training course for television · pdf filetelecommunications and broadcasting services...

TELECOMMUNICATIONS AND BROADCASTING SERVICES

TRAINING COURSE FOR TELEVISION BROADCASTING

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THE INTERNATIONAL TV STANDARDS Ten international TV standard exist at present, all based on the same principles:

- Physiology of vision.

- Line scanning.

- Field repetition.

- Colour transmission as separate luminance components.

VISION CHARACTERISTICS - Mean resolution 1’ (angle of sight).

- Optimum angle for picture observation without fatigue of eye muscle 10°.

- Optimum line number = observation angle / angle of sight = 10°/1’ = 600 lines. - Field frequency without motion blurred >12/s. - Field frequency without flicker >50/s.

NUMBER OF LINE PER PICTURE Frames of 525 and 625 lines are still in use. The resolution is too weak at 405 lines and the frequency required

is too high at 819 lines. These values have been superseded by 625 lines.

FIELD FREQUENCY Field frequencies of 50 Hz and 60 Hz in conjunction with 500 to 600 lines per frame led to a video frequency

band of more than 10 KHz. An ingenious trick cut the required frequency band down to half: interlaced scanning of a first field consisting of the odd lines and second field consisting of the even lines .Thus a

frequency of 50 field/s (flicker) together with only 25 frames/s (frequency band) is obtained.

COLOUR TRANSMISSION Three colour TV systems has been developed independently of each other regarding the number of lines and

frequency:

NTSC 1948 PAL 1961 SECAM 1957

The luminance signal is necessary for compatibility with the existing monochrome TV receivers. The three

primary signals RED, GREEN, BLUE, are transmitted in the form of colour difference signals (with reduced

bandwidth) relative to the luminance signal. Only two colour difference signals are necessary (the third being

produced by electronic calculation in the receiver). The two colour difference signals modulate a colour

subcarrier simultaneously with AM in the NTSC and PAL system and successively with FM in the SECAM

system. The modulation frequency spectrum of the colour subcarrier is inserted in the frequency spectrum

of the luminance signal at the upper end of the video frequency band (half-line or quarter-line offset).



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BASIC TV STANDARDS Two basic standards have been adopted for the international exchange of TV programs:

FCC Standard CCIR Standard Lines / frame 525 625 Fields/s 60 50 Colour system NTSC PAL/SECAM Video Bandwidth 4.2 MHz 5 / 5.5 / 6 MHz Colour Subcarrier 3.58 MHz 4.43 MHz The different video bandwidths of the CCIR standard are not so much due to field and line scanning

procedures, but rather to the bandwidth available in the TV transmitter channels.

STANDARD CONVERSION The main problem of standards conversion is the conversion of field frequency from 50 Hz to 60 Hz and vice

versa. For this purpose, the picture information must be stored and then scanned at the new frequency. The

display is picked up like an open scene in the new standard by a camera tube. A digital standards converter

converts the picture signal information from analog into digital form, reads it into a digital memory, reads it out

with a new scanning rate and reconverts it into analog form.

In the standards converter for colour television, the incoming signal must be divided into its luminance and

chrominance components, decoded and remodulated onto the other colour carrier. If only the colour system is

to be converted, e.g. PAL into SECAM, the number of lines and the field frequency being equal, no picture

memory is required. It then suffices to separate and transcode the chrominance signal and to modulate the new

carrier as required. (Transcode Principe).

Observation of international TV standard is necessary in view of:

- international exchange of programs.

- design of TV transmitters and transposers.

- design of video recorders.

- development of measuring instruments and systems.



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MOST USED BROADCASTING STANDARD

Standard FCC CCIR British OIRT Number of lines 525 625 625 625 Field frequency 60 Hz 50 Hz 50 Hz 50 Hz Standard code M B / G I D / K Channel width 6.0 MHz 7/8 MHz 8 MHz 8.0 MHz Vis/Sound carr. Spacing 4.5 MHz 5.5 MHz 6 MHZ 6.5 MHz 5.74 MHz Vestigial sideband 0.75 MHz 0.75 MHz 1.25 MHz 0.75 MHz (1.25 MHz) Vision IF 45.75 MHz 38.9 MHz 39.5 MHz 38.9 MHz (38 MHz) Vision / Sound ratio 5:1 10:1 5:1 10:1 20:1 20:1:0.2

BROADCASTING OF TV PROGRAMS The public television service is opened by broadcasting picture and sound from picture transmitters and

associated sound transmitters in three main frequency ranges in the VHF and UHF bands. By international

ruling of the UIT / ITU, these ranges are exclusively allocated to television broadcasting. Subdivision into

operating channels and their assignment by location are also ruled by international regional agreement .

