Fundamentals of Monochrome and Colour Tv System
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1 FUNDAMENTALS OF MONOCHROME AND COLOUR TV SYSTEM Picture formation A picture can be considered to contain a number of small elementary areas of light or shade which are called PICTURE ELEMENTS. The elements thus contain the visual image of the scene. In the case of a TV camera the scene is focused on the photosensitive surface of pick up device and a optical image is formed. The photoelectric properties of the pick up device convert the optical image to a electric charge image depending on the light and shade of the scene (picture elements). Now it is necessary to pick up this information and transmit it. For this purpose scanning is employed. Electron beam scans the charge image and produces optical image. The electron beam scans the image line by line and field by field to provide signal variations in a successive order. The scanning is both in horizontal and vertical direction simultaneously. The horizontal scanning frequency is 15,625 Hertz. The vertical scanning frequency is 50 Hz. The frame is divided in two fields. Odd lines are scanned first and then the even lines. The odd and even lines are interlaced. Since the frame is divided into 2 fields the flicker reduces. The field rate is 50 Hertz. The frame rate is 25 Hertz (Field rate is the same as power supply frequency). Number of TV Lines per Frame If the number of TV lines is high larger bandwidth of video and hence larger R.F. channel width is required. If we go for larger RF channel width the number of channels in the R.F. spectrum will be reduced. However, with more no. of TV lines on the screen the clarity of the picture i.e. resolution improves. With lesser number of TV lines per frame the clarity (quality) is poor. A compromise between quality and conservation of r.f. spectrum led to the selection of 625 lines in CCIR system B. Odd number is preferred for ease of sync pulse generator (SPG) circuitary to enable interlace of fields. The capability of the system to resolve maximum number of picture elements along scanning lines determines the horizontal resolution. It means how many alternate black and white elements can be there in a line. Let us also take another factor. It is realistic to aim at equal vertical and horizontal resolution. We have seen earlier that the vertical resolution is limited by the number of active lines. We have already seen that the number of active lines are 575. so for getting the same resolution in both vertical 1 and horizontal directions the number of alternate black and white elements on a line can be 575 multiplied by Kell factor and aspect ratio. Therefore, the number of alternate black and white dots on line can be 575 x 0.69 x 4/3 which is equal to 528. It means there are 528 divided by 2 cyclic changes i.e. 264 cycles. These 264 cycles are there during 52 micro seconds. Hence the highest frequency is 5 MHz. 2 6 4× 1 06 f = = 5 M H z h ig h e s t 5 2 Therefore the horizontal resolution of the system is 5 MHz. A similar calculation for 525 lines system limits the highest frequency to 4 MHz and hence the horizontal resolution of same value. In view of the above the horizontal bandwidth of signal in 625 lines system is 5 MHz. Viewing Distance Optimum viewing distance from TV set is about 4 to 8 times the height of the TV screen. While viewing TV screen one has to ensure that no direct light falls on the TV screen. Composite Video Signal (CVS) Composite Video Signal is formed with Video, sync and blanking signals. The level is standardized to 1.0 V peak to peak (0.7 volts of Video and 0.3 volts of sync pulse). The composite video signal (CVS) has been shown in figure 1. Front Porch 1.5µ 0.7V Sec. 1.0V 0.3V 4.7µ Sec. Back 5.8µ Sec. Porch 52µ Sec. 12µ Sec. Active Period Sync Tip H Blanking 64µ Sec. H Period Fig. 1 Composite Video Signal (CVS) RF Transmission of Vision and Sound Signals TV Transmission takes place in VHF Bands I and III and UHF Bands IV and V. Picture is amplitude modulated and sound is frequency modulated on different carriers separated by 5.5 MHz. Also for video amplitude modulation negative modulation is employed because of the following main advantages. • Pictures contain more information towards white than black and hence the average power is lower resulting in energy saving. (Bright picture points 2 correspond to a low carrier amplitude and sync pulse to maximum