TV Signal Transmission & Propagation Explained

Television Signal Transmission & Propagation

Contents
 Picture Signal transmission
 Positive and negative modulation
 Vestigial sideband transmission
 Standard channel BW
 Television transmitter
 TV Signal propagation
 Interference suffered by TV channels
 TV broadcast channels for terrestrial transmission

Picture Signal transmission
 In AM transmitters where efficiency is the prime
requirement, amplitude modulation is effected by
making the output current of a class C amplifier
proportional to the modulating voltage
 This amounts to applying a series of current pulses at
the frequency of the carrier to the output tuned (tank)
circuit where the amplitude of each pulse follows the
variations of the modulating signal

Picture Signal transmission
 The resonant frequency of the tuned circuit is set
equal to the carrier frequency
 The accumulative effect of this action of the resonant
circuit is generation of a continuous sine wave voltage
at the output of tank circuit
 The frequency of this voltage is equal to carrier
frequency having amplitude variations proportional to
magnitude of the modulating signal

Positive and negative modulation
 When the intensity of picture brightness causes
increase in amplitude of the modulated envelope, it is
called ‘positive’ modulation
 When the polarity of modulating video signal is so
chosen that sync tips lie at the 100 per cent level of
carrier amplitude and increasing brightness produces
decrease in the modulation envelope, it is called
‘negative modulation’

 Effect of Noise Interference on Picture Signal:
 In negative system of modulation, noise pulse extends in
black direction of the signal when they occur during the
active scanning intervals
 They extend in the direction of sync pulses when they
occur during blanking intervals
 In the positive system, the noise extends in the direction
of the white during active scanning i.e., in the opposite
direction from the sync pulse during blanking
 Obviously the effect of noise on the picture itself is less
pronounced when negative modulation is used

 Effect of Noise Interference on Synchronization:
 Sync pulses with positive modulation being at a lesser
level of the modulated carrier envelope are not much
affected by noise pulses
 However, in the case of negatively modulated signal, it is
sync pulses which exist at maximum carrier amplitude,
and the effect of interference is both to mutilate some of
these, and to produce lot of spurious random pulses
 This can completely upset the synchronization of the
receiver time bases unless something is done about it

 Peak Power Available from the Transmitter:
 With positive modulation, signal corresponding to
white has maximum carrier amplitude
 The RF modulator cannot be driven harder to extract
more power because the non-linear distortion thus
introduced would affect the amplitude scale of the
picture signal and introduce brightness distortion in
very bright areas of the picture

 Peak Power Available from the Transmitter:
 In negative modulation, the transmitter may be over-
modulated during the sync pulses without adverse
effects, since the non-linear distortion thereby
introduced, does not very much affect the shape of sync
pulses
 Consequently, the negative polarity of modulation
permits a large increase in peak power output and for a
given setup in the final transmitter stage the output
increases by about 40%

 Use of AGC (Automatic Gain Control) Circuits in the
Receiver:
 In negative system of modulation, peak level of
incoming carrier is the peak of sync pulses which
remains fixed at 100 per cent of signal amplitude and is
not affected by picture details
 This level may be selected simply by passing the
composite video signal through a peak detector

 Use of AGC (Automatic Gain Control) Circuits in the
Receiver:
 In the positive system of modulation the corresponding
stable level is zero amplitude at the carrier and obviously
zero is no reference, and it has no relation to the signal
strength
 The maximum carrier amplitude in this case depends
not only on the strength of the signal but also on the
nature of picture modulation and hence cannot be
utilized to develop true AGC voltage

Vestigial sideband transmission

 In the 625 line TV system where the frequency
components present in the video signal extend from dc
(zero Hz) to 5MHz
 A double sideband AM transmission would occupy a
total bandwidth of 10 MHz
 The actual band space allocated to the television
channel would have to be still greater, because with
practical filter characteristics it is not possible to
terminate the bandwidth of a signal abruptly at the
edges of the sidebands

 Therefore, an attenuation slope of 0.5 MHz is provided
at each edge of the two sidebands
 This adds 1 MHz to the required total band space
 In addition to this, each television channel has its
associated FM (frequency modulated) sound signal,
the carrier frequency of which is situated just outside
the upper limit of 5.5 MHz of the picture signal
 This, together with a small guard band, adds another
0.25 MHz to the channel width, so that a practical
figure for the channel bandwidth would be 11.25 MHz

 Such a bandwidth is too large, and if used, would limit
the number of channels in a given high frequency
spectrum allocated for TV transmission
 In the video signal very low frequency modulating
components exist along with the rest of the signal
 Therefore, as a compromise, only a part of the lower
sideband, is suppressed, and the radiated signal then
consists of a full upper sideband together with the
carrier, and the vestige (remaining part) of the
partially suppressed lower sideband

 This pattern of transmission of the modulated signal is
known as vestigial sideband or A5C transmission
 In the 625 line system, frequencies up to 0.75 MHz in the
lower sideband are fully radiated

 The picture signal is seen to occupy a bandwidth of
6.75 MHz instead to 11 MHz

Standard channel BW
 The sound carrier is always positioned at the extremity
of the fully radiated upper sideband and hence is 5.5
MHz away from the picture carrier
 The FM sound signal occupies a frequency spectrum of
about ± 75 KHz around the sound carrier
 However, a guard band of 0.25 MHz is allowed on the
sound carrier side of the television channel to allow for
adequate inter-channel separation
 The total channel bandwidth thus occupies 7 MHz
and this represents a band space saving of 4.25 MHz
per channel, when compared with the 11.25 MHz space

