Television Receiver


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Explanation of video signal processing at television receiver is discussed briefly. PAL receiver is studied.

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Television Receiver

  1. 1. Television Receiver
  2. 2. Contents  RF Tuner  IF Subsystem  Video amplifier  Sound section  Sync separation and processing  Deflection circuits  Scanning Currents in the yoke  DC power supplies  Electronic tuners  IF Subsystem
  3. 3. Contents  Y Signal channel  Chroma decoder  Separation of U and V colour phasors  Synchronous demodulators  Sub carrier generation and control  Matrixing for drive circuits  Receiver Servicing  Video pattern generator  Sweep & Marker generator  Colour TV Pattern Generator  Vectroscope
  4. 4. RF tuner
  5. 5. RF tuner  This section consists of RF amplifier, mixer and local oscillator  The purpose of the tuner unit is to amplify both 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 receiver uses superhetrodyne principle as used in radio receivers  The setting of the local oscillator frequency enables selection of desired station
  6. 6. RF tuner  The standard intermediate frequencies for the 625-B system are-Picture IF = 38.9 MHz, Sound IF = 33.4 MHz
  7. 7. RF tuner
  8. 8. Choice of selecting IF  1) Image Rejection Ratio:
  9. 9. Choice of selecting IF  The undesired signal which gets received is spaced at a gap of twice the IF frequency, and is known as ‘Image Signal’  The image rejection ratio is defined as the output due to desired station divided by output due to image signal  With RF amplifier the output due to image signal can be very much reduced or completely eliminated  Here as IF will be greater, the image frequency will be greater and there are more chances to eliminate it through well designed filter
  10. 10. Choice of selecting IF  2) Pick-up Due to Local Oscillator Radiation from TV Receivers  If the output from the local oscillator of a TV receiver gets coupled to the antenna, it will get radiated and may cause interference in another receiver  Here again advantage lies with higher IF frequency, because with higher IF there is a greater separation between the resonant circuits of local oscillator and RF amplifier circuits
  11. 11. Choice of selecting IF
  12. 12. Choice of selecting IF  3) Ease of Separation of Modulating Signal from IF Carrier at the Demodulator  For ease of filtering out the IF carrier freuency, it is desirable to have a much higher IF frequency as compared to the highest modulating frequency  In radio receivers the IF frequency is 455 KHz and the highest audio frequency is only 5 KHz  In TV receivers, with the highest modulating frequency of 5 MHz, an IF frequency of atleast 40 MHz is desirable
  13. 13. Choice of selecting IF  4) Image Frequencies Should Not Lie in the FM Band  The FM band is from 88 MHz to 110 MHz  With IF frequency chosen close to 40 MHz, the image frequencies of the lower VHF band fall between 121 to 148 MHz and thus cannot cause any interference in the FM band  Higher TV channels are much above the FM band
  14. 14. Choice of selecting IF  5) Interference or Direct Pick-Up from Bands Assigned for other Service  Amateur and industrial applications frequency band lies between 21 to 27 MHz  If the IF frequency is chosen above 40 MHz, even the second harmonics of this band will not cause any serious direct pick-up problems
  15. 15. Choice of selecting IF  6) Gain  The television receiver should produce enough gain at high IF frequencies  This is achievable through today’s highly accurate transistors
  16. 16. Video Detector
  17. 17. Video Amplifier  The video amplifier is dc coupled from the video detector to the picture tube, in order to preserve the dc component for correct brightness  However, in some video amplifier designs, on account of complexities of a direct coupled amplifier, ac coupling is instead used  The dc component of the video signal is restored by a diode clamper before feeding it to cathode or grid of the picture tube  In transistor amplifier designs, a suitable configuration of two transistors and a driver often becomes necessary to obtain the same gain
  18. 18. Video Amplifier  Besides gain, response of the amplifier should ideally be flat from dc (zero) to 5 MHz to include all essential video components  This needs rigorous design considerations because the band of frequencies to be covered extends from dc through audio range to radio frequencies  A loss in gain of high frequency components in the video signal would reduce sharpness of the picture whereas a poor low frequency response will result in loss of boundary details of letters etc  It is also essential that phase distortion in the amplifer is kept to a minimum
  19. 19. Sound section  The relatively weak FM sound signal is given at least one stage of amplification before feeding it to the FM detector  The FM detector is normally a ratio detector or a discriminator preceded by a limiter  The characteristics of a typical FM detector are shown in Fig
  20. 20. Sound section  A tuned amplifier, with enough bandwidth to pass the FM sound signal is used to boost the FM signal  The volume and tone controls form part of the audio amplifiers  The power amplifier is either a single ended or push-pull configuration employing transistors  Special ICs have been developed which contain FM demodulator and most parts of the audio amplifier
  21. 21. Sync separation and processing  The horizontal and vertical sync pulses that form part of the composite video signal are separated in the sync separator  A sync separator is a clipper that is suitably biased to produce output, only during sync pulse amplitude of the video signal  The pulse train as obtained from the sync separator is fed simultaneously to a differentiating and an integrating circuit
  22. 22. Sync separation and processing  The differentiator, being a high-pass filter, develops output in response to noise pulses in addition to the spiked horizontal sync pulses
