The
camera video signal is amplified and mixed with sync and blanking pulses and
transmitted to the receiving end over a cable in closed circuit television or
is used to amplitude modulate a radio frequency (RF) carrier which can be
radiated into space with the help of an antenna for transmission over long
distance. The large bandwidth of about 5MHz of the video signal necessitates
the use of very high frequencies in the very high frequency (VHF) and the ultra
high frequency (UHF) ranges since the carrier frequencies must be greater than
the modulating 5MHz signals. The bandwidth occupied by each TV channel is also
consequently large, greater than 5MHz. Propagation at the VHF and UHF range
frequencies is normally restricted to line-of-sight distance by the space waves
and hence the range of TV reception is also limited.
camera video signal is amplified and mixed with sync and blanking pulses and
transmitted to the receiving end over a cable in closed circuit television or
is used to amplitude modulate a radio frequency (RF) carrier which can be
radiated into space with the help of an antenna for transmission over long
distance. The large bandwidth of about 5MHz of the video signal necessitates
the use of very high frequencies in the very high frequency (VHF) and the ultra
high frequency (UHF) ranges since the carrier frequencies must be greater than
the modulating 5MHz signals. The bandwidth occupied by each TV channel is also
consequently large, greater than 5MHz. Propagation at the VHF and UHF range
frequencies is normally restricted to line-of-sight distance by the space waves
and hence the range of TV reception is also limited.
In
television transmission, along with the picture, sound also is to be
transmitted. Another RF carrier is used for transmitting the sound. Frequency
modulation (FM) is more commonly employed for sound carriers as it offers
noise-free reception with much less power than in amplitude modulation (AM),
and can provide high fidelity up to 15KHz if adequate bandwidth is used.
Frequency modulation is not suitable for video signals as the ghost
interference due to multipath reception of FM TV signals is more disturbing and
annoying than in AM, besides having a larger bandwidth requirement.
television transmission, along with the picture, sound also is to be
transmitted. Another RF carrier is used for transmitting the sound. Frequency
modulation (FM) is more commonly employed for sound carriers as it offers
noise-free reception with much less power than in amplitude modulation (AM),
and can provide high fidelity up to 15KHz if adequate bandwidth is used.
Frequency modulation is not suitable for video signals as the ghost
interference due to multipath reception of FM TV signals is more disturbing and
annoying than in AM, besides having a larger bandwidth requirement.
In
FM, the beat between two frequencies of the same signal arriving via two paths
with different propagation delays produce very objectionable interference
patterns in the form of multiple ghosts of Moire patterns. Spacing of Moire or
the frequency of reception of ghosts depends upon the contrast ratio between
adjacent areas, thus varying with the movements or changes in the scene.
FM, the beat between two frequencies of the same signal arriving via two paths
with different propagation delays produce very objectionable interference
patterns in the form of multiple ghosts of Moire patterns. Spacing of Moire or
the frequency of reception of ghosts depends upon the contrast ratio between
adjacent areas, thus varying with the movements or changes in the scene.
FM
has, however, the important advantage of noise interference reduction, and is
hence employed in microwave relay or satellite links for TV broadcasts where
multiple path reception is avoided by highly directional antennas for
transmission and reception.
has, however, the important advantage of noise interference reduction, and is
hence employed in microwave relay or satellite links for TV broadcasts where
multiple path reception is avoided by highly directional antennas for
transmission and reception.
The
block diagram of a basic television system is shown in figure 1 below.
block diagram of a basic television system is shown in figure 1 below.
The video signal from the camera pick-up tube,
amplified and mixed with the blanking and sync signal from sync pulse
generator, is supplied to the AM visual or picture transmitter. The transmitter
does not transmit both the sideband as usual. In order to keep down the
bandwidth of the channel, it transmits one sideband fully, while the other
sideband is transmitted partially as ‘vestigal sideband’.
amplified and mixed with the blanking and sync signal from sync pulse
generator, is supplied to the AM visual or picture transmitter. The transmitter
does not transmit both the sideband as usual. In order to keep down the
bandwidth of the channel, it transmits one sideband fully, while the other
sideband is transmitted partially as ‘vestigal sideband’.
The sound section of the TV system is separate.
The audio signals from the microphone are amplified suitable and are used to
frequency modulate the carrier of the aural or sound transmitter. The power
output of the picture and sound transmitter are combined in deplexer and fed to
a common transmitting antenna system to be radiated together.
The audio signals from the microphone are amplified suitable and are used to
frequency modulate the carrier of the aural or sound transmitter. The power
output of the picture and sound transmitter are combined in deplexer and fed to
a common transmitting antenna system to be radiated together.
At the receiving end, the picture is reproduced
on a cathode ray picture tube screen with the help of the focused electron beam
of varying intensity falling on the screen to produce a proportional glow at
the various positions on the phosphor coating of the screen.
on a cathode ray picture tube screen with the help of the focused electron beam
of varying intensity falling on the screen to produce a proportional glow at
the various positions on the phosphor coating of the screen.
The beam intensity is controlled by the picture
video signal brought over cables in the CCTV or obtained by the demodulation of
the RF waves received from the antenna and amplified by RF/IF amplifiers. The
beam also scans the picture raster in exact synchronism with the beam at the camera tube so that the intensity of
the glow produced at the various positions tallies with that of the
corresponding picture element on the picture scanned at the camera. The
scanning by the beam spot is so fast that the human eye is unable to follow the
spot movement and because of persistence of vision, views the picture as a
continuous one.
video signal brought over cables in the CCTV or obtained by the demodulation of
the RF waves received from the antenna and amplified by RF/IF amplifiers. The
beam also scans the picture raster in exact synchronism with the beam at the camera tube so that the intensity of
the glow produced at the various positions tallies with that of the
corresponding picture element on the picture scanned at the camera. The
scanning by the beam spot is so fast that the human eye is unable to follow the
spot movement and because of persistence of vision, views the picture as a
continuous one.