Monochrome tv

1. 1
Monochrome Television
Lecture - 3

2. Television
2
• The word television has its origin in two Greek words
„tele‟ and „vision‟.
• Tele means „at distance‟ and vision means „seeing‟

3. Development
• Television means “to see from a distance”
• The first demonstration of actual television was given by J. L. Baird in UK
and C.F. Jenkins in USA around 1927 using the technique of mechanical
scanning using rotating discs
• Real breakthrough occurred with the invention of Cathode Ray Tube (CRT)
and first camera tube based on storage principle (V.K. Zworykin of USA)
• By 1930 electromagnetic scanning of both CRT and camera were developed
with other secondary circuits: beam deflection, video amplification, etc.
• Television broadcast was started in 1935 but its progress was slowed down
due to Second World War
3

4. Cont.
 Initially due to the absence of any international standard three
monochrome systems:
• 525 line American,
• 625 line European, and
• 819 line French grew independently.
 Later initiatives have been taken for establishing a common 625 line
system. However, due to huge involvement to change equipment and
millions of Rx already in use for all the three systems.
 Three different standards of monochrome television have resulted in the
development of three different systems color television.
 In 1953 USA adopted on the recommendation of its National Television
Systems Committee and hence called NTSC system.
4

5. Cont.
5
 The other two color systems: PAL (Phase Alternating Line) and SECAM
(Sequential colour with memory) are later modifications of the NTSC
system to confirm the other two monochrome standards.
 Regular color transmission started in USA in 1954.
 In 1960 Japan adopted NTSC system followed by Canada and other
several countries
 The PAL color system compatible with 625 line monochrome system
developed by Telefunken Laboratory of Federal Republic of Germany
(FRG).
 PAL system reduces the color display errors that occurred in NTSC
system.

6. 6
 PAL system adopted by FRG and UK in 1967 and subsequently Australia,
Spain, Iran, India, Bangladesh, and several other west and south Asia
countries.
 The third color system in use is the SECAM system. This was initially
developed and adopted in France in 1967. Later versions known as
SECAM - IV and SECAM - V were developed in Russian National
Institute of Research (NIR) and sometimes referred to as NIR-SECAM
system.
 This system adopted by USSR, Germany Democratic Republic, Hungary,
some other East European countries, Algeria.
 The adaptation of a particular color system depends on the monochrome
system of the respective countries

7. Television
7
Television
Receiver
Speaker
Power Supply
Picture Tube
All Section
AC Input

8. Equipment - Television broadcasting requires
8
• Extensive lighting facilities, cameras, microphones, and control equipment
for television studios.
• Transmitting equipment for modulation, amplification and radiation of the
signals at the high frequencies.
• A wide variety of support equipment essential in broadcast studios and
control rooms.
• Besides the above video tape recorders, telecine machines, special effects
equipment plus all the apparatus for high quality sound broadcast.

9. Monochrome Television
9

10. 10
Monochrome Television Receiver

11. Detailed version of T.v. Receiver
11
Sound IF
Amplifier
Sound
Detector
Audio
Amplifier
Tuner IF Amplifier
Video
Detector Video Amplifiers
AGC
AFC Sync Separator
Horizontal
Oscillator
Vertical
Sweep
Horizontal
Output
High Voltage
Power
supply
Low Voltage
Power
Supply
Speaker
Picture
Tube
All Stages
AC line

12. Function of each Block Section
12
 Low Voltage Power Supply – this power supply DC voltage in all
section of the television. It is also a place used to convert AC voltage to
DC voltage.
 Sound IF Amp – received and synchronize Sound signal
 Sound detector – filter the signal received from the sound IF amp.
 Audio Amplifiers- Amplify signal from the sound detector
 Tuner – Select the station or channel that you desire.
 Sound IF Amplifiers – received the signal emitted from the sound trap
 Video Detector – filter the signal received from the video IF amp
 Video Amplifier - Amplify the video received from the video detector
section.

