Description international television standards. Their differences, advantages and disadvantages.
World broadcasting has a number of standards for color coding and transmission of sound signals, and synchronization. They are a combination of the three color coding systems (NTSC, PAL, SECAM) and ten standards for the transmission of signals and sweep: B,G,D,K,H,I,KI,N,M,L.
The parameters of the signal M N B,G H I D,K KI L The number of rows in the frame 525 625 625 625 625 625 625 625 The number of fields 60* 50 50 50 50 50 50 50 Bandwidth, MHz 6 6 7;8 8 8 8 8 8 Width of main sideband image, MHz 4.2 4.2 5 5 6 6 6 6 The spacing of bearing sound and image, MHz 4.5 4.5 5.5 5.5 6 6.5 6.5 6.5 The polarity of the modulation signal - - - - - - - + Modulation type sound FM FM FM FM FM FM FM AM The deviation of the sound carrier frequency, kHz 25 25 50 50 50 50 50 -Note: the standards B and G; D and K differ in the frequencies of TV channels (VHF and UHF, respectively).
The polarity of the modulation signal "-" negative "+" positive.
Because "drawing" of the image with interlacing, the true frame rate is half the frame rate - frequency shift of half-frames (fields).
* To be precise, the field frequency is equal 58.94 Hz.
Currently in operation are three compatible color TV system - SECAM, NTSC and PAL. Regardless of the type of system sensors signals (TV camera) is formed from three primary colors: Er - red, Eg green and Ed blue. These signals control currents rays in electronic projectors picture tube in the TV. By changing the ratio of the signals at the cathodes of a CRT, you can get any color tone within the color triangle defined by the color coordinates of the phosphors used.
The differences between the systems of color television (CG) methods are receiving signals from so-called primary colors full color video (PTSTS), which modulates the carrier frequency in a television transmitter.
This conversion is necessary in order to place the information about the color image in the frequency band of the black and white signal. In such a seal of spectrum signals is a feature of the human visual system, consisting in the fact that small parts of the image are perceived as unpainted .
The signals of primary colors is converted into a broadband luminance signal EY corresponding to the video signal is black-and-white television, and three narrowband signals carrying color information.
This so-called color difference signals. They are obtained by subtracting from the corresponding primary color signal of the luminance signal.
The brightness signal obtained by addition in a certain proportion of three signals of primary colors:
Ey= rEr+gEg+bEb (1)
In all color television systems transmit only the luminance signal EY and two color-difference signals Er-y and Eb-y. The signal Eg-y is recovered in the receiver from the expression (1). (It should be noted that prior to mixing the signals of the primary colors are chain gamma correction, compensating for distortion caused by nonlinear dependence of the brightness of the screen on the amplitude of the modulating signal).
System NTSC
System NTSC the first district heating system, which has practical application. Designed in USA and approved for broadcast in 1953. When creating system NTSC developed the main principles of the transfer of color images, in varying degrees, used in all subsequent systems.
In the system of NTSC PTSTS contains in each row of the brightness component and the color signal transmitted using subcarrier lying in the frequency band of the luminance signal. Subcarriers modulated in each row of the two color signals Er-y and Eb-y. The color signals did not create mutual interference, system NTSC used quadrature balanced modulation.
There are two main values of the chrominance subcarrier system NTSC: 3.579545 and 4.43361875 MHz. The second value is non-essential and is mainly used in the video for General use with a PAL system channel of the recording-playback.
System NTSC has a number of advantages: - high color resolution with a relatively narrow-band transmission channel; the structure of the spectra of the signals allows for the effective sharing of information through digital comb filters . The NTSC decoder is relatively simple and does not contain delay lines.
However system NTSC inherent disadvantages, the main of which is its high sensitivity to signal distortion in the transmission channel.
Signal distortion in the form of amplitude modulation (AM) are called differential distortion. As a result of such distortion, color saturation bright and dark areas. These distortions cannot be eliminated using the circuit of automatic gain control (AGC) of a color signal, because the differences in the amplitude of the color subcarrier appear in the same row.
Distortions in the form of phase modulation of the color subcarrier signal brightness is called differential phase distortion. They cause changes in the color tones depending on the brightness of the image area.