Band Frequency Channel Bandwidth

I (41) 47 to 68 MHz 2 to 4 7 MHz II 87.5 (88) to 108 MHz VHF FM sound III 174 to 223 (230) MHz 5 to 11 (12) 7 MHz IV 470 to 582 MHz 21 to 27 8 MHz V 582 to 790 (860) MHz 28 to 60 (69) 8 MHz VI 11.7 to 12.5 GHz satellite TV Special CHs. 68 to 82 (89) MHz 2 (3) S CHs. 7 MHz Digital sound 113 to 123 MHz S 2/3 5 MHz CATV 125 to 174 MHz S 4 to S 10 7 MHz CATV 230 to 300 MHz S 11 to S 20 7 MHz



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TIPES OF MODULATION Vision – CF3 (vestigial-sideband AM). 0.75 MHz – 4.2 MHz = 1:5.6 0.75 MHz – 5.0 MHz = 1:6.7 1.25 MHz – 5.5 MHz = 1:4.4 Sound – F3E FM. For better separation from vision signal in the receiver DUAL SOUND CARRIER SYSTEM System B/G. Channel 1 Channel 2 RF sound carrier………………. fvision + 5.5 MHz fvision + 5.7421875 (+/- 500 KHz) (+/- 500 KHz) Vision to sound power Ratio… 13 dB 20 dB Modulation………………………. FM FM Frequency deviation…………… < +/- 50 Hz < +/- 50 KHz Preemphasis………………….…. 50 us 50 us AF Bandwidth…………………… 40 to 15000 Hz 40 to 15000 Hz Sound modulation. Mono………………………………. mono mono Stereo……………………………… L+ R = M R 2 Dual sound……………………….. mono mono Pilot carrier frequency…………. 54.6875 KHz (+/- 5 Hz) equivalent to 3.5 fH) Modulation……………………….. AM with identification frequency 50% Modulation degree ident. Freq. Mono……………………………… none Stereo…………………………….. 117.5 Hz equivalent to fH/133 Dual sound……………………… 274.1 Hz eqvt. FH/57 Frequency deviation of transmitter carrier due to pilot tone. +/- 2.5 KHz +/- 0.5 KHz Syncronization………………….. pilot carrier and identification frequencies phase locked with fH.



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GROUP DELAY IN TV SYSTEMS The group-delay characteristics of TV systems are determined by various amplitude / frequency response

within the transmission path:

- in cables of extensive length in studios, switching centers and distribution points.

- In radio relay systems

- In TV transmitters, transposers and domestic receivers.

The group-delay error of a TV transmitter originates in the vestigial-sideband filter (IF-RF), in the video low

pass filters for limitation of the out-of-band radiation (VF) and in the diplexer combining the vision and sound

transmitters (RF). One-time correction to a residual error of 25 to 50ns are performed into the driver transmitter

precorrection circuit. With the introduction of colour television, a group-delay precorrection of –170 ns between

the luminance and chrominance signals was adopted on a quasi international level for the standards M, N and

B,G. As a group-delay measurements on TV transmitters are complex and require elaborate procedures, it

has been laid down in Technical Specifications that they should be switchable to two group –delay

characteristics:

- maximally flat for measuring purposes.

- Compensatory for precorrection in the transmitter, using the demodulator as a standard reference receiver to simulate the response of domestic receivers to the TV transmitter.



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RF TRANSMISSION OF VISION AND SOUND SIGNALS For radio transmission of the television signal and for some special applications, a RF carrier is modulated with

the composit video signal. For TV broadcasting and system including conventional TV receivers, amplitude

modulation is used, whereas frequency modulation is employed for TV transmission via microwave links

because of the higher transmission quality.

VESTIGIAL SIDEBAND AMPLITUDE MODULATION The advantage of amplitude modulation is the narrower bandwidth of the modulation products. With

conventional AM the modulating CVS of BW = 5 MHz, require an RF transmission bandwidth of BW RF equal

to 10 MHz.

In principle, one sideband could be suppressed since the two sidebands have the same signal content. This

would lead to single sideband amplitude modulation (SSB / AM ) (down figure center). Due to the fact that the

modulation signal reach very low frequencies, sharp cut off filters are required; however the group-delay

distortion introduced by these filters at the limits of the passband causes certain difficulties. The problem is

eluded using vestigial side band amplitude modulation (VSB / AM) instead of SSB / AM. In this case, one

complete sideband and part of the other are transmitted. (up figure bottom). In accordance with CCIR, 7 MHz

bands are available in the VHF range and 8 MHz bands in the UHF range for TV broadcasting. The picture

transmitter frequency response and the receiver passband characteristic are also determined by CCIR

standard.



