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Worldwide television has a number of standards for color coding and the organization of audio and synchronization signaling. They are a combination of three color coding systems (NTSC, PAL, SECAM) and ten signaling and scanning standards: B, G, D, K, H, I, KI, N, M, L.

Note: standards B and G; D and K differ in the frequency values ​​of TV channels (MV and UHF, respectively). The polarity of the video signal modulation is "-" negative, "+" positive. Since interlaced scanning is used when "drawing" the image, the true frame rate is half the frame rate - the frequency of changing half-frames (fields). * To be precise, the field frequency is 58.94 Hz.

There are currently three compatible color television systems in operation - SEKAM, NTSC and PAL. Regardless of the type of system, signal sensors (television cameras) form signals of three primary colors: Er - red, Eg - green and Ed - blue. The same signals control the beam currents in the electronic projectors of the kinescope in the TV. By changing the ratio of signals on the kinescope cathodes, any color tone can be obtained within the color triangle determined by the color coordinates of the phosphors used. The differences between color television (CT) systems are in the methods of obtaining the so-called total color video signal (PCTS) from the primary color signals, which modulates the carrier frequency in the television transmitter. Such a transformation is necessary in order to place information about a color image in the frequency band of a black and white signal. At the heart of such a compaction of the signal spectra is a feature of the human visual system, which consists in the fact that small details of the image are perceived as uncolored. 

Primary color signals are converted into a wideband luminance signal Ey, corresponding to a black and white television video signal, and three narrowband signals carrying color information. These are the so-called color-difference signals. They are obtained by subtracting the luminance signal from the corresponding base color signal. The luminance signal is obtained by adding in a certain proportion of the three primary color signals:

Ey=rEr+gEg+bEb (*)

In all color television systems, only the luminance signals Eu and two color difference signals, Er-y and Eb-y, are transmitted. The signal Eg-y is restored in the receiver from the expression (*). (It should be noted that prior to mixing, the primary color signals go through gamma-correction circuits that compensate for distortions caused by the non-linear dependence of the screen glow brightness on the amplitude of the modulating signal).

NTSC system

The NTSC system is the first DH system that has found practical application. Developed in the USA and accepted for broadcast in 1953. When creating the NTSC system, the basic principles of color image transmission were developed, which were used to one degree or another in all subsequent systems.

In the HTSC system, the PTTS contains in each line a luminance component and a chrominance signal transmitted using a subcarrier lying in the luminance signal bandwidth. The subcarrier is modulated in each line with two chromaticity signals Er-y and Eb-y. To prevent color signals from creating mutual interference, quadrature balanced modulation is used in the HTSC system. 

There are two main HTSC chrominance subcarrier values: 3.579545 and 4.43361875 MHz. The second value is minor and is used mainly in video recording to use the common recording-playback channel with the PAL system. The NTSC system has a number of advantages: -- high color clarity with a relatively narrow-band transmission channel; The structure of the signal spectra makes it possible to effectively separate information using comb digital filters. The HTSC decoder is relatively simple and contains no delay lines. At the same time, the NTSC system also has some disadvantages, the main of which is its high sensitivity to signal distortions in the transmission channel. 

Signal distortion in the form of amplitude modulation (AM) is called differential distortion. As a result of such distortions, the color saturation of bright and dark areas is different. These distortions cannot be eliminated using the automatic gain control (AGC) circuit of the chrominance signal, since differences in the amplitude of the color subcarrier appear within the same line. Distortions in the form of phase modulation of a color subcarrier by a luminance signal are called differential phase distortions. They cause changes in color tone depending on the brightness of a given area of ​​the image. 

For example, human faces are painted reddish in shadows and greenish in highlights. To reduce the visibility of d-f distortion, NTSC TVs are provided with an operational color tone control, which allows you to make more natural coloring of parts with the same brightness. However, color tone distortion in brighter or darker areas increases. High requirements to the parameters of the transmission channel lead to the complexity and cost of HTSC equipment or, if these requirements are not met, to a decrease in image quality. The main goal in the development of the PAL and SECAM systems was to eliminate the shortcomings of the NTSC system.

