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    CHAPTER 1

    INTRODUCTION TO AIR

    1.1INTRODUCTION

    All India Radio (abbreviated as AIR), officially known as Akashvani is the radio

    broadcaster ofIndia and a division of Prasar Bharati (Broadcasting Corporation of

    India), an autonomous corporation of the Ministry of Information and Broadcasting,

    Government of India. Established in 1936, today, it is the sister service of Prasar

    Bharati's Doordarshan, the national television broadcaster. The word Akashvani was

    coined by Professor Dr. M.V. Gopalaswamy for his radio station in Mysore during

    1936.

    All India Radio is one of the largest radio networks in the world. The headquarters is at

    the Akashvani Bhavan, New Delhi. Akashvani Bhavan houses the drama section, the

    FM section and the National service. The Doordarshan Kendra (Delhi) is also located

    on the 6th floor of Akashvani Bhavan.

    A famous thing that happened with the AIR was that during his regular broadcasts from

    the Azad Hind Radio, Subhash Chandra Bose used to refer to the pre-independence

    AIR as Anti Indian Radio.

    The first radio program in India was broadcast by the Radio club of Bombay in June

    1923. It was followed by the setting up of a broadcasting service that began

    broadcasting in India in July 1927 on an experimental basis at Mumbai and Kolkata

    simultaneously under an agreement between government of India and a private

    company called the Indian Broadcasting company ltd. The operation of AIR began

    formally in 1936, as a government organization, with clear objectives to inform,

    educate and entertain the masses.

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    http://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Indiahttp://en.wikipedia.org/wiki/Prasar_Bharatihttp://en.wikipedia.org/wiki/Government_of_Indiahttp://en.wikipedia.org/wiki/Doordarshanhttp://en.wikipedia.org/wiki/New_Delhihttp://en.wikipedia.org/wiki/Doordarshanhttp://en.wikipedia.org/wiki/Delhihttp://en.wikipedia.org/wiki/Azad_Hind_Radiohttp://en.wikipedia.org/wiki/Subhas_Chandra_Bosehttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Indiahttp://en.wikipedia.org/wiki/Prasar_Bharatihttp://en.wikipedia.org/wiki/Government_of_Indiahttp://en.wikipedia.org/wiki/Doordarshanhttp://en.wikipedia.org/wiki/New_Delhihttp://en.wikipedia.org/wiki/Doordarshanhttp://en.wikipedia.org/wiki/Delhihttp://en.wikipedia.org/wiki/Azad_Hind_Radiohttp://en.wikipedia.org/wiki/Subhas_Chandra_Bose
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    Serve the rural, illiterate and underprivileged population, keeping in mind the

    special needs and interests of the young, social and cultural minorities, the tribal

    population, and of those residing in border regions, backward or remote areas.

    Promote national integration.

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    CHAPTER 2

    STUDIO CHANNEL IN A TYPICAL AIR STATION

    2.1 INTRODUCTION

    The broadcast of a programme from source to listener involves use of studios,

    microphones, announcer console, switching console, telephone lines / STL and

    Transmitter. Normally the programmes originate from a studio centre located inside the

    city/town for the convenience of artists. The programme could be either live or

    recorded. In some cases, the programme can be from OB spot, such as commentary of

    cricket match etc. Programmes that are to be relayed from other Radio Stations are

    received in a receiving centre and then sent to the studio centre or directly received at

    the studio centre through RN terminal/telephone line. All these programmes are then

    selected and routed from studio to transmitting centre through broadcast quality

    telephone lines or studio transmitter microwave/VHF links. A simplified block

    schematic showing the different stages is given in Fig. 2.1.

    Fig. 2 .1 Simplified block schmatic of broadcasting chain

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    2.2 Studio Centre

    The Studio Centre comprises of one or more studios, recording and dubbing room, a

    control room and other ancilliary rooms like battery room, a.c. rooms, switch gear

    room, DG room, R/C room, service room, waiting room, tape library, etc. The size of

    such a centre and the number of studios provided depend on the programme activities

    of the station. The studio centres in AIR are categorised as Type I, II, III and IV. The

    number of studios and facilities provided in each type are different. For example a type

    I studio has a transmission studio, music studio with announcer booth, a talks studio

    with announcer booth, one recording/dubbing room and a Read Over Room. Type II

    has one additional drama studio. The other types have more studios progressively.