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RF TRANSMISSION In most cases both modulation and demodulation take place at the IF, the vision IF being 38.9 MHz and the

sound IF 33.4 MHz. The modulation of the RF carrier by the CVS is in the form of negative AM, bright picture

points corresponding to a low carrier amplitude and the sync pulse to maximum carrier amplitude.

Negative amplitude modulation of RF vision carrier by CVS

A residual carrier (white level) of 10% is required because of the intercarrier sound method used in the

receiver. One advantage of negative modulation is optimum utilization of the transmitter, since maximum power

is necessary only briefly for the duration of the sync pulse and at the maximum amplitude occurring periodically

during the sync pulses to serve as a reference for automatic gain control in the receiver.

SOUND SIGNAL TRANSMISSION In TV broadcasting the sound signal is transmitted by frequency-modulating the RF sound carrier. The In

accordance with the receiver CCIR standard the sound carrier is 5.5 MHz above the associated vision carrier.

The maximum frequency deviation is 50 KHz. Due to certain disturbances in colour transmission, the

original sound / vision carrier power ratio of 1:5 was reduced to 1:10 or 1:20 [2]. Even in the latter case no

deterioration of the sound quality was apparent if the signal was sufficient for a satisfactory picture. With the

dual-sound carrier method, an additional sound carrier 250 KHz above the actual sound carrier is frequency

modulated its power level being 6 dB lower than that of the first sound carrier.

TV TRANSMITTER The RF television signal can be produced by two different methods. If the modulation takes place in the output

stage of the transmitter, the RF vision carrier is first brought to the required driving power ,and than, with

simultaneous amplitude modulation, amplified in the final stage to the nominal vision carrier output power of the

transmitter. The modulation amplifier boosts the wideband CVS to the level required for amplitude modulation

in the output stage. The sound carrier is frequency-modulated with a small deviation at a relatively low

frequency. The final frequency and the actual frequency deviation are produced via multiplier stages. The

picture transmitter and sound transmitter output stages are fed to the common antenna via the vision / sound

diplexer.



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TV TRANSMITTER When using IF modulation, first the IF vision carrier 38.9 MHz is amplitude-modulated. The subsequent filter

produces vestigial sideband AM. One or two sound carriers are frequency-modulated, also at the IF. Next,

mixing with a common carrier takes place both in the vision and in the sound channel so that the vision /sound

carrier spacing of 5.5 MHz is maintained at the RF. Linear amplifier stages boost the vision and sound carrier

powers to the required level.

The advantage of the second method is that the actual processing of the RF television signal is carried out at the IF , thus at a lower frequency, and band and channel-independent. However, for further amplification,

stages of high linearity are required, at least in the picture transmitter.



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RF TV MONITORING EQUIPMENTS Owing to the fact that a Television signal is composed of time varying levels of brightness interrupted by

periodic sync signals, two different measurement techniques have evolved since the inception of television:

- Time-domain – based on the wave form. - Frequency-domain – based on frequency response of amplitude and phase.

The most used equipments are:

- TV demodulator associated to a field strength meter.

- Spectrum analyser.

- In line Watt/Power meter.

- Frequency counter.

- Wave form and Vector scope.

Peripheral accessories are also very important. The most used are:

- Dummy load (50 Ohm).

- Terminations (50 Ohm).

- Directional coupler. (50 Ohm).

- Impedence bridge.(SWR Bridge) (50 Ohm).

TV DEMODULATOR

It convert the RF signal of a TV transmitter or transposer into the video and sound signals. Analysis or comparison with the original video and sound signals permit distortions to be recognized and faults to be found based on the type and amount of distortion. Field intensity can be also detected. SPECTRUM ANALYZER It operates by a principle of search frequency analysis, in which the frequency range to be analysed is sampled

by varying an internal oscillator frequency using an analysis filter with matched resolution bandwidth and

defined by any combination of the start, centre and stop frequencies as well as the span. The analysis is

displayed on a colour monitor, in the frequency in the X direction and the associated amplitudes in the Y

direction.

IN LINE WATTMETER It measure the power flowing into the RF line at various levels of power, by means of interchanging sensors of

different power levels. It can measure direct forward power and reflected power in coaxial transmission lines as

well as peak envelope power (PEP) measurements of SSB and AM signals.