13. Function of each Block Section
13
 AGC
 AFC
 Sync separator
 Vertical Sweep
 HorizontalOscillator
 HorizontalOutput
 High Voltage Power Supply
 Picture tube
 Deflection Yoke

14. Picture Elements
A still picture is fundamentally an arrangement of many small dark and
light areas. Each small area of light or shade is a picture element or picture
details. All the elements contain the visual information in the scene. If they
are transmitted and reproduce in the same degree of light or shade as the
original and proper position, the picture will be reproduce.
A Still Picture
Reproducing a picture by
Duplicating its picture elements
Magnified view to show
Picture elements
14

15. 15
• The television picture is scanned in a sequential series of horizontal lines
one under the other. This scanning makes it possible for one video signal
to include all the elements for the entire picture. The sequence for
scanning all the picture elements is as follows:
 The electron beam sweeps across one horizontal line, covering all the
picture elements in that line
• At the end of each line, the beam is returned very quickly to the left side
to begin scanning the next horizontal line.
Horizontal and vertical scanning

16. 16
• The return time is called retrace or flyback. No picture information is
scanned during retrace because both the camera tube and picture tube
are blanked out for this period.
• The retraces must be very rapid since they are wasted time in terms of
picture information.
• When the beam is returned to the left side, its vertical position is lowered
so that the beam will scan the next lower line and not repeat over the
same line.
• This is accomplished by the vertical scanning motion of the beam, which
is provided in addition to horizontal scanning.
Cont.

17. 17
Typical H-scanning pattern

18. 18
Typical V-scanning pattern

19. 19
Number of scanning lines per frame
Maximum number of alternate light and dark
elements (lines) which can be resolved by the eye is
given by

1
vN
= minimum resolving angle of the eye expressedin radians
= viewing distance /picture height =D/H
Experimentally it is found that D/H=4
=one minute=1/60 degree
8604)601180(11  vN

20. 20
Number of scanning lines per frame
6027.0860  kNN vrThe effective number of lines
• The values of k lies in between 0.65 to 0.75. While viewing picture having
motion eye can detect effective sharpness if line number is 500. Also the
channel bandwidth increases with increasing line numbers.
Frame per second
• The vertical scanning is at the rate of 30 Hz for the frame frequency of 30
frames per second. The frame rate of 30 per second means that 625 lines
for one complete frame are scanned in 1/30 second.
• Standard commercial motion picture practice 25 frames per second.

21. 21
Persistence of vision
• The impression made by light seen by the eye persists for a small fraction
of a second after the light source is removed.
• Therefore, if many views are presented to the eye during this interval of
persistence of vision, the eye will integrate them and give the impression
of seeing all the images at the same time.

22. 22
Flicker and elimination of flicker
• The rate of 25 frames per second, however, is not rapid enough to allow
the brightness of one picture to blend smoothly into the next during the
time when screen is black between the frame. The result is a definite
flicker of light as the screen is made alternatively bright and dark.
• If it is possible to project each frame twice in screen the flicker can be
eliminated.

23. 23
Interlaced Scanning
• To achieve this the horizontal sweep oscillator is made to work at a frequency
of 15625 Hz (312.5 × 50 = 15625)
• to scan the same number of lines per frame (15625/25 = 625 lines),
• but the vertical sweep circuit is run at a frequency of 50 instead of 25 Hz.

24. In all then, the beam scans 625 lines (312.5 × 2
= 625) per frame at the same rate of 15625 lines
(312.5 × 50 = 15625)per second.
Therefore, with interlaced scanning the flicker
effect is eliminated without increasing the
speed of scanning, which in turn does not need
any increase in the channel bandwidth.
24
Cont.

25. Simplified cross-sectional view of a vidicon TV
camera tube
25

26. Cont.
 Two simultaneous motions of the beam, one from left to right and the
other from top to bottom encounters a different resistance across the
target plate.
 Depending on the resistance of the photoconductive coating the current
passes through a load resistance RL, which is connected to the
conductive coating on one side and to a dc supply source on the other.
 Depending on the magnitude of the current a varying voltage appears
across the resistance RL and this corresponds to the optical information
of the picture.
 The electrical information obtained from the TV camera tube is video
signal.
26

27. A TV camera, the heart of which is a camera tube, is used to convert the
optical information into a corresponding electrical signal, the amplitude of
which varies in accordance with the variations of brightness
Different TV camera tubes are
• Vidicon
• Plumbicon
• Orthicon
T.v. Camera Tube
27

28. 28
Vidicon

29. 29
Vidicon
 A vidicon is the most commonly used TV camera tube because its
simplicity and based on the principle of photoconductivity.
 An optical image of the scene to be transmitted is focused by a lens
assembly on the rectangular glass face-plate of the camera tube.
 The inner side of the glass face-plate has a transparent conductive coating
on which is laid a very thin layer of photoconductive material layer of
either selenium or anti-mony compounds..
 The photo layer has a very high resistance when no light falls on it, but
decreases depending on the intensity of light falling on it.
 Thus depending on the light intensity variations in the focused optical
image, the conductivity of each element of the photo layer changes
accordingly

30. 30
Vidicon
 An electron beam is used to pick-up the picture information now
available on the target plate in terms of varying resistance at each point.
 The beam is formed by an electron gun in the TV camera tube. On its way
to the inner side of the glass faceplate it is deflected by a pair of deflecting
coils mounted on the glass envelope and kept mutually perpendicular to
each other to achieve scanning of the entire target area.
 To achieve scanning the deflecting coils are fed separately from two
sweep oscillators which continuously generate saw-tooth waveforms,
each operating at a different desired frequency.

