Unit : I Basics of TV & Fundamentals of Television Television basics

1. Unit : I • Basics of TV & Fundamentals of Television • Television basics • Elements of TV system • Block diagram • Production of luminance • Colour difference signal • Composite video signal • Channel bandwidth • Color TV systems • Colour fundamentals • Mixing of colors • Color perception • Chromaticity diagram

2. 1.1Block Diagram of TV Receiver

3. Elements of Television : 1. Receives signal from Transmitter 2. Feed to Receiver through Feeder cable 3. Yagi Antenna is used 3

4. 1. Antenna: A dipole with reflectors and directors, called as “Yagi Antenna” is used as its response is more uniform over a band of frequencies. • It picks up signal from electromagnetic waves travelling through space & feeds to tuner in the receiver. • Since it is not possible for one dipole antenna to cover both upper and lower VHF band channels effectively; high and low band dipoles are mounted together and connected to a common transmission line.

5. Design : Length of Dipole : 143 / f Mhz Length of Reflector : 152 / f Mhz Length of First Director: 137 / f Mhz f= Centre Freq of Channel Length : in meter Length of subsequent directors : Reduces 2.5%

6. 2. Balun:

8. It is a matching transformer, designed for impedance matching. • A transmission line connects antenna to the receiver input terminals for the RF tuner. • A Balun is used for matching balanced & unbalanced impedance at Antenna input side & RF section at output side.

9. It consists of two 150 ohm quarter-wave sections of bifilar windings on aferrite core. • At one end the two quarter-wave lines are connected inparallel to give 75 ohm impedance and at the other end they are connected in series to providean impedance equal to 300 ohms. • Either side of the Balun can be used for input with the opposite side for output.

10. 3. Tuner : Tuner RF Amplifier : • Select desired channel with the help of tuned circuit • Provides gain to the selected RF signal • Decouples the local oscillator from antenna • prevents local oscillator freq from Radiating in space 10

11. Tuner Local Oscillator : • It provides RF of constant amplitude & constant • frequency for the channel selected for reception. • Uses a very high stability crystal & variable • frequency oscillator to change channels to receive • different frequency 11

12. Tuner Mixer : • It receives two signal – one is the channel freq signal from RF amplifier & other from Local oscillator. • Mixer produces several inter-modulation products of frequencies. • One of the product is the difference freq of RF & local osc,which is called as “ Intermediate freq.(IF)” • Other freq are rejected by filter used after mixer 12

13. Functions of Tuner : • It selects the desired station & • rejects others. • 2. It converts all incoming channel freq. to • a common IF Frequency. • 3. It provides gain to the weak input • signal picked up by the antenna. • 4. It isolates the local oscillator from • the feeder circuit to prevent undesired • radiationthrough the antenna.

14. 5. It provides matching between the antenna and input circuits of the receiver, thuseliminating the appearance of ghost images on the screen. 6.It rejects any pick-up from station operating in the FM band. 7. It has provision to select any channel out of the various allotted in the VHF and UHFbands for TV transmission. 8. It has a fine tuning control for accurate setting of the selected station.

15. 3. IF Amplifier: • Tuner output feeds to the first IF amplifier. IF Amplifier Balun Tuner Antenna • 3 to 4 IF stages are needed to provide sufficient gain with an input of • about 0.5 mV signal from the mixer to deliver about 4 V into the video • detector • To amplify modulated (Amplitude modulated video & Frequency modulated Audio) IF signal over its entire bandwidth.

16. 4. Video Detector & Amplifier: Antenna Balun Tuner If Amplifier Video Det. & Ampl. • IF signals after amplification in the IF section are fed to • the video detector. • It recovers composite video signal & sound signal. • It is an envelope detector using diode. • Standard output of detector is 1 V peak to peak.

17. S Sound Section Video Det. Amplifier CRT Section Sync Seperator Section

18. 5. CRT Section:

19. The glass envelope contains an electron-gun structure that produces a beam of electrons aimed at the fluorescent screen. The video signal is fed to cathode Volt from EHT Section Beam is deflected by a pair of deflecting coils mounted on the CRT Anode

21. 6. Sound Section: Sound Section : It consists of stages – Sound IF Amp., Detector, output amp. & speaker to recover original audio signal.

22. 7. Sync Sepearator:

23. 7. Sync Sepearator: • It Separates Vertical & Horizontal pulses. • Vertical Oscillator generates 50 Hz • Hori oscillator generates 15625 Hz frequency • Output of both section is given to Deflection • Circuit / Yoke mounted on neck of CRT.

24. 9. Horinzontal Deflection Section: 8. Vertical Deflection Section: Deflects the beam Vertically & Horizontally to reproduce the Picture of screen of CRT

25. It generates Anode Voltage to CRT – 23KV, • Grid & Focus voltage for CRT 10. EHT Section: • It generates Auxiliary supplies required for • other different sections : 12V,32V,175V,5V • from 110V of power supply.