    2.3 Broadcast Studio

    A broadcast studio is an acoustically treated room. It is necessary that the place where

    a programme for broadcast purposes is being produced should be free of extraneous

    noise. This is possible only if the area of room is insulated from outside sound.

    Further, the microphone which is the first equipment that picks up the sound, is not able

    to distinguish between wanted and unwanted signals and will pick up the sound not

    only from the artists and the instruments but also reflections from the walls marring the

    quality and clarity of the programme. So the studios are to be specially treated to give

    an optimum reverberation time and minimum noise level. The entry to the studios is

    generally through sound isolating lobby called sound lock. Outside of every studio

    entrance, there is a warning lamp, which glows Red when the studio is ON-AIR.

    The studios have separate announcers booths attached to them where first level fading,

    mixing and cueing facilities are provided.

    2.4 Studio Operational Requirements

    Many technical requirements of studios like minimum noise level, optimum

    reverberation time etc. are normally met at the time of installation of studio. However

    for operational purposes, certain basic minimum technical facilities are required for

    smooth transmission of programmes and for proper control. These are as follows:

    Programme in a studio may originate from a microphone or a tape deck, or a

    turntable or a compact disc or a R-DAT. So a facility for selection of output of any

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    of these equipments at any moment is necessary. Announcer console does this

    function.

    Facility to fade in/fade out the programme smoothly and control the programme

    level within prescribed limits.

    Facility for aural monitoring to check the quality of sound production and sound

    meters to indicate the intensity (VU meters).

    For routing of programmes from various studios/OB spots to a central control room,

    we require a facility to further mix/select the programmes. The Control Console in

    the control room performs this function. It is also called switching console.

    Before feeding the programmes to the transmitter, the response of the programmeshould be made flat by compensating HF and LF losses using equalised line

    amplifiers.(This is applicable in case of telephone lines only)

    Visual signalling facility between studio announcer booth and control room should

    also be provided.

    If the programmes from various studios are to be fed to more than one transmitter, a

    master switching facility is also required.

    2.5 Mixing

    As already mentioned, various equipments are available in a studio to generate

    programme as given below:

    Microphone, which normally provides a level of 70 dBm.

    Turntable which provides an output of 0 dBm.

    Tape decks which may provide a level of 0 dBm.

    CD and R-DAT will also provide a level of 0 dBm.

    The first and foremost requirement is that we should be able to select the output of any

    of these equipments at any moment and at the same time should be able to mix output

    of two or more equipments. However, as we see, the level from microphone is quite

    low and need to be amplified, so as to bring it to the levels of tape recorder/ tape decks.

    Audio mixing is done in following two ways:

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    i) Required equipments are selected and then outputs are mixed before feeding to

    an amplifier. This is called low level mixing (Fig. 2.2). This is not commonly

    used now days.

    Fig. 2.2 Low level mixing

    ii) Low-level output of each equipment is pre-amplified and then mixed. This is

    called high level mixing.

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    Fig. 2.3 High level mixing

    Low level mixing system may look economical since it requires one single pre-

    amplifier for all low level inputs, but quality of sound suffers in this system as far as

    S/N ratio is concerned. Noise level at the input of best designed pre-amplifier is of the

    order of 120 dBm and the output levels from low level equipment 70dBm. In low

    level mixing, there is signal loss of about 10 to 15 dB in mixing circuits. Therefore, the

    S/N ratio achieved in low level mixing is 35 to 40 dB only.

    High level mixing system requires one pre-amplifier in each of the low level channels

    but ensures a S/N of better than 50 dB. All India Radio employs High level mixing.

    2.6 Announcer Console

    Most of the studios have an attached booth, which is called transmission booth or

    Announcer booth or play back studio. This is also acoustically treated and contains a

    mixing console called Announcer Console. The Announcer Console is used for mixing

    and controlling the programmes that are being produced in the studio using artist

    microphones, tape playback decks and turn tables/CD players. This is also used for

    transmission of programmes either live or recorded.

