Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, brightness, color, and sound are represented by the amplitude, phase, and frequency of the signal.
The strength of an analog signal varies over a continuous range of possible values, meaning that electronic noise and interference may be introduced. Thus, a moderately weak signal becomes snowy and subject to interference. In contrast, picture quality from a digital television (DTV) signal remains good until the signal level drops below a certain threshold (the "digital cliff"), where reception is either no longer possible or becomes intermittent.
Analog television may be wireless (as in terrestrial and satellite television) or distributed over a cable network (cable television).
Traditionally, all broadcast television systems used analog signals. Starting after the year 2000, motivated by the lower bandwidth requirements of compressed digital signals, a digital television transition has been underway in most of the world, with different deadlines for the cessation of analog broadcasts. Countries that still primarily use analog systems are mostly in Africa, Asia, and South America.
Development
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. A similar disk reconstructed the image at the receiver. Synchronization of the receiver disc rotation was handled through sync pulses broadcast with the image information. Camera systems used similar spinning discs and required intensely bright illumination of the subject for the light detector to work. The reproduced images from these mechanical systems were dim, very low resolution and flickered severely.
Analog television did not begin in earnest 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. 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. Also, far less maintenance was required of an all-electronic system compared to a mechanical spinning disc system. All-electronic systems became popular with households after World War II.
Standards
Broadcasters of analog television encode their signal using different systems. 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.Final acts of the European Broadcasting Conference in the VHF and UHF bands. Stockholm, 1961. These systems determine the number of scan lines, frame rate, channel width, video bandwidth, video-audio separation, and so on. A color encoding scheme (NTSC, PAL, or SECAM) could be added to the base monochrome signal. Using RF modulation the signal is then modulated onto a very high frequency (VHF) or ultra high frequency (UHF) carrier wave. Each frame of a television image is composed of scan lines drawn on the screen. The lines are of varying brightness; the whole set of lines is drawn quickly enough that the human eye perceives it as one image. The process repeats, and the next sequential frame is displayed, allowing the depiction of motion. 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 first commercial television systems were black-and-white; the beginning of color television was in the 1950s.
A 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 transmission system must include a means of television channel selection.
Analog broadcast television systems come in a variety of frame rates and resolutions. Further differences exist in the frequency and modulation of the audio carrier. The monochrome combinations still existing in the 1950s were standardized by the International Telecommunication Union (ITU) as capital letters A through N. When color television was introduced, the chrominance information was added to the monochrome signals in a way that black and white televisions ignore. In this way backward compatibility was achieved.
There are three standards for the way the additional color information can be encoded and transmitted. The first was the American NTSC system. The European and Australian PAL and the French and former Soviet Union SECAM standards were developed later and attempt to cure certain defects of the NTSC system. PAL's color encoding is similar to the NTSC systems. SECAM, though, uses a different modulation approach than PAL or NTSC. PAL had a late evolution called PALplus, allowing widescreen broadcasts while remaining fully compatible with existing PAL equipment.
In principle, all three color encoding systems can be used with any scan line/frame rate combination. Therefore, in order to describe a given signal completely, it is necessary to quote the color system plus the broadcast standard as a capital letter. For example, the United States, Canada, Mexico and South Korea used (or use) NTSC-M, Japan used NTSC-J, the UK used PAL-I, France used SECAM-L, much of Western Europe and Australia used (or use) PAL-B/G, most of Eastern Europe uses SECAM-D/K or PAL-D/K and so on.
Not all of the possible combinations exist. NTSC is only used with system M, even though there were experiments with NTSC-A (405-line) in the UK and NTSC-N (625-line) in part of South America. PAL is used with a variety of 625-line standards (B, G, D, K, I, N) but also with the North American 525-line standard, accordingly named PAL-M. Likewise, SECAM is used with a variety of 625-line standards.
For this reason, many people refer to any 625/25 type signal as PAL and to any 525/30 signal as NTSC, even when referring to digital signals; for example, on DVD-Video, which does not contain any analog color encoding, and thus no PAL or NTSC signals at all.
Although a number of different broadcast television systems are in use worldwide, the same principles of operation apply.