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IN LINE WATTMETER



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IN LINE WATTMETER



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INTERMODULATION MEASUREMENTS A television transmitter emit several frequencies at the same time: vision-carrier frequency, colour-subcarrier frequency and sideband frequency (video content) as well as one or two sound-carrier frequencies. If the vision and sound signals are amplified together, intermodulation between the individual

frequencies produce additional sum and difference frequencies.. Although these interfering frequencies have

the same origins, the intermodulation products within the useful channel are covered by the term

intermodulation and those outside it are called spurious emissions. Intermodulation products only occur when there is joint amplification of the vision and sound signals.

For determining intermodulation products, the vision transmitter is fully modulated with a sinusoidal signal (in

addition to the sync pulses) (red field). The sound transmitter (carrier) is left unmodulated. The spectrum of this

can be displayed on a spectrum analyser and the intermodulation became visible. The measurement of

spurious emission differs from that of intermodulation only in the frequency scale (outside the channel).

VIDEO DEPHT OF MODULATION The maximum percentage of video modulation allowed by the CCIR rules is 87,5 % .( 100% - 87,5% = 12,5% ).

If we assume that 100 is the level of reference (corresponding to the 0 dB reference of the spectrum analyser),

when the white peak level of the video signal will drop to –18dB referred to the sync peak level, we obtain the

maximum of video modulation allowed.

C = 87.5% A = 12,5% 20 log 100/A = 18 dB



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ANALIZER DISPLAY



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POWER SUPPLY AND GROUNDING OF THE EQUIPMENTS Every body knows that many plant’s fault are originated by the electric line, either due to the interruption of the

energy supply or owing to the intervention of the protections or for some component being burnt due to

overvoltage. If for the energy’s supply interruption there is no remedy, unless we resort to the subsidiary direct

current power supply , on the contrary it is possible to do a lot in order to avoid the faults due to overvoltage.

We start from the choice of the spot of the plant’s installation by avoiding the top of the hill, if this is not

necessary. It means that, if the area to be served is seen also from a point along the slope, we suggest to

chose thi one, thus permitting to reduce the number of probabilities to be attacked by atmospheric

disturbances.

When making the patch with the electric power distribution line of your national company, it is necessary to bear in mind the recommendations listed here below. The aerial part of the line will terminate at 30 or 40 meters far from the plant, then it will continue with a highly

insulated cable (from 3 to 4 KV) in a duct under ground made with PVC tube of adequate dimensions and

thickness to reach the cabinet. At this point it will be necessary to separate the circuit of the low voltage line

from the equipment’s one by setting a transformer preceded by a switch or automatic switch that is only a

thermal one. This transformer must be highly insulated between the primary and the secondary windings as

well as to the mass, because it shall have to support without damages coming from the electrical line and from

the metal structure holding the antennas.

On the market there are transformers with the required characteristics and tested with the wave shapes

simulating the ones inducted by the atmospheric disturbances. The housing of this transformer may be either

outside or inside of the equipment’s cabinet according to the size of the cabinet itself. If the same is located

outside, we shall adequately protect it from atmospheric agents. The output of our transformer will terminate in

a small board equipped with some sockets to power supply the equipments and all sockets will be protected by

a single automatic switch whose magnetic protector is not gated.

The plant is not protected yet, then it is necessary to equip it with an efficient grounding, which will act as a

dispersion device for the atmospheric disturbances which attack the plant and as protector for casual contacts

and dispersions toward the mass of the power supply line.

The grounding must be made in such a way as to guarantee an equipotentiality among the different parts of

the plant and a fairly low value of dispersion impedence at the pulse currents in order to have a total voltage to

ground within controlled limits.



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POWER SUPPLY AND GROUNDING OF THE EQUIPMENTS The diagram to realize the grounding will depend on the geoelectrical characteristics of the ground, namely on

its resistivity grounds, the grounding will be formed by small sized elements, while in high resistivity grounds

dispersion devices of adequate length will be used in order to obtain low impedance values at the pulse

currents.

So before making the grounding, it is necessary to know the average resistivity of the ground where we fix the

plant. This can be easily obtained an instrument (MEGGER). The same will be absolutely necessary also for

measuring the ground resistance, when the plant is finished. By dividing the ground into two categories, those

with resistivity below 100 ohm/m and those with resistivity over 100 ohm/m, we can give the following general

rules for making the grounding.

For the resistivity lower than 100 ohm/m the drawing may consist of a loop circuit of a diameter of 2 or 4 meters

set around the antennae’s support and integrated with 3 or 4 small rods with leads of some meters placed

radially. All thi must be connected to the cabinet, to the antennae’s support, to the separator transformer and so

on. The ground resistance, measured at 50 Hz, must not be over 10 ohm.



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