    The technical facilities provided in a typical announcer booth, besides an Announcer

    Console are one or two microphones for making announcements, two turn tables for

    playing the gramophone records and two playback decks or tape recorders for recorded

    programmes on tapes. Recently CD and Rotary Head Digital Audio Tape Recorder (R-

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    DAT) are also included in the Transmission Studio. Audio block schematic of

    transmission studio is shown in Fig. 2.4.

    Fig. 2.4 Announcer Console

    2.7 Control Room

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    For two or more studios set up, there would be a provision for further mixing which is

    provided by a control console manned by engineers. Such control console is known as

    switching console. Broad functions of switching console in control room are as follows:

    Switching of different sources for transmission like News, O.Bs. other

    satellite based relays, live broadcast from recording studio.

    Quality monitoring.

    Signalling to the source location.

    Communication link between control room and different studios.

    Audio block schematic of control room is shown in Fig.2. 5.

    Fig. 2.5 Block Schematic of Control Room

    CHAPTER 3

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    TRANSMITTER

    The electronic equipment used to produce radio Frequency for radio transmission is

    called transmitter.

    The function of the transmitter is to generate the R.F carrier of proper frequency and of

    sufficient power. The output of a transmitter is applied to the antenna, which radiates

    the signal into space.

    Basically, a transmitter consists of two sections.

    1. RF section

    2. AF section

    3.1 RF Section

    a) Master oscillator: - An oscillator is the heart of a transmitter. It generates radio

    frequency voltage with high degree of frequency stability. Crystal oscillator hasmaximum degree of frequency stability and it is used as master oscillator. The

    frequency generated depends upon the thickness of the crystal.

    b) Buffer amplifier: - A buffer amplifier is used to isolate an oscillator so that the

    frequency of the oscillator is not affected by the operation of succeeding stages.

    c) Int. Power amplifier: - It is a class c amplifier used to increase the power level and

    deliver to succeeding stages.

    d) Power amplifier: - This is a plate modulated class c amplifier. Its O/P is fed to the

    transmitting antenna through a feeder line.

    3.2 AF Section

    This section is designed to convert the information Into corresponding electrical

    variations of sufficient magnitude. The electrical signals are amplified, modulated and

    fed to the power amplifier.

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    passed to the low pass filter fixed in baseband interconnection unit. The signal is fed to

    the VCO unit where oscillator frequency of 415 MHz and at the same time music is

    modulated. This frequency is multiplied by 4 times in RF amplifier. The exact

    frequency 1440 MHz is adjusted in reference oscillator. The signal is further amplified;

    the power of transmitter is connected to the antenna filter which is then given to

    antenna port. When power of one transmitter is connected to antenna port, the other

    transmitter is connected to dummy load (stand by).

    3.4 RECEIVER PATH

    The transmitted signal at the other station is received through antenna feeder cable .It is

    then passed through antenna filter tuned for receiver frequency. The signal is then given

    to RF front-end amplifier where it amplified by 12 db. The RF amplifier is consists of a

    mixer, local oscillator and a multiplexer. The local oscillator frequency is generated in

    VCO unit is fed to the RF amplifier (+10 dbm) and then it is multiplied by 4 times for

    mixing with received frequency. The output I.F. 35 MHz with a gain of 20-25 db is then

    given to the I.F. amp. Where it is amplified and demodulated. The detected music

    signal is then amplified, filtered and then given to line transformer.

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    Fig 3.2 Reciever

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    3.5 SPECIFICATIONS

    1.Frequency range -1427- 1660 MHz (1440 MHz working)

    2. FM modulation.

    3. 3 Watt power output.

    4. Output impedance 50 unbalanced.

    5. Total harmonic distortion

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    CHAPTER 4

    MICROPHONES

    4.1 Introduction

    Microphone plays a very important role in the art of sound broadcasting. It is a device

    which converts accoustical energy into electrical energy. In the professional

    broadcasting field microphones have primarily to be capable of giving the highest

    fidality of reproduction over audio bandwidth.

