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Communication Systems (EC-326)
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EC 326 COMMUNICATION SYSTEMS
UNIT - III
T Srinivasa Rao
Dept. of ECE
Bapatla Engineering College
Part 1
Vision characteristics and scanning systems,
Composite video signal.
Part 2
Camera tubes: Principle of operation, Image
Orthicon, Vidicon, Plumbicon.
Part 3
Block diagram of broad cast TV transmitter
Block diagram of broadcast TV receiver.
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Simplified cross-sectional view of a
Vidicon TV camera tube
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Elements of a picture tube
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Signal transmission paths illustrating compatibility between colour
and monochrome TV systems.
R, G and B represent three camera tubes which develop video
signals corresponding to the red, green and blue contents of the
scene being televised
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A Rectangular Picture Tube
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Monochrome Picture Tube
 One Electron Gun
 Continuous phosphor coating
Colour Picture Tube
 Three electron beams
 Three phosphors.
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 Electron Gun
 Control Grid
 Accelerating Grid
 Focusing Grid
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Vidicon camera tube cross-section
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Circuit for output signal from a Vidicon camera tube
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Leaky Capacitor Concept
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THE PLUMBICON
Target Details
Output Signal Current
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Characteristics
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A sync generator driving
several camera units
Switcher for selecting any camera output to
one monitor.
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Block diagram of a drive and sync pulse generator
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Block Diagram of a Television Transmitter
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Rear view of a black and white receiver with the back removed
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Block diagram of a monochrome television receiver
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CHOICE OF INTERMEDIATE FREQUENCY IN TV
(i) Image Rejection Ratio
(ii) Pick-up Due to Local Oscillator Radiation from TV
Receivers
(iii) Ease of Separation of Modulating Signal from IF
Carrier at the Demodulator
(iv) Image Frequencies Should Not Lie in the FM Band
(v) Interference or Direct Pick-Up from Bands Assigned
for other Services
(vi) Gain
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V S B CORRECTION
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VIDEO DETECTOIN
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Detector and filter circuit of a radio receiver
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REQUIREMENTS
(a) The detector load must provide a suitable impedance as seen
through the diode, at input of the detector to tune and damp
secondary of the last IF coupling circuit correctly.
(b) The detector load must remove from the output, the IF content in
the signal as much as possible. For this purpose the load usually
includes one or two low-pass filter sections.
(c) The detector load should have a trap circuit (a series rejector circuit)
for separating the intercarrier sound signal.
(d) The detector load must also include a provision to boost the higher
video frequencies to compensate for the loss due to input
capacitance of the video amplifier.
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FUNCTIONS OF THE COMPOSITE VIDEO SIGNAL
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SOUND SIGNAL SEPARATION
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Block diagram of AGC system
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High Voltage (EHT) Supply
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Deflection Circuits
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Operating Controls of a T V Receiver
 Channel Selector
 Fine Tuning
 Volume
 Tone
 Contrast
 Brightness
 Vertical Hold
 Horizontal Hold.
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