Displaying an image
A CRT television displays an image by scanning a beam of electrons across the screen in a pattern of horizontal lines known as a raster. At the end of each line, the beam returns to the start of the next line; at the end of the last line, the beam returns to the beginning of the first line at the top of the screen. As it passes each point, the intensity of the beam is varied, varying the luminance of that point. A color television system is similar except there are three beams that scan together and an additional signal known as chrominance controls the color of the spot.
When analog television was developed, no affordable technology for storing video signals existed; the luminance signal had to be generated and transmitted at the same time at which it is displayed on the CRT. It was therefore essential to keep the raster scanning in the camera (or other device for producing the signal) in exact synchronization with the scanning in the television.
The physics of the CRT require that a finite time interval be allowed for the spot to move back to the start of the next line (horizontal retrace) or the start of the screen (vertical retrace). The timing of the luminance signal must allow for this.
The human eye has a characteristic called phi phenomenon. Quickly displaying successive scan images creates the illusion of smooth motion. Flickering of the image can be partially solved using a long persistence phosphor coating on the CRT so that successive images fade slowly. However, slow phosphor has the negative side effect of causing image smearing and blurring when rapid on-screen motion occurs.
The maximum frame rate depends on the bandwidth of the electronics and the transmission system, and the number of horizontal scan lines in the image. A frame rate of 25 or 30 hertz is a satisfactory compromise, while the process of interlacing two video fields of the picture per frame is used to build the image. This process doubles the apparent number of video frames per second and further reduces flicker and other defects in transmission.
Receiving signals The television system for each country will specify a number of television channels within the UHF or VHF frequency ranges. A channel actually consists of two signals: the picture information is transmitted using amplitude modulation on one carrier frequency, and the sound is transmitted with frequency modulation at a frequency at a fixed offset (typically 4.5 to 6 MHz) from the picture signal.
The channel frequencies chosen represent a compromise between allowing enough bandwidth for video (and hence satisfactory picture resolution), and allowing enough channels to be packed into the available frequency band. In practice, a technique called vestigial sideband is used to reduce the channel spacing, which would be nearly twice the video bandwidth if pure AM was used.
Signal reception is invariably done via a superheterodyne receiver: the first stage is a tuner which selects a television channel and frequency-shifts it to a fixed intermediate frequency (IF). The signal amplifier performs amplification to the IF stages from the microvolt range to fractions of a volt.
Extracting the sound At this point, the IF signal consists of a video carrier signal at one frequency and the sound carrier at a fixed offset in frequency. A demodulator recovers the video signal. Also at the output of the same demodulator is a new frequency-modulated sound carrier at the offset frequency. In some sets made before 1948, this was filtered out, and the sound IF of about 22 MHz was sent to an FM demodulator to recover the basic sound signal. In newer sets, this new carrier at the offset frequency was allowed to remain as intercarrier sound, and it was sent to an FM demodulator to recover the basic sound signal. One particular advantage of intercarrier sound is that when the front panel fine-tuning knob is adjusted, the sound carrier frequency does not change with the tuning, but stays at the above-mentioned offset frequency. Consequently, it is easier to tune the picture without losing the sound.
So the FM sound carrier is then demodulated, amplified, and used to drive a loudspeaker. Until the advent of the NICAM and MTS systems, television sound transmissions were monophonic.
Structure of a video signal The video carrier is demodulated to give a composite video signal containing luminance, chrominance and synchronization signals. The result is identical to the composite video format used by analog video devices such as VCRs or CCTV cameras. To ensure good linearity and thus fidelity, consistent with affordable manufacturing costs of transmitters and receivers, the video carrier is never modulated to the extent that it is shut off altogether. When intercarrier sound was introduced later in 1948, not completely shutting off the carrier had the side effect of allowing intercarrier sound to be economically implemented.
Each line of the displayed image is transmitted using a signal as shown above. The same basic format (with minor differences mainly related to timing and the encoding of color) is used for PAL, NTSC, and SECAM television systems. A monochrome signal is identical to a color one, with the exception that the elements shown in color in the diagram (the color burst, and the chrominance signal) are not present.