Chapter 6

Chapter 6Television Signal Transmission & Propagation

ContentsPicture Signal transmissionPositive and negative modulationVestigial sideband transmissionStandard channel BWTelevision transmitterTV Signal propagationInterference suffered by TV channelsTV broadcast channels for terrestrial transmission

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

Picture Signal transmission

Positive and negative modulationWhen the intensity of picture brightness causes increase in amplitude of the modulated envelope, it is called positive modulationWhen 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

Positive and negative modulation

Positive and negative modulationEffect 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 intervalsThey extend in the direction of sync pulses when they occur during blanking intervalsIn 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 blankingObviously the effect of noise on the picture itself is less pronounced when negative modulation is usedPositive and negative modulation

Positive and negative modulation

Positive and negative modulationEffect 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 pulsesHowever, 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 pulsesThis can completely upset the synchronization of the receiver time bases unless something is done about itPositive and negative modulationPeak Power Available from the Transmitter:With positive modulation, signal corresponding to white has maximum carrier amplitudeThe 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

Positive and negative modulationPeak 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 pulsesConsequently, 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%

Positive and negative modulationUse 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 detailsThis level may be selected simply by passing the composite video signal through a peak detector

Positive and negative modulationUse 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 strengthThe 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 voltageVestigial sideband transmission

Vestigial sideband transmissionIn the 625 line TV system where the frequency components present in the video signal extend from dc (zero Hz) to 5MHzA double sideband AM transmission would occupy a total bandwidth of 10 MHzThe 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

Vestigial sideband transmissionTherefore, an attenuation slope of 0.5 MHz is provided at each edge of the two sidebandsThis adds 1 MHz to the required total band spaceIn 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 signalThis, 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 MHzVestigial sideband transmissionSuch a bandwidth is too large, and if used, would limit the number of channels in a given high frequency spectrum allocated for TV transmissionIn the video signal very low frequency modulating components exist along with the rest of the signalTherefore, 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

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

Vestigial sideband transmissionThe picture signal is seen to occupy a bandwidth of 6.75 MHz instead to 11 MHz

Standard channel BWThe sound carrier is always positioned at the extremity of the fully radiated upper sideband and hence is 5.5 MHz away from the picture carrierThe FM sound signal occupies a frequency spectrum of about 75 KHz around the sound carrierHowever, a guard band of 0.25 MHz is allowed on the sound carrier side of the television channel to allow for adequate inter-channel separationThe 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 spaceStandard channel BW

Standard channel BWFigure shows allocation of two channel on spectrum band

Channel bandwidth for colour transmissionFollowing figure shows location of colour signal band in video signal spectrum

Television transmitter

TV Signal propagationRadio waves are electromagnetic waves, which when radiated from transmitting antennas, travel through space to distant places, where they are picked up by receiving antennasAlthough space is the medium through which electromagnetic waves are propagated, but depending on their wavelengths, there are three distinctive methods by which propagation takes placeThese are: (a) ground wave or surface wave propagation, (b) sky wave propagation, and (c) space wave propagationTV 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 propagatedSuch waves are called ground or surface wavesThe attenuation of ground waves, as they travel along the surface of the earth is proportional to frequency, and is reasonably low below 1500 kHz

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 wavesWhen such waves are transmitted high up in the sky, they travel in a straight line until the ionosphere is reachedThis 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 wavesTV Signal propagationThe 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

TV Signal propagation

TV Signal propagationWith 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 earthFor ordinary ionospheric conditions this frequency occurs at about 35 to 40 MHzAbove this frequency, the sky waves cannot be used for radio communication between distant points on the earth

TV Signal propagation(c) Space Wave PropagationThe 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 receiverThis is known as space wave propagationFor 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

TV Signal propagation

TV Signal propagation

TV Signal propagationEffect of Earths Curvature:Earths curvature limits the maximum distance between the transmitting and receiving antennasThe maximum line of sight distance d between the two antennas can be easily found outNeglecting (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

TV Signal propagationRange of TransmissionA sample calculation shows that for a transmitting antenna height of 225 meters above ground level the radio horizon is 60 kmIf the receiving antenna height is 16 meters above ground level the total distance is increased to 76 kmDepending 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 channelsTV Signal propagationBooster StationsSome areas are either shadowed by mountains or are too far away from the transmitter for satisfactory television receptionIn 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

TV Signal propagationSignal strength is a function of power radiated, transmitting and receiving antenna heightsThe 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 pointA peak signal to r.m.s. noise ratio of 45 db is generally considered adequate to produce a good quality picture

TV Signal propagationField 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 lengthThe 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

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

Interference suffered by TV channels(b) Adjacent Channel InterferenceStations located close by and occupying adjacent channels, present a different interference problemAdjacent channel interference may occur as a result of beats between any two of these frequencies or between a carrier and any sidebandsA coarse dot structure is produced on the screen if picture carrier of the desired channel beats with sound carrier of the lower adjacent channelInterference suffered by TV channels(c) Ghost InterferenceGhost interference arises as a result of discrete reflections of the signal from the surface of buildings, bridges, hills, towers etc

Interference suffered by TV channels

Interference suffered by TV channelsThe 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 rightSuch displaced pictures are known as trailing ghost picturesThe 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 transmissionBelow are the band rages approved by Consultative Committee on International Radio(CCIR)