PAL system

The PAL system to eliminate the main one was developed by the "Telefunken" company in 1963. The purpose of its creation was a drawback later found out, HTSC - sensitivity to differential - phase distortion. In what the PAL system has obvious. a number of advantages that initially did not seem In the PAL system, as in the NTSC, quadrature modulation of the color subcarrier by chrominance signals is used. But if in the NTSC system the angle between the total vector and the BY vector axis, which determines the color tone when the color field is transmitted, is constant, then in the PAL system its sign changes every line. Hence the name of the system -- Phase Alternation Line. Reduction of sensitivity to differential - phase distortions is achieved by averaging the color signals in two adjacent lines, which leads to a decrease in vertical color clarity by a factor of two compared to HTSC. This feature is a disadvantage of the PAL system. Advantages: low sensitivity to diff - phase distortions and asymmetry of the color channel passband. (The latter feature is particularly valuable in countries that adopt the G standard with a 5.5 MHz video/audio carrier spacing, which always results in upper chrominance sideband clipping.) The PAL system also has a signal-to-noise gain of 3dB over HTSC. PAL60 is an HTSC video playback system. In this case, the HTSC signal is easily transcoded into PAL, but the number of fields remains the same (that is, 60). The TV set must support this frame rate value.

SECAM system

The SEKAM system in its original form was proposed in 1954. French inventor Henri de France. The main feature of the system is sequential, through a line, transmission of color difference signals with further restoration of the missing signal in the receiver using a delay line for the time of the line interval. The name of the system is formed from the initial letters of the French words SEquentiel Couleur A Memoire (alternate colors and memory). In 1967, broadcasting on this system began in the USSR and France. 

Color information in the SECAM system is transmitted using frequency modulation of the color subcarrier. The rest frequencies of the subcarriers in rows R and B are different and are Fob=4250kHz and For=4406.25kHz. Since in the SECAM system chrominance signals are transmitted sequentially through a line, and in the receiver it is restored using a delay line, i.e. If the information from the previous line is repeated, then the vertical color sharpness is halved, as in the PAL system. The use of FM provides low sensitivity to the action of distortions of the "differential gain" type.

 The sensitivity of SECAM to diff-phase distortions is also low. On color fields, where the brightness is constant, these distortions do not appear in any way. On the color transitions, a spurious increase in the subcarrier frequency occurs, which causes their delay. However, when the transition duration is less than 2 µs, the correction circuits in the receiver reduce the effect of these distortions. Typically, after bright areas of the image, the fringing is blue, and after dark areas, it is yellow. The tolerance for "differential phase" distortion is about 30 degrees, i.e. 6 times wider than in HTSC.

D2-MAC system

In the late 70s, improved color television systems were developed that used time division with compression of the luminance and chrominance components. These systems are the basis for high-definition television (HDTV) systems, and have received the name MAK (MAS) - "Multiplexed Analog Components". In 1985, France and Germany agreed to use one of the modifications of the MAC systems, namely D2-MAC / Paket, for satellite broadcasting. Main features: the initial line interval of 10 μs is reserved for the transmission of digital information: line sync, sound and teletext. In the digital package, cudgel coding is used using a three-level signal, which reduces the required bandwidth of the communication channel by a factor of two. This coding principle is reflected in the name - D2. Two stereo audio channels can be transmitted simultaneously. The rest of the line is occupied by analog video signals. First, a compression string of one of the color-difference signals (17 µs) is transmitted, then a luminance string (34.5 µs). The principle of color coding is approximately the same as in SEKAM. To transmit a complex D2-MAC signal, a channel with a bandwidth of 8.4 MHz is required. The D2-MAC system provides substantially better color image quality than all other systems. There is no interference from the color subcarriers in the image, there is no crosstalk between the luminance and chrominance signals, and the image clarity is noticeably improved.

Publication: radioman.ru

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