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 signalThe 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 theactive 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

� In negative modulation, the transmitter may be overmodulated 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 theReceiver:

� 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

� In the positive system of modulation the corresponding stable level is zero amplitude at the carrier and obviouslyzero 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 foradequate 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

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

Television transmitter

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 theconducting 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 theionosphere

� (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 distancebetween 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-ofsight 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 necessaryInterference suffered by TVchannels

� (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

� (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

� (c) Ghost Interference

� Ghost interference arises as a result of discrete reflections of the signal from the surface of buildings, bridges, hills, towers etc

� 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 buildingsTV broadcast channels for terrestrial transmission

� Below are the band rages approved by Consultative Committee on International Radio(CCIR)