    4.2 Microphone Classification

    Depending on the relationship between the output voltage from a microphone and the

    sound pressure on it, the microphones can be divided into two basic groups.

    4.2.1 Pressure Operated Type

    In such microphones only one side of the diaphragm is exposed to the sound wave. The

    output voltage is proportional to the sound pressure on the exposed face of the

    diaphragm with respect to the constant pressure on the other face. Moving coil, carbon,

    crystal and condenser microphones are mostly of this type. In their basic forms, the

    pressure operated microphones are omni-directional.

    4.2.2 Velocity or Pressure Gradiant Type

    In these microphones both sides of the diaphragm are exposed to the sound wave. Thus

    the output voltage is proportional to the instantaneous difference in pressure on the twosides of the diaphragm. Ribbon microphone belongs to this category and its polar

    diagram is figure of eight.

    4.3 Types of Microphones

    There are many types of microphones. But only the most common types used in

    broadcasting have been described here.

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    4.3.1 Dynamic or Moving Coil MicrophoneThis is common broadcast quality

    microphone which is rugged and can be carried to outside broadcast/recording etc. It

    consists of a strong permanent magnet whose pole extensions form a radial field within

    a round narrow gap. A moving coil is supported within this gap and a dome shaped

    diaphragm usually of aluminium foil is attached to the coil. The coil is connected to a

    microphone transformer whose secondary has sometimes tapings to select proper

    impedance for matching.

    With sound pressure changes, the diaphragm and coil move in the magnetic field,

    therefore, emf is induced in the speech coil, which is proportional to the incoming

    sound.

    The primary impedance of the matching transformer is generally high (5 to 6 times of

    thespeech coil impedance so that low frequencies are not lost and rising impedance

    frequency characteristic is avoided as best as possible. The resonant frequency is

    generally damped with special arrangements of absorption in acoustic cavity,

    Bass/boost arrangements are provided by an equalising tube connecting the rear side of

    diaphragm i.e. inside of microphone with the atmosphere. The diameter and length of

    the tube is critically adjusted for achieving good frequency response. The output of the

    microphone is 65 to 68 dBv and various shapes of the body make it OMNI UNI or

    SEMI directional.

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    Fig.4. 1 Dynamic Microphone (Moving coil)

    4.3.2 Ribbon/ Velocity Microphone

    Corrugated aluminium foil about 0.1 mm thick forms a ribbon which is suspended with

    in two insulated supports. The ribbon is placed within the extended poles of a strong

    horse shoe magnet. The ribbon moves due to the difference in pressure (at right angles

    to its surface) i.e. from the front or rear of the mike. There exists the maximum

    pressure difference between the front and rear of ribbon because of maximum path

    difference.

    The sound does not develop any pressure gradient when it comes from the sides of the

    microphones because there is no path difference. It reaches the front and rear of ribbon

    at the same time, hence no movement of ribbon. Thus, this microphone is bi-directional

    and follows figure of eight directivity pattern with no pick up from sides.

    Such a microphone has a clarity filter. This is a series resonant circuit at low

    frequencies across the primary of microphone transformer. When switched to the

    Talk or Voice position, the response is modified cutting down low frequencies by

    about 8 dB at 50 Hz. This filter should therefore not be in circuit during music

    performances.

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    This microphone delivers 80 dBv with a very good frequency response. The output

    impedance of this microphone is high. The popular method of providing d.c. voltage to

    the condenser is known as Phantom Powering .

    Variable directivity capacitor microphones are becoming popular these days.

    Fig 4.3 Condenser microphone

    4.3.3 Electret Microphone

    It is a modified form of condenser microphone in which the polarising voltage is

    avoided. In fact a plastic polymer containing metallic dust keeps the metal particles

    permanently charged with in the plastic insulation and such a polymer within the

    diaphragm foil or fixed plate delivers the electrical signal on the principle of the

    condenser mike. The hissing noise gets avoided since there is no external polarising

    resistor as a load. The microphone has high impedance and is generally having FET

    pre-amplifier. The microphone costs very little but developes excellent quality designs

    in many forms.

    Perhaps this microphone is going to flourish most in comparison to all other.