31. 31
Vidicon
 The magnetic deflection caused by the current in one coil gives horizontal
motion to the beam from left to right at a uniform rate and then brings it
quickly to the left side to commence the trace of next line.
 The other coil is used to deflect the beam from top to bottom at a uniform
rate and for its quick retrace back to the top of the plate to start this
process all over again.
 Two simultaneous motions are thus given to the beam, one from left to
right across the target plate and the other from top to bottom thereby
covering the entire area on which the electrical image of the picture is
available.

32. 32
Vidicon
 As the beam moves from element to element, it encounters a different
resistance across the target-plate, depending on the resistance of the
photoconductive coating. The result is a flow of current which varies in
magnitude as the elements are scanned.
 This current passes through a load resistance RL, connected to the
conductive coating on one side and to a dc supply source on the other.
Depending on the magnitude of the current a varying voltage appears
across the resistance RL and this corresponds to the optical information
of the picture.

33. Picture Reception
The signal that carries the picture information is amplified and coupled to the picture
tube which converts the electrical signal back into picture elements of the same
degree of black and white.
33

34. • The picture tube is very similar to the CRT used in an oscilloscope.
• The glass envelope contains an electron gun structure that produces a beam of
electrons aimed at the fluorescentscreen.
• When the electron beam strikes the screen, light is emitted. The beam is
deflected by a pair of deflecting coils mounted on the neck of the picture tube
in the same way and rate as the beam scans the target in the camera tube.
• The amplitudes of the currents in the horizontal and vertical deflecting coils
are so adjusted that the entire screen, called raster, gets illuminated because of
the fast rate of scanning.
• The video signal is fed to the grid or cathode of the picture tube. When the
varying signal voltage makes the control grid less negative, the beam current
is increased, makingthe spot of light on the screenbrighter.
Picture Reception
34

35. • More negative grid voltage reduces the brightness. If the grid voltages is
negative enough to cut-off the electron beam current at the picture tube there
will be no light.
• This state corresponds to black. Thus the video signal illuminates the fluorescent
screen from white to black through various shades of grey depending on its
amplitude at any instant.
• This corresponds to the brightness changes encountered by the electron beam of
the camera tube while scanning the picture details element by element.
• The rate at which the spot of light moves is so fast that the eye is unable to follow
it and so a complete picture is seen because of the storage capability of the
human eye.
Cont.
35

36. Sound Reception
• The path of the sound signal is common with the picture signal from
antenna to the video detector section of the receiver.
• Here the two signals are separated and fed to their respective channels.
• The frequency modulated audio signal is demodulated after at least one
stage of amplification.
• The audio output from the FM detector is given due amplification before
feeding it to the loudspeaker.
• To ensure perfect synchronization between the scene being televised and
the picture produced on the raster, synchronizing pulses are transmitted
during the retrace, i.e., fly-back intervals of horizontal and vertical motions
of the camera scanning beam.
36

37. 37
Cont.
• Thus, in addition to carrying picture detail, the radiated signal at the
transmitter also contains synchronizing pulses.
• These pulses which are distinct for horizontal and vertical motion control,
are processed at the receiver and fed to the picture tube sweep circuitry
thus ensuring that the receiver picture tube beam is in step with the
transmitter camera tube beam.

38. Construction: Electron gun motion:
 The electron gun unit has a cathode, control grid and accelerating anode.
The cathode (K) is a small metallic oxide disk placed at the end of a narrow
tube that covers the heater. It is heated to produce thermionic emission and
thus serves as source of electrons for the beam current.
 The control grid (G1) is used to control the flow of electrons from the
cathode (K). The control grid (G1) is maintained at negative potential with
respect to cathode (K).
 The grids that follow the control grid are the accelerating grid (or) screen
(G2) and focusing grid (G3).these are maintained at different positive
potentials with respect to cathode (K).
 All the grids, cathode, heater elements of the electron gun are connected to
the base pins. Through this base pins only necessary voltages are applied.