Standard channel BW
 Figure shows allocation of two channel on spectrum
band

Channel bandwidth for colour
transmission
 Following figure shows location of colour signal band
in video signal spectrum

TV Signal propagation
 Radio waves are electromagnetic waves, which when
radiated from transmitting antennas, travel through
space to distant places, where they are picked up by
receiving antennas
 Although space is the medium through which
electromagnetic waves are propagated, but depending
on their wavelengths, there are three distinctive
methods by which propagation takes place
 These are: (a) ground wave or surface wave
propagation, (b) sky wave propagation, and (c) space
wave propagation

 (a) ground wave or surface wave propagation:
 Vertically polarized electromagnetic waves radiated at
zero or small angles with ground, are guided by the
conducting surface of the ground, along which they
are propagated
 Such waves are called ground or surface waves
 The attenuation of ground waves, as they travel along
the surface of the earth is proportional to frequency,
and is reasonably low below 1500 kHz

 (b) Sky Wave Propagation:
 Most radio communication in short wave bands up to
30 MHz (11 meters) is carried out by sky waves
 When such waves are transmitted high up in the sky,
they travel in a straight line until the ionosphere is
reached
 This region which begins about 120 km above the
surface of the earth, contains large concentrations of
charged gaseous ions, free electrons and neutral
molecules
 The ions and free electrons tend to bend all passing
electromagnetic waves

 The angle by which the wave deviates from its straight
path depends on
 (i) frequency of the radio wave
 (ii) angle of incidence at which the wave enters the
ionosphere
 (iii) density of the charged particles in the ionosphere at
the particular moment
 (iv) thickness of the ionosphere at the point

 With increase in frequency, the allowable incident
angle at the ionosphere becomes smaller until finally a
frequency is reached, when it becomes impossible to
deflect the beam back to earth
 For ordinary ionospheric conditions this frequency
occurs at about 35 to 40 MHz
 Above this frequency, the sky waves cannot be used for
radio communication between distant points on the
earth

 (c) Space Wave Propagation
 The only alternative for transmission in the VHF and
UHF bands, despite large attenuation, is by radio
waves which travel in a straight line from transmitter
to receiver
 This is known as space wave propagation
 For not too large distances, the surface of the earth can
be assumed to be flat and different rays of wave
propagation can reach the receiver from transmitter

 Effect of Earth’s Curvature:
 Earth’s curvature limits the maximum distance
between the transmitting and receiving antennas
 The maximum line of sight distance d between the two
antennas can be easily found out
 Neglecting (hr)2 and (ht)2, being very small as
compared to R, the radius of the earth, the line-of-
sight distance d ≈ 4.22(√ht + √hr ) km

 Range of Transmission
 A sample calculation shows that for a transmitting
antenna height of 225 meters above ground level the
radio horizon is 60 km
 If the receiving antenna height is 16 meters above
ground level the total distance is increased to 76 km
 Depending on the transmitter power and other factors
the service area may extend up to 120 km for the
channels in the VHF band but drops to about 60 km
for UHF channels

 Booster Stations
 Some areas are either shadowed by mountains or are
too far away from the transmitter for satisfactory
television reception
 In such cases booster stations can be used. A booster
station must be located at such a place, where it can
receive and rebroadcast the program to receivers in
adjoining areas

 Signal strength is a function of power radiated,
transmitting and receiving antenna heights
 The acceptable signal to noise ratio at the picture tube
screen is measured in terms of peak-to-peak video
signal voltage (half tone), injected at the grid or
cathode of the picture tube versus the r.m.s. random
noise voltage at that point
 A peak signal to r.m.s. noise ratio of 45 db is generally
considered adequate to produce a good quality picture

 Field strength is indicated by the amount of signal
received by a receiving antenna at a height of 10 meters
from ground level, and is measured in microvolts per
meter of antenna dipole length
 The field strength for very good reception in thickly
populated and built-up areas is 2500 µV/ meter for
channels 2 to 4 (47 to 68 MHz), and 3550 µV/meter for
channels 5 to 11 (174 to 223 MHz)
 For channels in the UHF band, a field strength of
about 5000 µV/meter becomes necessary

Interference suffered by TV
channels
 (a) Co-channel Interference
 Two stations operating at the same carrier frequency, if
located close by, will interfere with each other
 This phenomenon which is common in fringe areas is
called co-channel interference
 As the two signal strengths in any area almost
equidistant from the two co-channel stations become
equal, a phenomenon known as ‘venetian-blind’
interference occurs

channels

channels
 (b) Adjacent Channel Interference
 Stations located close by and occupying adjacent
channels, present a different interference problem
 Adjacent channel interference may occur as a result of
beats between any two of these frequencies or between
a carrier and any sidebands
 A coarse dot structure is produced on the screen if
picture carrier of the desired channel beats with sound
carrier of the lower adjacent channel

channels
 (c) Ghost Interference
 Ghost interference arises as a result of discrete
reflections of the signal from the surface of buildings,
bridges, hills, towers etc

channels
 The direct signal is usually stronger and assumes
control of the synchronizing circuitry and so the
picture, due to the reflected signal that arrives late,
appears displaced to the right
 Such displaced pictures are known as ‘trailing ghost’
pictures
 The effect of such reflected signals (ghost images) can
be minimized by using directional antennas and by
locating them at suitable places on top of the buildings

TV broadcast channels for
terrestrial transmission
 Below are the band rages approved by Consultative
Committee on International Radio(CCIR)

TV Signal Transmission & Propagation Explained

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