    4.4 Special Microphones

    4.4.1Combined Microphones

    More than one microphone is placed within the same unit to achieve a particular

    purpose e.g. Western Electric 639 combines a dynamic (OMNI) microphone with a

    Ribbon (Bi-directional) microphone to get a cardioid pattern. Thus we have a choice of

    three patterns by switching in either or both units. AKG 202 consists of two dynamic

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    microphones to obtain a flat frequency response. In fact the dual arrangement is

    coaxially mounted with the smaller in the front and the bigger diaphragm in the rear in

    a single housing. The two are connected electrically with a phase correcting network.

    The response shows a cross-over at 500 Hz. One unit is adjusted for frequencies above

    and the other unit for frequencies below to achieve good response. A switch is provided

    to cut down low frequencies with 50 Hz dropping upto 20 dB with respect to 1 kHz.

    The sensitivity is 53 dBv and mike is uni-directional resembling a cardioid pattern.

    4.4.2 Lip Ribbon Microphone

    It is also called noise-cancelled mike since the ribbon even if held close, does not pick

    up breathing noises due to a guard in between. The stainless steel mesh acts as a wind

    shield. The design and other features resemble the ribbon mike.

    4.4.3 Lapel Microphone

    Both carbon and ribbon types are available. The microphone is very small and light-

    weight and is suspended around the neck keeping the mike just below the chin. It is

    most suitable for running commentary or in a lecture.

    4.4.4 Contact Microphone

    It is generally a dynamic microphone of lower sensitivity. It is normally placed close to

    the source of sound, when it is not supposed to pick up other stray noises.

    4.4.5 Gun Mike

    It has two forms, (short gun and long gun). A dynamic mike placed at the end of a

    perforated tube extends its directivity in the front. The short gun about 18 long can

    pick up a talk from about 10 feet distance and a long gun with a tube of about 3 feet

    length can pick up sound from a distance of about 20 to 25 feet. The quality suffers but

    is

    intelligible. This microphone is useful when sound from a distant spot is to be picked

    up. An example is the picking of the sound of bat hitting a cricket ball.

    4.5 Important Characteristics

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    Important characteristics of the microphones are as under :

    4.5.1Frequency Response

    This characteristic indicates the relative signal output voltage of a microphone at

    different frequencies for a constant acoustic level input at all the frequencies. These

    days it is possible to attain an almost flat frequency response over the audio range of 20

    Hz to 20 kHz. Frequency response of a microphone depends on :-

    Direction of arrival of sound, and

    Distance between the source and the microphone

    Frequency response specified by manufacturers is generally that obtained by using a

    calibrated sound source at a specified distance in an anechoic test room or duct. The full

    20 Hz to 20 kHz spectrum may not be necessary or required in some applications. In

    some microphones a roll off at low frequency end is provided to cut off low frequency

    noise. If a microphone covers the essential audio range 100 Hz to 7 kHz within + 1 dB

    it is considered to be a broadcast quality microphone.

    4.5.2 Directivity

    Microphones can be designed either to respond equally to sounds from an angle or to

    discriminate those arriving from specific directions. Microphones which respond

    equally at all angles are called omni-directional. The microphones which pick up

    equally from front and rear and have very little pick up equally from sides are called Bi-

    directional and have a polar diagram as figure of eight. The microphones which pick up

    maximum from the front with slight reduction in the sides and very less pick up from

    the rear are called Cardioid (means heart shape). Microphones directivity is often a

    principal reason for choosing between different models for particular applications.

    4.5. 3Sensitivity

    The ability to pick up weak sound and to deliver more electrical signal determines the

    sensitivity. It is measured in dBs below 1 volt as the electrical output from a

    microphone when a standard sound pressure of one microbar i.e. 1 dyne per sqr. cm. is

    applied at the diaphragm of the microphone. The velocity or Ribbon Mike gives an

    output of about 70 dBv and dynamic Mike 65 to 68 dBv.

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    4.5.4 Distortion

    The microphone should not add or subtract the amplitude or frequency of the sound

    during conversion. The maximum change in complex wave form cannot be measured,

    as such the tests are conducted under sine wave conditions and within the broadcasting

    range offrequencies, the distortion is not allowed to exceed beyond a specified value,

    typically less than 0.5% at 1000 Hz.

