Thông tin thiết kế mạch P6

THE TELEVISION TRANSMITTERThe transmission of video images depends on a scanning device that can break up the image into a grid and measure the brightness of each element of the grid. This information can be sent serially or in parallel to a distant point and used to reproduce the image. It is evident that the smaller the size of the grid element, the better the definition of the image. One of the simplest devices which can measure the brightness of light is the phototube
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Thông tin thiết kế mạch P6 Telecommunication Circuit Design, Second Edition. Patrick D. van der Puije Copyright # 2002 John Wiley & Sons, Inc. ISBNs: 0-471-41542-1 (Hardback); 0-471-22153-8 (Electronic) 6 THE TELEVISION TRANSMITTER6.1 INTRODUCTIONThe transmission of video images depends on a scanning device that can break upthe image into a grid and measure the brightness of each element of the grid. Thisinformation can be sent serially or in parallel to a distant point and used to reproducethe image. It is evident that the smaller the size of the grid element, the better thedefinition of the image. One of the simplest devices which can measure the brightness of light is thephototube. It consists of a cathode which is coated with a material which gives offelectrons when light is shone on it and an anode which can collect the emittedelectrons when a suitable voltage is applied to it. The cathode and anode areenclosed in an evacuated glass envelope. The number of electrons emitted by thecathode is proportional to the intensity of the light impinging on it. Assumingcomplete collection of the electrons, the current in the resistor R shown in Figure 6.1will be proportional to the light intensity and so will the voltage across R. A primitive video signal can be generated by using a 3 Â 3 matrix made up ofphototubes as shown in Figure 6.2. For simplicity we assume that the tree is black and its background is white. Asuitable lens focuses the image of the tree onto the matrix of phototubes. It is clearthat the voltage output from phototubes (1,1), (1,3), (3,1) and (3,3) will be high; allothers will be low. The voltages so obtained can be transmitted and used to controlthe brightness of a corresponding 3 Â 3 matrix of lights at a distant point giving avague idea of what the tree looks like! The picture detail can be improved byincreasing the number of elements in the matrix so that each element corresponds tothe smallest area possible. The assumption of a black tree on a white background isno longer necessary since, with increasing detail, different shades of grey can beaccommodated. The information may be sent along individual wires linking the phototube to thelight matrix (parallel transmission) but this would be very expensive and impractical 161162 THE TELEVISION TRANSMITTER Figure 6.1. The phototube with its added circuitry to convert light intensity to voltage.for any system other than the simple one described here. A better system would beone in which the voltage from each phototube is scanned in some given order and thevoltage and position of each phototube are sent on a single wire to the receiving endfor reconstruction (serial transmission). The price to be paid for reducing the numberof wires is the increased complexity introduced by the scanner and a system forcoding and decoding the voltage and position information at the transmitter andreceiver, respectively.6.2 SYSTEM DESIGNFigure 6.3 shows the basic components of a television transmitter. A system oflenses focus the image onto a camera tube which collects and codes the informationabout the brightness and position of each element of the matrix forming the pictureby scanning the matrix. A pulse generator supplies pulses to the camera to controlthe scanning process. The output from the camera goes to a video amplifier foramplification and the addition of extra pulses to be used at the receiver for decodingpurposes. Figure 6.2. Generation of a primitive video signal using a 3 Â 3 matrix of phototubes. 6.3 COMPONENT DESIGN 163 Figure 6.3. A block diagram of the television transmitter. A microphone picks up the sound associated with the picture and afteramplification the signal is fed to the audio terminal of a frequency modulator. Thecarrier signal supplied to the modulator is a 4.5 MHz signal, generated by a crystal-controlled oscillator at a lower frequency and multiplied by an appropriate factor.The FM signal carrying the audio information is added to the video signal. Theoutput of the video amplifier consisting of the video signal, receiver control pulses,and the frequency modulated signal is fed to the amplitude modulator. The carrier ofthe amplitude modulator is supplied by a second crystal oscillator and associatedmultiplier which produce a signal of frequency which lies within the band 54–88 MHz (VHF). The radiofrequency power amplifier boosts the power to the legallydetermined value and the vestigial sideband filter removes most of the lowersi ...