- 1.Television Signal Transmission & Propagation
2. 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
3. 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 4. 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 5. Picture Signal transmission
6. 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 7. Positive and negative modulation 8. Positive and
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 9. Positive and
negative modulation 10. Positive and negative modulation 11.
Positive and negative modulation 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 12. Positive and negative
modulation 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 13.
Positive and negative modulation 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% 14. Positive and negative modulation 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 15. Positive and negative modulation 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 16. Vestigial sideband transmission 17.
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
18. Vestigial sideband transmission 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 19. Vestigial sideband transmission 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 20. Vestigial sideband transmission 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
21. Vestigial sideband transmission The picture signal is seen to
occupy a bandwidth of 6.75 MHz instead to 11 MHz 22. 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 23. Standard channel BW 24. Standard channel BW
Figure shows allocation of two channel on spectrum band 25. Channel
bandwidth for colour transmission Following figure shows location
of colour signal band in video signal spectrum 26. Television
transmitter 27. 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 28. TV Signal 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 29.
TV Signal propagation (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 30. TV Signal
propagation 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 31. TV Signal
propagation 32. TV Signal propagation 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 33. TV Signal propagation (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 34. TV
Signal propagation 35. TV Signal propagation 36. TV Signal
propagation Effect of Earths Curvature: Earths 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 37. TV Signal propagation 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
38. TV Signal propagation 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 39. TV Signal propagation 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 40. TV Signal propagation 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 V/ meter for channels 2 to 4 (47 to 68 MHz),
and 3550 V/meter for channels 5 to 11 (174 to 223 MHz) For channels
in the UHF band, a field strength of about 5000 V/meter becomes
necessary 41. 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 42. Interference suffered by TV
channels 43. Interference suffered by TV 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 44. Interference suffered by TV 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 45. Interference suffered by TV channels
46. Interference suffered by TV 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 47. TV
broadcast channels for terrestrial transmission Below are the band
rages approved by Consultative Committee on International
Radio(CCIR)