For example, human faces are painted in a reddish color in the shadows and greenish in highlights.
To reduce the visibility of d-f distortion, NTSC TVs, an operational controller color tone, which allows more natural color parts with identical brightness. However, distortion of the color tone brighter or darker areas are higher in this case.
High requirements to the parameters of the transmission channel leads to the complication and cost of equipment NTSC or, if these requirements are not met, reduce the quality of the image.
The main objective when developing the system, PAL and SECAM was addressing the weaknesses in the system of NTSC.
PAL
PAL system developed by the company "Telefunken" in 1963. Its aim was to eliminate the main drawback NTSC - sensitivity differential phase distortion. Later it became clear that the PAL system has a number of advantages that are not initially obvious.
In the PAL system, as in NTSC applies quadrature modulation of the color subcarrier of the chrominance signals. But if the system is NTSC, the angle between the total vector and the axis vector B-Y, which determines the color tone when the color field is constant, then in the PAL system changes its sign each line. Hence the name of the system - Phase Alternation Line.
Reduction of sensitivity to the differen - phase distortion is achieved by averaging the color signals in two adjacent lines, reducing the vertical definition color two times as compared to NTSC. This feature is a drawback of the system FELL.
Advantages : low sensitivity to difference and phase distortion and asymmetry of the bandwidth of the color channel. (The latter property is particularly valuable for countries that adopted the standard G spacing of bearing picture and sound 5.5 MHz, which always causes a restriction of the upper sideband of the chrominance signal.)
PAL system also has a winning ratio signal / noise by 3dB relative to NTSC.
PAL60 - playback system NTSC video. The NTSC signal easy is transcoded into FELL, but the number of fields remains the same (i.e., 60). The TV must support the frame rates.
The SECAM system
SECAM system in its original form proposed in 1954. French inventor Henri de France. The main feature of the system - alternately, through the line, the transfer of color-difference signals with the further recovery in the receiver of a missing signal using the delay line during the line interval.
The name of the system formed from the initial letters of the French words SEquentiel Couleur A Memoire (sequential color and memory). In 1967 it was launched on this system in the USSR and France.
The color information in the SECAM system is transmitted using frequency modulation of the color subcarrier. The rest frequency subcarriers in rows R and B are different and are Fob=kg and For=4406.25 kHz .
Because in the SECAM system, the color signals are passed alternately through the line, and the receiver is restored by using delay lines, i.e., repeats information from the previous line, the color resolution in the vertical cut in half, as in the PAL system.
The use of FM provides a low sensitivity to the effect of distortion type "differential reinforcement". Small sensitivity SECAM and differential phase distortions. The color fields, where the brightness is constant, this distortion does not occur. On same color transitions occur spurious increment of the subcarrier frequency, causing their delay. However, when the duration of the transition is less 2mks correction circuit in the receiver to reduce the effects of these distortions.
Usually after the bright areas of the image border is blue, and after dark - yellow. The tolerance on distortion type "differential phase" is about 30 degrees, i.e. 6 times wider than NTSC.
System D2-MAC
At the end of the 70 years, we have developed an improved color television system, using a temporal separation of the sealing components of brightness and colour. These systems are the basis for systems of high-definition television (HDTV), and received the name of the MAC (MAC) - "Multiplexed Analog Components.
In 1985, France and Germany agreed to use a satellite feed of one of the modifications of MAC systems, namely D2-MAC / package.
Main features: the initial size of the interval 10mks reserved for the transmission of digital information: signal lines, sound and Teletext. In digital package applied Dubinino coding using a three-level signal, which is twice reduces the required bandwidth of the communication channel.
This coding principle is reflected in the title - D2. At the same time can be transmitted to two stereo sound channels.
The rest of the line is analog video signals. Is sent first seal line of one of the color-difference signals (MX), then the luminance line (34.5 µs). The principle of encoding a color approximately the same as that in Scame. For transmission of a complex signal D2-MAC requires a channel with a bandwidth of 8.4 MHz.
System D2-MAC provides significantly better color quality than all other systems. In the image there is no interference from the color subcarrier, there is no crosstalk between the luminance and chrominance markedly improved the clarity of the image.
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