26. 10. EHT Section:

27. Sound CRT IF,Video Det.,Video Amp. Tuner EHT Sync Seperator Block Diagram : Colour Television

28. 1.2Basics of TV Receiver

29. Television basics

30. Basic Factors affecting Television Transmission & Reception 1. Gross structure 2. Image continuity 3. Flicker 4. Number of scanning lines 5. Fine structure 6. Tone gradation

31. 1. Gross Structure • Frame adopted is rectangle with Aspect Ratio (Width/Height) = 4 : 3 Reasons: • Most of the motion in scene occurs in horizontal plane • Horizontal dimension of scene is more than its vertical dimension. • Eyes can view more easily and comfortably : 4. Motion pictures use a rectangular frame with width/height ratio of 4:3 – so adopted this aspect ratio in TV Eye’s Horizontal Viewing Angle : 155 degree Eye’s Vertical Viewing Angle : 120 degree

32. Requirements: 1. Aspect ratio of the size of the picture produced on the receiver screen and the picture being telecast must be the same. Achieved by setting the magnitude of the current in the deflection coils to correct values both at the TV camera and the receiving picture tube 2. Same coordinated should be scanned at any instant both by the camera tube beam and the picture tube beam Achieved by transmitting synchronizing pulses along with the picture information

33. 2. Image Continuity Image TV Camera TV Receiver Electrical Signal “Scanning”

34. 312.5 1 313 2 312.5 Scanning Sequence

36. Original image 1st Field 2nd Field Recovered image

37. 2. Image Continuity • For Film Camera : Captures the image & give still picture. • Present day motion pictures – 24 still picturesof the scene are taken per second and projected on the screen at the same rate. • In TV camera lens continuously focuses the scene. • Image is converted in to an electrical signals for each points. • This process of extraction of points to signal is called, “Scanning”

38. Scanning in Television systems • The scene is scanned rapidly both in the horizontal and vertical directions simultaneously • Frame repetition rate is 25 per second

39. Horizontal scanning • Linear rise of current in the deflection coils deflects the beam across the screen with a continuous uniform motion for the trace from left to right . • At the peak of the rise, the saw tooth wave reverses its direction and decreases rapidly to its initial value, producing the retrace or fly back

40. Vertical Scanning • Saw tooth current in the vertical deflection coils moves the electron beam from top to bottom of the raster at uniform speed while the electron beam is being deflected horizontally

41. Because of the motion in the scene being televised, the information or brightness at the top of the target plate or picture tube screen normally changes by the time the beam returns to the top to recommence the whole process. This information is picked up during the next scanning cycle and the whole process is repeated 25 times to cause an illusion of continuity • During the horizontal and vertical retrace intervals, the scanning beams at the camera tube and the picture tube are blanked and no picture information is either picked up or reproduced during retrace. • Synchronizing pulses are transmitted during this period – resulting in distortionless reproduction of the picture details.

42. 3.Flicker • In Television 25 frames per second are scanned to cause an illusion of continuity. • Between two frames, observe darkness, called as “Flicker” • Dark interval between two bright pictures become visible for a very short time & appear as a flicker. Reduced with “Interlaced Scanning”

43. Interlaced scanning One Frame Divided Two Field 625 Lines

44. Interlaced scanning • In Television 25 frames per second are scanned to cause an illusion of continuity. • Downward rate of travel of scanning of electron beam in increased so that every alternate line gets scanned instead of every successive line.

45. When the beam reaches the bottom of the picture frame, it quickly returns to the the top to scan those lines that are missed in the previous scanning. • Thus the total number of lines are divided into two groups called as “fields”. • Each field is scanned alternately. • This method of scanning is known as; “Interlaced Scanning”

46. 625 lines of each frame /picture are divided into sets of 312.5 lines • Horizontal sweep oscillator is made to work at a frequency of 15625 Hz, (312.5 × 50 = 15625) • Vertical sweep circuit run at a frequency of 50 Hz. • After scanning both field, total 625 lines are scanned (312.5 x 2= 625) per frame at the same rate of 15625 lines per second. • Hence with interlaced scanning flicker is eliminated without increasing speed of scanning. 46

48. Interlaced Images

49. Scanning periods Freq: 50 Hz` V. Retrace V. Trace H.Trace H.Retrace Vertical Waveform Freq: 15625 Hz 18.720 ms 1.280 ms Horizontal Waveform 20ms 12 µs 52 µs 40 Lines are Lost During V.retrace. 64µs No of Active Lines = (625-40)=585

50. Scanning periods • Normal duration of the Horizontal line is, • 64µs (1/15625 = 64µs) out of which active line period (Trace Period) is 52 µs remaining is the blanking period (Retrace period) = 12µs • The beam returns during this short interval to extreme left side of the frame to start tracing the next line. • Normal duration of the Vertical trace is, • 20 ms (1/50 = 20 ms). • Out of this 20ms, 18.720 ms are spent for bringing the beam from top to bottom (Trace Period) • Remaining 1.280 ms is taken by the beam to return back to the top to start scanning next cycle (Retrace Period) 50