    4.5.5Termination Impedance

    The microphone must have a proper impedance and a balanced or unbalanced output

    suited to the pre-amplifier. In the broadcast chain the microphone lines cover long

    distances, therefore, the impedance is chosen in the range of 50 ohms to 60 ohms at the

    microphone terminals. The commercial microphones in public address system do not

    require lengthy mike cables and prefer high electrical output across high impedance

    which is generally above 5 k ohms.

    Moreover broadcasting microphones use balanced output with Mike cable containing

    two live conductors and a earth shield commercial microphones have unbalanced output

    connected to single core of mike cable which is shielded.

    The above arrangement used in commercial practice is not suitable for broadcasting

    set-up mainly because, the noise pick up on unbalanced lines and high impedance of

    circuit become objectionable and prone to loss of high frequencies when the cable is

    long. Therefore, the termination of broadcast type of microphone will have balanced

    output with impedance in the range of 50, 70, 100, 200, 300 or 600 ohms to suitably

    match the input impedance of the pre-amplifier.

    In some modern microphones, the pre-amplifier is an integral part of microphone and

    high level output is brought out. In another modern variety the cable is not used at all.

    The sound picked by microphone is modulated on miniature FM transmitter and a

    power of 100 mW or so is radiated. Such microphones do not have any termination but

    an antenna and are called cordless or RF microphones or Radio Microphones, Sound

    signals are available on demodulation at the receiver for mixing with other

    microphones.

    4.6 Guidelines for choice and placement of Microphones

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    4.6.1 Choice of Microphone

    a) Frequency response : A flat frequency response (50-10000 Hz) is preferred for music

    and drama programmes whereas a gentle roll on low frequency side below 200 Hz is

    preferred for talks and announcement.

    For OBS, the above requirements can be somewhat relaxed further.

    b) Directivity pattern: For announcements, in modern practice cardioid microphones

    without proximity effect (like AKG-2-way cardioids are preferred though at one time

    we were using omni-directional microphones.

    For talks/discussions and music cardioid or bi-directional microphones are preferred.

    An omni directional microphone can sometimes be used for discussions. For OBs

    having PA system cardioid with very good front to back ratio only should be used. For

    open air discussions/interview etc. omni directional microphones can be used. Special

    type of microphones such as lavalier microphones should be used where situation

    demands use of such a microphone

    4.6.2 Placement of Microphones

    Placement of microphone has important bearing on the quality of its output. A few

    general guidelines given in the following paragraphs should help in improvement of

    programme production.

    a) As far as possible, microphones should be placed with its 0o axis facing the source of

    sound to avoid off axis colouration.

    b) Phasing of Microphone

    Whenever two or more microphones are used with their outputs mixed together, it

    should be ensured that their outputs are in phase. A simple test for above is as follows:

    Fade in one microphone and monitor the sound level output for any programme. With

    all other controls undisturbed, fade in the second microphone. If the outputs of the two

    microphones are in phase, an increase in the overall sound level output should result. If

    there is loss of sound level and deterioration in quality (attenuation of low frequencies)

    leads of one of the microphones should be reversed in the microphone socket and the

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    test repeated. In the case of ribbon bi-directional microphone the same result can be

    achieved by rotating the microphone through 180o.

    If more than two microphones are used simultaneously second and third microphones

    and so on should also be tested similarly.

    c) Working Distance

    Whenever a directional microphone is kept fairly close to the source of sound low

    frequencies in the output of the microphone may get disproportionately boosted thereby

    giving rise to boomy sound. This effect known as proximity effect is most pronounced

    in bi-directional ribbon microphones such as RCA 44 Bx. This effect should be

    normally avoided by placing the microphones fairly away (30-45 cm) from the source

    of sound. However, proximity effect can be used to advantage for special sound effects

    (warmth, intimacy, etc.) by placing the microphones closer to the sound source.