  23. 23. Sync separation and processing  This results in occasional wrong triggering of the horizontal oscillator which results in diagonal tearing of the reproduced picture  To overcome this difficulty, a special circuit known as automatic frequency control (AFC) circuit is employed  The AFC circuit employs a discriminator arrangement which compares the incoming horizontal sync pulses and the voltage that develops across the output of the horizontal deflection amplifier
  24. 24. Deflection circuits  The necessary sawtooth voltage for vertical and horizontal is developed by charging and discharging a capacitor with different time constants  For vertical deflection, the frequency of the oscillator is controlled by varying the resistance of the RC coupling network and is locked in synchronism by the vertical sync pulses  A part of the coupling network resistance is a potentiometer that is located on the front panel of the receiver  This is known as ‘Vertical Hold Control’
  25. 25. Deflection circuits  The frequency of horizontal oscillator is controlled by dc control voltage developed by the AFC circuit  Since the noise pulses in the control voltage are completely suppressed, most receivers do not provide any horizontal frequency (hold) control  Since the deflection coils need about one amp of current to sweep the entire raster, the output of the oscillator is given one stage of power amplification (as for vertical deflection) and then fed to the horizontal deflection coils
  26. 26. Scanning Currents in the yoke  Vertical scanning current: The vertical oscillator uses an RC network to develop a sawtooth waveform  With this as an input to the vertical amplifier, a sawtooth current flows through the vertical deflection coil to cause vertical scanning  The vertical output stage is a power amplifier which acts as a current source to produce a linear rise of magnetic field in the deflection coil  The stage is cut off for brief period of retrace only
  27. 27. Scanning Currents in the yoke  Horizontal scanning current: The horizontal coil is given a current changes of several amperes in 52us  Due this large change a self induced voltage is generated across the coil which creates the horizontal trace  The horizontal amplifier is continuously switched ON and switched OFF  The horizontal output stage consumes more than 75 percent of the total power used by the receiver
  28. 28. DC power supplies  Various DC sources needed in a typical television receiver are as under  Low voltage: about 12 to 35 volts for IC and small signal amplifiers  Medium voltages: about 150 v for horizontal output stage, 300 to 400v for the screen and focus grid of picture tube and about 175 v for the video amplifier  High voltage: 15 to 18KV for final anode of picture tube
  29. 29. PAL-D Colour Decoder
  30. 30. Electronic tuners for colour television  Varactor is a special silicon diode, the junction capacitance of which is used for tuning  This capacitance varies inversely with the amount of reverse bias applied across the diode  The resonant frequency of the tuned circuits in which they are connected, is controlled merely by changing the reverse bias across the varactor  Figure shows a basic circuit for varactor diode tuning
  31. 31. Electronic tuners for colour television
  32. 32. IF Subsystem for colour television  IF system for colour television consists of  band shaping filter circuit  IF amplifiers(AGC controlled)  AGC(Automatic gain control)  AFT(Automatic frequency tuning)  Intercarrier sound IF detector  Video detector  Buffer video amplifier
  33. 33. Block diagram of AFT
  34. 34. IF Subsystem  After passing through IF subsystem the colour signal is given in to 4 blocks  Luminance or Y channel  Chroma decoder  AGC circuit  Sync-separator and Raster circuit
  35. 35. Y signal Channel
  36. 36. Y signal Channel  Y signal represents the brightness of the picture signal  The colour signals get added on Y signal to reproduce coloured scene on raster  To recover colour information from Y signal, we need to use comb filter  It selects frequencies that need to be passed and rejects the other frequencies in Y signal band  To achieve this video signal v is applied to a delay line of 64 us and inverter  Output of delay line and inverter are added and chroma signal frequencies are recovered
  37. 37. Y signal Channel
  38. 38. Y signal Channel
  39. 39. Chroma decoder
  40. 40. Chroma decoder  The main function of chroma decoder is to recover U and V colour difference signals which are combined with Y to obtain R,G and B video signal  For this the decoder has to perform following functions  Chroma signal seperation and amplification  Separation of U and V  Demodulation of U and V  Generation of subcarriers for two demodulators  To develop ‘ident’ signal
  41. 41. Separation of U and V colour phasors
  42. 42. Synchronous demodulators
  43. 43. Synchronous demodulators
  44. 44. Synchronous demodulators
  45. 45. Sub carrier generation and control
  46. 46. Sub carrier generation and control  To extract the colour burst signal from composite video signal, a burst phase discriminator is used  It determines the phase difference of input colour burst and sets the reference oscillator accordingly to use it as a reference subcarrier for U and V demodulators  Also a colour killer block and burst phase identification block is used to turn ON the second stage amplifier only when a colour signal is received  In absence of colour burst, the amplifier will be OFF and the signal is considered as monochrome signal
  47. 47. Matrixing for drive circuits
  48. 48. Matrixing for drive circuits  The value of R1, R2 and R3 are so chosen that the G-Y amplifier block receives input as per following equation  (G-Y) = -0.51(R-Y) – 0.186(B-Y)  Once all colour difference signals are extracted, they are applied to RGB matrix for cathode drive  Luminance signal Y is added to these difference signals to obtain the Vr, Vb and Vg colour voltages