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TV Transmission Principles


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Problem Statement


Efficient transmission of color information in a TV signal
Type of modulation appropriate for signals that have a strong low-frequency component, such as video signals
Image/Picture transmission in TV signals


Baseband vs. Passband Transmission

Baseband signals:
Voice (0-4kHz)
TV (0-6 MHz)
A signal may be sent in its baseband format when a dedicated wired channel is available.
Otherwise, it must be converted to passband.
Modulation & Demodulation


TV broadcast

The audio signal
symmetrical signal
without continuous current
frequency does not exceed 20 kHz.
The video signal
consists of a logical component, the sync and an analogue part according to the line picture scanning
unsymmetrical signal
continuous component
frequency bandwidth extends from 0 to 5 MHz.


Modulation Type


In television though positive modulation was adopted in initial stages, negative modulation is generally adopted (PAL’B uses negative modulation) now a days, as there are certain advantages over positive modulation.


Advantages of Negative Modulation


The peak level representing the blanking or sync level may be maintained constant, thereby providing a reference for AGC in the receivers.
In negative modulation, the peak power is radiated during the sync-tip. As such even in case of fringe area reception, picture locking is ensured, and derivation of inter carrier is also ensured.

TELEVISION TRANSMISSION

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BACKGROUND

Television was developed prior to World War II
monochrome television
Color television began to come on the market about 1960
The next step in television evolvement was high-definition
television (HDTV), and 1998 is considered to be the year
when HDTV was launched

OBJECTIVES

provide the reader with a clear understanding of how TV
works
describe how television is transmitted and distributed over
long distances
provide an overview of digital television, and we cover
several generic methods of digitizing original analog
television signals

AN APPRECIATION OF VIDEO TRANSMISSION

A video transmission system must deal with four factors
when transmitting images of moving objects:
1. Perception of the distribution of luminance or simply the distribution of light and shade
2. Perception of depth or a three-dimensional perspective
3. Perception of motion relating to the first two factors above
4. Perception of color (hues and tints)
Monochrome TV deals with the first three factors. Color
TV includes all four factors.

Scanning process from TV camera to receiver display.

National Television Systems Committee (US) (NTSC) practice divides an image into 525 horizontal scanning lines
The aspect ratio used almost universally is 4:3 and an image
divided into 525 (491) vertical elements would then have 700 (652) horizontal elements
rate of display is called the frame rate
60 frames per second. In Europe it is 50 frames per second

Following North American practice, some other important parameters

1. A field period is 1/60 sec. This is the time required to scan a full picture on every horizontal line.
2. The second scan covers the lines not scanned on the first period, offset one-half horizontal line.
3. Thus 1/30 sec is required to scan all lines on a complete picture.
4. The transmit time of exploring and reproducing scanning elements or spots along each scanning line is 115,750 sec (525 lines in 1/30 sec) = 63.5 µsec.