Priyadarshini College of Engineering Department of Electronics & Telecommunication Engineering Semester: VII Subject: Television and Video Engineering Experiment No. 1 Aim: - To identify different components and section in TV receiver. Objective: Apparatus: Television receiver trainer kit, Cathode ray oscilloscope. Theory: Television means ‘to see from a distance’. In the early years of the twentieth century many scientists experimented with the idea of using selenium photosensitive cells for converting light from pictures into electrical signals and transmitting them through wires. Figure (1) Block schematic of Monochrome Television receiver Figure(1) Illustrates block schematic of Monochrome Television receiver. The explanation of different stages of Monochrome TV receiver is as follows: 1. Antenna: The main function of the antenna is to accept the electromagnetic waves coming from the television transmitter. All TV antennas are mounted in horizontal position for better reception and favorable Signal to Noise Ratio. For channels located in the VHF band, a half wave length antenna is mostly used. Various antennas in use are dipole type with reflectors and directors. A Yogi antenna i.e. a dipole with one reflector and two or more directors is also used. For channels located in UHF band, special antennas Fan dipole, rhombic and parabolic reflector type are used. 2. RF Section: RF Tuner OR Front end This section consists of RF amplifier, Mixer and Local oscillator. The function of this section is to amplify both the sound and picture signals picked up by the antenna and to convert the carrier frequencies and their associated bands into the intermediate frequencies and their Sidebands. The setting of the local oscillator frequency enables Selection of desired station. The standard intermediate frequencies for the 625-B system are:Picture IF = 38.9 MHz Sound IF = 33.4 MHz 3.IF Amplifier Section: Video IF Amplifier This section is also called Video IF amplifier since composite video signal is the envelope of the modulated picture IF signal. All the gain and selectivity of the receiver is provided by the IF section. The main function of this section is to amplify modulated IF signal over its entire bandwidth with an input of about 0.5 mV signal 5 form the mixer to deliver about 4V into the video detector. This needs an overall gain of about 8000. 4. AGC Section: AGC circuit controls gain of RF and IF stages to deliver almost constant signal voltage to the video detector, despite changes in the signal picked up by the antenna. The change in gain is achieved by shifting the operating point of the amplifying devices used in the amplifiers. The operating point is changed by a bias voltage that is developed in the AGC circuit. 5. Sound IF section: The Picture and sound signals on their respective carriers are amplified together in the IIF section. In Video detector the picture IF 38.9 MHz acts as the carrier and beats with the sound carrier 33.4 MHz and its associated FM side band frequencies to produce difference 5.5 MHz + 50 KHz components. This is called Inter carrier beat signals which is amplified by the IF amplifier and then given to FM detector. 6. Audio Amplifier: The output of FM detector is amplified by the Audio amplifier stage. 7. Loud Speaker : The loudspeaker converts the electrical output of audio amplifier Stage into sound signal. 8. Video Pre-amplifier: The output of IF section is given to video detector stage to recover the composite video signal and to transform the sound signal to lower carrier frequency. 9. Video Amplifier: The picture tube needs video signal with peak to peak amplitude of 80 to 100 volts for producing picture with good contrast. Video amplifier provides gain of 40 to 60 to the output of video detector having 2 volts magnitude. The response of the amplifier should be ideally flat from dc to 5 MHz to include all essential video components. Contrast Control: It is used to adjust contrast between black and white parts of the picture. 10. Picture tube circuitry: The output from the video amplifier can be fed either at the cathode or control grid of the picture tube . The cathode drive is preferred and grid is left free to receive retrace blanking pulses to ensure that no retrace lines are seen on the screen. 11. Sync Separator circuit: This circuit separates horizontal and vertical sync pulses from Composite video signal. 12. Horizontal Frequency Oscillator: It is set to develop sweep drive voltage at 15625 Hz. Its frequency is controlled by dc control voltage developed by the AFC circuit. 6 13. Horizontal output amplifier: The oscillator output is wave shaped to produce linear rise of current in the horizontal deflection coils. As deflection coils need about one 1 ampere of current to sweep the entire raster, the output of the oscillator is amplified by the amplifier and then given to the horizontal deflection coils. 14. Vertical Frequency Oscillator: Blocking Oscillators and cathode coupled multivibrators are used as Vertical deflection oscillators. The output of this stage is saw tooth waveform having frequency 50 Hz. The controlling time constant are chosen to develop output according to trace and retrace periods. 15. Vertical output amplifier: The output from the oscillator cum wave shaping circuit is fed to power amplifier which is then fed to the vertical deflection coils to produce vertical deflection of the beam on the picture tube screen. 16. E.H.T Supply: An anode voltage of the order of 15 kV is needed for sufficient in Black and white picture tubes. This is known as HV or EHT (Extra High Tension) supply. This is obtained from the Horizontal output transformer. 17. Low voltage Power supply: The B+ or low voltage power supply is obtained by rectifying and Filtering the ac mains supply by stepping it up or down before rectification and then provided to various sections. Conclusion: Thus, we identified different components and section in TV receiver. Discussion Questions: i) Discuss the factors affecting choice of IF. ii)Discuss what is trap circuit. Where the trap circuit used in TV Receiver? iii) How is FM signal demodulated in TV Receiver?