131). Write short notes on CATV.
CATV stands for community antenna television systems. The CATV
system is a cable system distributes good quality television signal to a very large
number of receivers throughout an entire community. Generally this system gives
increased TV programmes to subscribers who pay a fee for this service. A cable
system may have many more active VHF and UHF channels than a receiver tuner
can directly select. .
132). What do you understand by satellite TV?
Satellite TV is a TV from space. Broadcasters from earth transmit their
programmes to specified satellites. Then, the transmissions are returned to
receiving equipment on the ground. Therefore, the better the receiving equipment
,the higher the quality of the reception.
133). List some of the applications of satellite TV?
The shear range of programmes currently available on satellite channel is
very much impressive such as 24-hour music videos, news, and feature films. A
variety of general entertainment programmes, sports ,children’s programmes ,
foreign language broadcasts and cultural programmes are all available for the
keyboard dish owner. Some of these come through subscription channels and
others by free to watch channels which are sponsored by advertising.
134). State the merits of satellite TV. The picture quality from satellite systems is surprisingly good and
compares well with conventional land based TV transmissions. In addition unlike
terrestrial broadcasts it is free from the spectrum of picture ghosting.
135). Mention the limitations of satellite TV.
The limitation of the satellite TV is varying picture and audio quality .By
adopting de-emphasis circuits we can avoid the variations.
136). Give the applications of video tape recorders.
Smaller and lower priced video tape recorders using ½ inch tape are
available for closed type circuit TV or for use in the home. They can record and
playback programs on a television receiver in color and monochrome. In addition
to that small portable cameras are provided for a complete television system with
the recorder. These portable systems are also employed for taping television
programs from a remote are also employed for taping television programs from a
remote location for away from the TV broadcast studio.
137). List some merits of high definition television.
Improvement in both vertical and horizontal resolution of the reproduced
picyure by approximately 2:1 over existing standards.
Much improved colour rendition
Higher aspect ratio of atleast 5:3
Stereophonic sound
138). What do you mean by Longitudinal video recording?
A method in which video signals are recorded on atleast several tracks
along the length of the tape.
139). What do you mean by Quadruplex (Transverse) scan recording?
In transverse scan recording ,four recording heads are spaced 90 apart and
are mounted on a rotating drum and the tape moves past it,transversely.Each head
comes in contact with the tape as the previous one leaves it.
140). What do you mean by helical scan recording ?
In helical scan recording ,the two recording heads ‘look at ‘ the tape
surface as it is drawn past them through two tiny rectangular slits mounted on
opposite sides of the drum.The heads thus trace out diagonal tracks across the
tape,one track per head.
141). What are two types of video disc system?
Laser or optical disc system
Capacitance disc system
142). List the fundamental components of DVD player.
A drive motor to spin the disc.
A laser and lens system to focus in on the bumps and read them.
Tracking mechanism that can move the laser assembly so that the lasers
beam can follow the spiral track.
Electronic circuitry
143). What are the advantages of DVD players over VCR’s.
The quality of picture and sound in a DVD is better than on a video
tape,and DVD’s maintain their high quality over time,because there is no
physical contact with the disc as it revolves.
144). List 4 merits of digital TV receivers.
Reduced Ghosts
Reduction of 50Hz flicker
High resolution pictures
Slow motion action
145). Describe how the flicker is solved by interlaced scanning?
Interlaced Scanning Effective rate of 50 vertical scans per second is utilized to reduce the
flicker.
Increase the downward rate of travel of the scanning electron beam.
Every alternate line gets scanned instead of every successive line.
Scanning Periods
Duration of one horizontal line is 64µs
Duration of one vertical trace is 20ms
146). Explain the structure and the generation of video output from a vidicon camera.
• it functions on the principle of photo conductivity
• in photoconductive method where the conductivity or resistivity of the
photoconductive surface varies with in proportion to the intensity of light
focused on it.
• With a B+ source of 40V ,an area with high illumination may attain a
potential of about +39V on the beam side.
• Dark areas may attain a potential of about +35 on the beam side.
• As the beam scans the target plate, it encounters different positive
potentials on the side of photo layer that faces the gun.
• Number of electrons from the beam is then deposited on the photo layer
surface to reduce to reduce the potential of each element.
• Sudden change in potential on each element causes a current flow in the
signal electrode circuit producing a varying voltage across the load
resistance
147). Give the constructional details of a monochrome picture tube and explain the
beam landing
Employs electrostatic focusing and electro magnetic deflection
Composite video signal that is injected either at the grid or cathode of the
tube
Electron Gun:-emits the electron beam with proper energy to cause it to fluoresce,
another anode called final anode is included within the tube.
Low Voltage Focusing:
It needs less voltage at the focusing electrodes.
High Voltage Focusing:
It requires high voltage at the focusing electrode. This focus system is
called ‘unipotential lens’ system.
Picture Tube Screen:
Screen phosphor: -light metals such as zinc and cadmium in the form of
sulphide, sulphate, and phosphor compounds are used.