39. Monochrome picture tube:

40. Focusing anode section:
 Electrostatic focusing method is used here, to focus the electron beam. This
section also brings all the electrons in the stream into small spot. It is
considered as first electrostatic lens action.
 The second lens system consists of screen grid are so selected that the
second convergence point is on the screen of the picture tube.

41. Deflection coil section:
 Here we are using electromagnetic system to deflect the electron beam in
horizontal and vertical direction.
 one pair of deflection coils is placed left and right side neck of the picture
tube to produce vertical deflection and one pair is placed top and bottom
side of the neck to produce horizontal deflection.
 The two pairs of coils are collectively called deflection yoke. The magnetic
field in the coil reacts with the electron beam to make the deflection.
 In the deflection yoke centering magnet and pin cushion magnet are also
provided for centering the electron beam and adjusting the movement of
the electron beam at the corners.

42. Deflection angle:
 This is the angle through which the beam can be deflected without striking
the side of the picture tube (or) bulb.
 A final anode is included in the tube, to provide sufficient velocity and
energy for the electron beam.
 Here a black graphite material coating called aquadag, it is used as final
anode.
 It is connected through a specially provided pin at the top or side of the
glass bell to a very high potential of over 15kv.
 The secondary electrons emitted from the screen are attracted by these
aquadag coating.
Final anode section:

43.  The phosphor chemicals are generally light materials such as zinc and cadmium
in the form of sulphate and phosphate compounds.
 This material is proceeded to produce very fine particles which are then applied
on the inside of the glass plate.
 The high velocity electrons of the beam on hitting the phosphor excite its atoms
with the result that corresponding spot fluorescesand emit light.
 Aquadagis also coated on the outer surface of the glass bell.
 A spring clip is used to connect this coating with the chasis ground.
 This coating is used to filter the AC ripples in high voltage and to provide a
perfect higher voltage.
Phosphor screen:
External conductive coating:

44.  An AC supply of 6.3V is given to the heater. This filament heats the cathode (K) and
the cathode emits electrons.
 The control grid (G1) controls the flow of electrons. By varying the control grid
voltage, The number of electrons in the beam is also controlled.
 The accelerating anode (G2) increases the velocity of the moving electrons. The
focusing anode (G3) merges the electron beam so that they merge at a point and
strike the phosphor coatingon the screen.
 The aquatic coating inside the tube is given a high voltage in the order of about
10kv to 15kv. This high voltage coating accelerates the electrons and also collect the
secondary emissions.
 Using the deflection coils we can deflect the electrons in both vertical and
horizontal directions. A saw tooth current is used for this, when the electron beam
strikes the phosphor coating it emits light. Depending on the video signal voltage
the emitted light is bright (or) dark.
Working

45. 45
Television Broadcast Channels
The band of frequencies assigned for transmission of the and sound
signals is a television channel. FCC assigned 6 MHz for a channel
FrequencyRange ChannelNo. FrequencyBand (MHz)
1 Not used
Low band VHF 2-4 54-60, 60-66, 66-72,
72-76 MHz Air Navigation 72-76
Low band VHF 5,6 76-82, 82-88
88-108 FM band 88-108
High band VHF 7,8,9,10,11,12,13 174-180,180-186,186-192,192-
198,198-204, 204-210,210-216
UHF 14-83 470-890

46. 46
TV parts- for reference

47. 47
Tuner
Tuner : Receive input signal (TV wave) from antena and convert to frecuency IF
sinyal.

48. 48
IF Amplifier
Functions brave as a signal coming from the tuner up to 1000 times. Output signal
generated Tuner is a weak signal and is highly dependent on the distance relay,
the position of the recipient and the expanse of nature.

49. 49
Sound Series
IF sound carrier signal will be detected by the
frequency modulator (FM).
Power Amplifier (Sound Output) : Amplifying audio
signal until be able vibrate loudspeaker.
Loudspeaker : convert audio signal audio to sound.
Separated information signal (audio) from signal audio carrier and amplifying
until can listening by audience.

50. 50
High Voltage Generator
Produce high voltage for activate CRT until be able to produce electrons for
video display.

51. 51
Horizontal Deflection Yoke
• Generate enough current deflection for
used scanning electric beam horizontal
direction.
• Generate high voltage through the coil
and fly back to continue to the anode
electrode and CRT focus electrode.