carrier amplitude). • Interference such as car ignition interfering signals appear as black which is less objectionable. • Picture information is in linear portion of modulation characteristic and hence does not suffer compression. Any compression that may take place is confined to sync pulse only. • The design of AGC circuit for TV Receiver is simpler. AM produces double side bands. The information is the same in both side bands. It is enough to transmit single side band only. Carrier also need not be transmitted in full and a pilot carrier can help. However, suppressing the carrier and one complete side band and transmitting a pilot carrier leads to costly TV sets. A compromise to save RF channel capacity is to resort to vestigial side band system in which one side band in full, carrier and a part of other side band are transmitted. Picture Carrier Sound Carrier B . S . L U.S.B f o t r a P -1.25 –0.75 0 5 5.5 Frequency MHz Fig. 5 Theoretical representation of the side bands in VSB transmission. 3 THE PAL COLOUR TELEVISION SYSTEM The Colour Television It is possible to obtain any desired colour by mixing three primary colours i.e. Red, Blue and green in a suitable proportion. Additive Colour Mixing The figure 10 shows the effect of projecting red, green, blue beams of light so that they overlap on screen. Y= 0.3 Red + 0.59 Green + 0.11 Blue Fig. 10 Additive Colour Mixing The Colour Television It is possible to obtain any desired colour by mixing three primary colours i.e., red, blue and green in suitable proportion. Thus it is only required to convert optical information of these three colours to electrical signals and transmit it on different carriers to be decoded by the receiver. This can then be converted back to the optical image at the picture tube. The phosphors for all the three colours i.e. R, G and B are easily available to the manufacturers of the picture tube. So the pick up from the cameras and output for the picture tube should consists of three signals i.e. R, G and B. It is only in between the camera and the picture tube of the receiver we need a system to transmit this information. 4 Colour television has the constraint of compatibility and reverse compatibility with the monochrome television system which makes it slightly complicated. Compatibility means that when colour TV signal is radiated the monochrome TV sets should also display Black & White pictures. This is achieved by sending Y as monochrome information along with the chroma signal. Y is obtained by mixing R,G & B as per the well known equation : Y = 0.3 R + 0.59 G + 0.11 B Reverse compatibility means that when Black & White TV signal is radiated the colour TV sets should display the Black & White pictures. If we transmit R, G, B, the reverse compatibility cannot be achieved. Let us see how : If we transmit Y, R & B and derive G then : Since Y = 0.3R + 0.59G + 0.11 B G = 1.7Y - 0.51 R - 0.19 B In such a case what happens with a colour TV set when we transmit black and white signal. R and B are zero, but G gun gets 1.7 Y. The net result is black & white pictures on a colour TV screen appear as Green pictures. So reverse compatibility is not achieved. Colour Difference Signals To achieve reverse compatibility, when we transmit Y, R-Y and B-Y instead of Y, R & B, we do not take G-Y as this will always be much lower than R-Y and B-Y and hence will needs more amplification and will cause more noise into the system. G-Y can be derived electronically in the TV receiver. In the previous paragraph we have seen G = 1.7 Y - 0.51 R - 0.19 B So G-Y = -0.51 (R-Y) - 0.19 (B-Y) Thus, colour difference signals fulfill the compatibility and reverse compatibility. Because in this case the colour difference signals are zero if the original signal is monochrome (i.e. R = B = G) So if we take R - Y R - Y = R - (0.3 R + 0.59 R + 0.11 R) = 0 Similarly B - Y = 0 As such colour difference signals are zero for white or any shade of gray whereas, Y carries the entire Luminance information. It is to be noted while R, G, B signals always have positive value R-Y, B-Y and G-Y signals can either be positive or negative or even zero. 5 The R-Y and B - Y chrominance signals may be recovered at the television receiver by suitable synchronous demodulation. But sub-carrier is to be generated by a local oscillator. This generated sub-carrier in the receiver must have same frequency as that of transmitted sub-carrier and also the same phase.