Screen Brightness:-picture tubes employ a very thin coating of aluminium
on the back surface of the screen .This improves the screen brightness.
148). Explain in detail the silicon diode array vidicon camera tube.
It uses photovoltaic type where the target is prepared from a thin n-type silicon
wafer instead of deposited layers on the glass faceplate.
Oxidation:-to form a film of silicon dioxide(Sio2)which is an insulator.
Photomasking and etching: - an array of fine openings is made in the
Oxide layer.
Boron is vapourized through the array of holes.
Thin layer of gold is deposited on each p type opening to form contacts for
signal output.
The silicon target plate thus formed is typically 0.003cm thick,1.5 cm
square having an array of 540x540 photodiodes.
The Target plate is mounted in a vidicon type of camera tube.
149). Write notes on composite video signal.
evelop retrace.
To rectify the drawback which occurs on account of half-line
discrepancy five narrow pulses are added on either side of the
vertical sync pulse. These are called pre-equalising and postequalising pulses
150). Draw Block Diagram of RF Tuner and explain how incoming signals from
different sections are translated to common picture IF and sound IF frequencies.
RF tuner section consists of RF amplifier ,mixer and local oscillator and is
normally mounted on a separate subchasis,called the front end.
The purpose tuner unit is to amplify both picture and sound signals picked
up the antenna and to convert the carrier frequencies and their associated
side bands into intermediate frequencies.
The receiver uses superheterodyne principle.The signal voltage or
information from various stations modulated over different carrier
frequencies is heterodyned in the mixer with the output from a local
oscillator to transfer original information on a common fixed carrier
frequency called intermediate frequency(IF)
The setting of local oscillator frequency enables selection of desired
station.
151). Describe briefly the factors that influenced the choice of picture IF =38.9 and
sound IF =33.4MHz in the 625 line system.
The factors that influence the choice of intermediate frequencies in TV receivers
are:
Image Rejection Ratio:
Image rejection ratio is defined as the output due to desired station
divided by output due to image signal.
Pickup due to local oscillator radiation from TV receivers
Image Frequencies should not lie in the FM band
Interference or direct pickup from bands assigned for other services
Gain
152). Explain how composite video signal is detected ?How is the polarity of video
output signal decided?
Modulated IF signals after due amplification in the IF section are fed to
the video detector .
The video detector is designed to recover composite video signal and to
transform the sound signal to another lower carrier frequency.
This is done by rectifying the input signal and filtering out unwanted
frequency components.
A diode is used ,which is suitably polarized to rectify either positive or
negative peaks of the input signal.
153).Describe with a circuit Burst phase discriminator.
The burst seperator circuit has the function of extracting 8 to 10 cycles of
reference color burst which are transmitted on the back porch of every horizontal
pulse. The circuit is tuned to the subcarrier frequency and is keyed on during the
flyback time by pulses derived from the horizontal output stage.
154).Write the need and working of Automatic Gain control Circuit.
AGC circuit controls the gain of RF and IF stage to enable almost constant
signal voltage at the output of video detector, despite changes in the signal picked
up by the antenna. The change in gain is achieved by shifting the operating point
of transistors used in the amplifiers. The operating point is changed by a bias
voltage that is developed in the bias circuit.
155) Tuner section consists [ ]
a) R.F amplifier b) L.O c) Mixer d) All of the above
156) What is standard P.I.F. in 625 lines T.V. system [ ]
a) 33.4 KHz. b) 38.9 KHz. c) 33.4 MHz. d) 38.9 MHz.
157) The separation between sound carrier and picture carrier in 625 lines T.V system is
a) 4.43 MHz. b) 5.5 MHz. c) 5 MHz. d) 4.5 MHz. [ ]
158) In which deflection system AFC is present in T.V. Receiver [ ]
a) Vertical b) Horizontal c) Both d) None
159) Which is used to separate the Horizontal sync pulses [ ]
a) LPF b) HPF c) BPF d) BSF
160) Which is used to separate the Vertical sync pulses [ ]
a) LPF b) HPF c) BPF d) BSF
161) In electronic tuner section channel tuning is done by [ ]
a) Switching diodes b) Varactor diodes c) Both d) All
162) In electronic tuner section band selection is done by [ ]
a) Switching diodes b) Varactor diodes c) Both d) All
163) Eight or ten samples of colour sub carrier frequency is called [ ]
a) Line sync pulse b) Colour burst signal c) chromo signal d) None
164) The operating point is shifting towards the saturation due to AGC bias is [ ]
a) Reverse AGC b) Forward AGC c) Delayed AGC d) All
165) In a F.M detectors the phase angle between primary (e1) and secondary (e2)
Voltages is
(i) At resonance [ ]
a) 900 b) <900 c) >900 d) None
(ii) Above resonance [ ]
a) 900 b) <900 c) >900 d) None
(iii) Below resonance [ ]
a) 900 b) <900 c) >900 d) None
166) Expand AFC____________________________
167) Expand ACC____________________________
168) Which circuit is used to separate the sync pulses [ ]
a) Clipper b) Clamper c) Differentiator d) Integrator
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