TV Transmission- TV Transmitter Block Diagram|Television Transmission- Picture and Sound Transmission
Picture transmission
The
picture information is optical in character and may be thought of as an
assemblage of a large number of bright and dark areas representing picture
details.
These elementary areas into which the picture details may be broken up
are known as ‘picture elements’, which when viewed together, represent the
visual information of the scene.
At any instant there are almost an infinite
number of pieces of information, existing simultaneously, each representing the
level of brightness of the scene to the reproduced.
In other words the
information is a function of two variables, time and space.
Ideally then, it
would need an infinite number of channels to transmit optical information
corresponding to all the picture elements simultaneously. So a method known as
scanning is used instead.
In the
scanning process, the conversion of optical information to electrical form and
its transmission are carried out element by element, one at a time and in a
sequential manner to cover the entire scene which is to be televised.
Scanning
of the elements is done at a very fast rate and this process is repeated a
large number of times per second to create an illusion of simultaneous pick-up
and transmission of picture details.
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.
The above Fig. Shows very elementary details of one type of Camera
tube to illustrate this principle.
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.
The photolayer 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 photolayer changes accordingly.
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.
Scanning
is done in the same way as one reads a written page to cover all the words in
one line and all the lines on the page (
To achieve this, the
deflecting coils are fed separately from two sweep oscillators which
continuously generate saw-tooth waveforms, each operating at a different desired
frequency.
Interlaced Scanning
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.
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.
The
electrical information obtained from the TV camera tube is generally referred
to as video signal
. This signal is amplified and
then amplitude modulated with the channel picture carrier frequency.
The
modulated output is fed to the transmitter antenna for radiation along with the
sound signal.
Sound transmission
The
microphone converts the sound associated with the picture being televised into proportionate electrical signal, which is
normally a voltage.
This electrical output, regardless is a single valued
function of time and so needs a single channel for its transmission.
The audio
signal from the microphone after amplification is frequency modulated,
employing the assigned carrier frequency.
Frequency modulation
In FM,
the amplitude of the carrier signal is held constant, whereas its frequency is
varied in accordance with amplitude variations of the modulating signal.
As
shown in the above Fig, output of the
sound FM transmitter is finally combined with the AM picture transmitter
output, through a combining network, and fed to a common antenna for radiation
of energy in the form of electromagnetic waves.
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