    It may however, be noted that some of the directional microphones such as AKG type

    D200, D202, D222 and D224 do not exhibit the above proximity effect and hence these

    can be used closer to the source of sound

    . In outdoor locations because of the higher ambient noise level, the working distancemust be kept less than the corresponding distance when working indoors.

    d) Balancing

    There are two aspects of balancing in microphones :

    - When single microphone is used for more than one source of sound, distance between

    the microphone and different sources of sound be adjusted suitably so that the desired

    sound levels are achieved from all sources.

    - When a microphone is used in an enclosed space (as in studios) reverberant energy or

    indirect sound plays an important role in the quality of the output of the microphone.

    When the microphone is placed close to the sound source, direct sound is predominant

    and the output appears to be dry . On the other hand

    when the microphones is far away from the sound source the reverberant (indirect)

    sound is predominant and the output lacks in clarity. Hence for proper balancing ofdirect to indirect sound pick up the microphone should be kept at an optimum distance.

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    In an acoustically treated room like a studio, a normal distance of 30-45 cms, is found

    to be satisfactory. An important Note : Balancing should always be judged by

    monitoring the microphone output at a location outside the studio (say from the

    attached announcers both or recording room etc.) Recording of a programme should not

    be started till a proper balance is achieved.

    e) Microphone should not be placed very close to a reflecting surface such as announcer

    table top or bare walls of a studio.

    f) A talker should not hold the script between his face and the microphone otherwise

    shadowing effect will occur at high frequencies.

    g) For placement of microphone for pick up of musical instruments following

    guidelines may be kept in view :

    - For stringed instruments (violin, sitar, sarangi, etc.) 0o axis of the microphone should

    be preferably placed normal to the front face of the instrument.

    - For instruments with large sound output (like drums and other percussion and bass

    instruments) the microphone should be placed well away from the source of sound.

    - For wood wind instruments where the instrument is not particularly directional (such

    as flute) the microphone may be placed about 60 cms. away so that instrument does not

    speak straight at it.

    h) If a source of sound is placed on the dead axis of a microphone it sounds as if this

    source of sound is placed at a considerable distance from the microphone. This effect

    could be made use of in a drama production.

    i) Talking very close to a microphone may cause blasting on explosive consonants such

    as P . Hence it should be avoided.

    4.7 Care of Microphones

    Microphone needs a very careful handling as it is a delicate equipment. Some important

    precautions in handling microphones are listed below :

    - Do not allow it to fall or get any type of knock.

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    - If microphones are to be carried, pack each one of them separately in a cushioned box

    and keep them apart. Do not pack any tools, magnetic substances or types in the same

    box.

    - Do not open a microphone nor test the continuity of the microphone or its transformer.

    If the microphone is connected to the mike cable, test the mike cable continuity only

    after isolating the mike. Multimeter current is enough to disturb the ribbon or

    diaphragm resulting in major damage.

    - While testing the mike in a studio do not speak very loud nor blow into it. Speak

    gently, rubbing the mike on its side in a gentle manner and announce its type, the stand

    on which it is resting and the channel to which it is connected, so that the control

    engineer may know the complete details of the test being given. He may verify that the

    sound picked up and the rubbing noise are both from the same microphone under test.

    - If there are folk singers or loud instruments like Nagaswaram/drums etc. present, keep

    sensitive mikes at a distance from the source of sound.

    - Do not allow the microphones to get wet during rain. Use a wind shield or PVC

    coverings depending upon the situation.

    A list of different types of microphones used in All India Radio & Doordarshan is given

    in table 1. Table 1 Different Types of Microphones

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    CHAPTER 5

    THE DOORDARSHAN KENDRA DELHI

    5.1 AN OVERVIEW

    Doordarshan Kendra DELHI is part of the DD India, the largest television network inthe world. Doordarshan with over 5 high power TerrestrialTransmitters,62 low

    power,5 very low power transmitter and 3 production centers serve DELHI Inaugurated

    on 28th may 2000 by the then broadcast minister mr. ARUN JATELY. Doordarshan

    Kendra delhi currently produces and telecasts 168 hrs of local programmes per week.

