signal. The radiation of the television signal into space for propagation is done by a single antenna (Collins 2001, p.2).
At the receiver end, the combined audio and video signals are intercepted by a single antenna. The video signal is demodulated and amplified using RF or intermediate frequency (IF) amplifiers. The resulting signal is then used to control the beam intensity of the picture being reproduced on the screen. The beam also scans the picture raster to ensure synchronization of camera image brightness and that of the final image. This scanning is usually very fast such that the human eye cannot follow the spot movements. This is also attributed to consistence of vision- hence the many single still images making up the video are viewed as a single motion picture (Benson & Whitaker, 2000, p.56). With advances in technology, the receiver output screen maybe a cathode ray tube (CRT) screen, plasma screen, an LCD screen or even an LED screen. The audio signal is simply demodulated and amplified to be output by the receiver speech output devices.
The purpose of this report was to explain the process of television signal transmission and reception. The discussion of the main evolution stages of this process was to appreciate the origin of the process and relate it to the current state of affairs. The report appreciates the fact that the television signal is made up of two separate signals that are transmitted and received separately using a single antenna. This is because the video signal has a wide bandwidth of up to 5 MHz where-as the sound has a comparably very small bandwidth. The video information is transmitted as a composite video signal composed of the video signal, blanking and sync pulses. This composite video signal is used to amplitude modulate a very high frequency or an ultra-high frequency carrier. This enables it to be transmitted in the RF spectrum. On the other hand, the sound signal is used to frequency modulate an RF carrier before transmission. The two resulting powers are superimposed on one another in the diplexer before they are radiated by a single transmission antenna. At the receiver end, the two signals are received by a single antenna and then demodulated separately to give a motion picture and sound.
Collins, G, 2000. Fundamentals of Digital Television Transmission, John Wiley & Sons, New York.
Benson, B & Whitaker, J 2000. Standard Handbook of Video and Television Engineering, Volume 1 McGraw-Hill, New York.
Fischer, W 1961, Digital Video and Audio Broadcasting Technology: a Practical Engineering Guide, TBA Books. New York.