28-01-2013, 01:52 PM
Monochrome and Colour Television
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INTRODUCTION
Development of Television
Television* means ‘to see from a distance’. The desire in man to do so has been there for ages.
In the early years of the twentieth century many scientists experimented with the idea of
using selenium photosensitive cells for converting light from pictures into electrical signals
and transmitting them through wires.
The first demonstration of actual television was given by J.L. Baird in UK and C.F.
Jenkins in USA around 1927 by using the technique of mechanical scanning employing rotating
discs. However, the real breakthrough occurred with the invention of the cathode ray tube and
the success of V.K. Zworykin of the USA in perfecting the first camera tube (the iconoscope)
based on the storage principle. By 1930 electromagnetic scanning of both camera and picture
tubes and other ancillary circuits such as for beam deflection, video amplification, etc. were
developed. Though television broadcast started in 1935, world political developments and the
second world war slowed down the progress of television. With the end of the war, television
rapidly grew into a popular medium for dispersion of news and mass entertainment.
Television Systems
At the outset, in the absence of any international standards, three monochrome (i.e. black and
white) systems grew independently. These are the 525 line American, the 625 line European
and the 819 line French systems. This naturally prevents direct exchange of programme between
countries using different television standards. Later, efforts by the all world committee on
radio and television (CCIR) for changing to a common 625 line system by all concerned proved
ineffective and thus all the three systems have apparently come to stay. The inability to change
over to a common system is mainly due to the high cost of replacing both the transmitting
equipment and the millions of receivers already in use. However the UK, where initially a 415
line monochrome system was in use, has changed to the 625 line system with some modification
in the channel bandwidth. In India, where television transmission started in 1959, the 625-B
monochrome system has been adopted.
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. Thus the problem of picture
transimission is fundamentally much more complex, because, 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. Presently
the practical difficulties of transmitting all the information simultaneously and decoding it at
the receiving end seem insurmountable and so a method known as scanning is used instead.
Here 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.
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
of the complexity of its waveform, 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. 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 Fig. 1.1 (a), 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.
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.
SYNCHRONIZATION
It is essential that the same coordinates be scanned at any instant both at the camera tube
target plate and at the raster of the picture tube, otherwise, the picture details would split and
get distorted. 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. 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.
[attachment=48603]
INTRODUCTION
Development of Television
Television* means ‘to see from a distance’. The desire in man to do so has been there for ages.
In the early years of the twentieth century many scientists experimented with the idea of
using selenium photosensitive cells for converting light from pictures into electrical signals
and transmitting them through wires.
The first demonstration of actual television was given by J.L. Baird in UK and C.F.
Jenkins in USA around 1927 by using the technique of mechanical scanning employing rotating
discs. However, the real breakthrough occurred with the invention of the cathode ray tube and
the success of V.K. Zworykin of the USA in perfecting the first camera tube (the iconoscope)
based on the storage principle. By 1930 electromagnetic scanning of both camera and picture
tubes and other ancillary circuits such as for beam deflection, video amplification, etc. were
developed. Though television broadcast started in 1935, world political developments and the
second world war slowed down the progress of television. With the end of the war, television
rapidly grew into a popular medium for dispersion of news and mass entertainment.
Television Systems
At the outset, in the absence of any international standards, three monochrome (i.e. black and
white) systems grew independently. These are the 525 line American, the 625 line European
and the 819 line French systems. This naturally prevents direct exchange of programme between
countries using different television standards. Later, efforts by the all world committee on
radio and television (CCIR) for changing to a common 625 line system by all concerned proved
ineffective and thus all the three systems have apparently come to stay. The inability to change
over to a common system is mainly due to the high cost of replacing both the transmitting
equipment and the millions of receivers already in use. However the UK, where initially a 415
line monochrome system was in use, has changed to the 625 line system with some modification
in the channel bandwidth. In India, where television transmission started in 1959, the 625-B
monochrome system has been adopted.
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. Thus the problem of picture
transimission is fundamentally much more complex, because, 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. Presently
the practical difficulties of transmitting all the information simultaneously and decoding it at
the receiving end seem insurmountable and so a method known as scanning is used instead.
Here 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.
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
of the complexity of its waveform, 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. 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 Fig. 1.1 (a), 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.
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.
SYNCHRONIZATION
It is essential that the same coordinates be scanned at any instant both at the camera tube
target plate and at the raster of the picture tube, otherwise, the picture details would split and
get distorted. 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. 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.