When color television was introduced, the chrominance information was added to the monochrome signals in a way that black and white televisions ignore. The monochrome combinations still existing in the 1950s were standardized by the International Telecommunication Union (ITU) as capital letters A through N. Further differences exist in the frequency and modulation of the audio carrier. The transmission system must include a means of television channel selection.Īnalog broadcast television systems come in a variety of frame rates and resolutions. Ī practical television system needs to take luminance, chrominance (in a color system), synchronization (horizontal and vertical), and audio signals, and broadcast them over a radio transmission. The first commercial television systems were black-and-white the beginning of color television was in the 1950s. The analog television signal contains timing and synchronization information so that the receiver can reconstruct a two-dimensional moving image from a one-dimensional time-varying signal. The process repeats and next sequential frame is displayed, allowing the depiction of motion. The lines are of varying brightness the whole set of lines is drawn quickly enough that the human eye perceives it as one image. Each frame of a television image is composed of scan lines drawn on the screen. Using RF modulation the signal is then modulated onto a very high frequency (VHF) or ultra high frequency (UHF) carrier wave. A color encoding scheme ( NTSC, PAL, or SECAM) could be added to the base monochrome signal. These systems determine the number of scan lines, frame rate, channel width, video bandwidth, video-audio separation, and so on.
The official systems of transmission were defined by the ITU in 1961 as: A, B, C, D, E, F, G, H, I, K, K1, L, M and N. All-electronic systems became popular with households after World War II.įurther information: Broadcast television systemīroadcasters of analog television encode their signal using different systems. Also, far less maintenance was required of an all-electronic system compared to a mechanical spinning disc system. The electron beam could be swept across the screen much faster than any mechanical disc system, allowing for more closely spaced scan lines and much higher image resolution. The reproduced images from these mechanical systems were dim, very low resolution and flickered severely.Īnalog television did not really begin as an industry until the development of the cathode-ray tube (CRT), which uses a focused electron beam to trace lines across a phosphor coated surface. Camera systems used similar spinning discs and required intensely bright illumination of the subject for the light detector to work. Synchronization of the receiver disc rotation was handled through sync pulses broadcast with the image information. A similar disk reconstructed the image at the receiver. The earliest systems of analog television were mechanical television systems that used spinning disks with patterns of holes punched into the disc to scan an image. Motivated by the lower bandwidth requirements of compressed digital signals, beginning in the 2000s, a digital television transition is proceeding in most countries of the world, with different deadlines for the cessation of analog broadcasts. In contrast, picture quality from a digital television (DTV) signal remains good until the signal level drops below a threshold where reception is no longer possible or becomes intermittent.Īnalog television may be wireless ( terrestrial television and satellite television) or can be distributed over a cable network as cable television.Īll broadcast television systems used analog signals before the arrival of DTV. Thus with analog, a moderately weak signal becomes snowy and subject to interference. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, phase and frequency of an analog signal.Īnalog signals vary over a continuous range of possible values which means that electronic noise and interference may be introduced. Early monochrome analog receiver with large dials for volume control and channel selection, and smaller ones for fine-tuning, brightness, contrast, and horizontal and vertical hold adjustmentsĪnalog television is the original television technology that uses analog signals to transmit video and audio.
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