    Now more than 85 per cent of the 60,385,118 populations of M.P., With the

    introduction of DTH almost cent percent of the population can now receive DDK delhi

    programmes without cable connection. Doordarshan studios have been established at

    Gwalior, Bhopal and Indore to foster regional diversity. People all over India are

    watching Doordarshans programmes. It is also received in 64 countries spread over

    the continents of Asia, Africa, Europe, Australia and America.

    5.2 TV Scenario in Delhi.

    As per the 2001 census there are 60,385,118(5.5 million) house holds in M.P., 74.9 per

    cent of them are in the rural sector (44,42550) the remaining 25.1 per cent (13,52656)

    are in the urban sector. In 2001, 38.8 per cent of the households owned TV sets . Of

    these 62.3 per cent were in rural areas and the remaining 37.7 per cent in urban areas.

    Even if we estimate 10 15 per cent growth per annum. Of these estimated 3 million

    TV households 40 45 per cent is estimated to have cable connection i.e., 1.3 million

    and the remaining 1.7 million are without cable connection, and totally depend on DDK

    Bhopal for their TV viewing. The introduction of DTH, DD Direct Plus has

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    considerably increased DD viewership in MP. From the available sales estimates of set

    top boxes and receivers it is estimated that MP has 3 to 4 lakh DTH households.

    5.3 TECHNICAL INFORMATION OF TRANSMITTING

    FACILITIES AT DDK, DELHI:

    Doordarshan Kendra, delhi is equipped with studio, two terrestrial transmitters and one

    digital up-link station. The two terrestrial transmitters are of 10 KW power each.

    One is for DD-National and the other is for DD-News telecasting.

    5.4 TERRESTRIAL TRANSMITTER PARAMETERS:

    DD-NEWS :CH #31 (VHF-Band-III) Pictures IF: 551.25 MHz, Sound IF: 556.75

    MHz

    DOWNLINK PARAMETERS OF DD-NEWS SATELLITE

    PROGRAMMES

    Latitude Co-ordinates 23 1425 ( North )

    Longitude Co-ordinates 77 2320 ( East)

    Main Sea Level 300 Mtrs.

    Antenna Hight 100 Mtrs.

    Effective height of the antenna above

    sea level400 Mtrs.

    Peak power (Both DD-I & DD-II) 10 KW each

    Black power 06 KW each

    Antenna gainArt Direction8 Db wide band,

    Jampro Antenna.

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    FREQUENCY OF OPERATION

    Band DD-II

    Band III

    Channel 31(.)

    Video carrier 551.25 MHz

    Audio carrier 556.75 MHz

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    CHAPTER 6

    SYSTEM FUNDAMENTAL OF MONOCHROME

    AND COLOUR TV

    6.1Picture formation

    A picture can be considered to contain a number of small elementary areas of light or

    shade which are called PICTURE ELEMENTS. The elements thus contain the visual

    image of the scene.

    In the case of a TV camera the scene is focused on the photosensitive surface of pick

    up device and a optical image is formed. The photoelectric properties of the pick up

    device convert the optical image to a electric charge image depending on the light and

    shade of the scene (picture elements). Now it is necessary to pick up this information

    and transmit it. For this purpose scanning is employed. Electron beam scans the charge

    image and produces optical image. The electron beam scans the image line by line and

    field by field to provide signal variations in a successive order.

    The scanning is both in horizontal and vertical direction simultaneously.

    The horizontal scanning frequency is 15,625 Hertz.

    The vertical scanning frequency is 50 Hz.

    The frame is divided in two fields. Odd lines are scanned first and then the even lines.

    The odd and even lines are interlaced. Since the frame is divided into 2 fields the

    flicker reduces. The field rate is 50 Hertz. The frame rate is 25 Hertz (Field rate is the

    same as power supply frequency)

    6.2 Number of TV Lines per Frame

    If the number of TV lines is high larger bandwidth of video and hence larger R.F.

    channel width is required. If we go for larger RF channel width the number of channelsin the R.F. spectrum will be reduced. However, with more no. of TV lines on the

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    screen the clarity of the picture i.e. resolution improves. With lesser number of TV lines

    per frame the clarity (quality) is poor.

    The capability of the system to resolve maximum number of picture elements along

    scanning lines determines the horizontal resolution. It means how many alternate black

    and white elements can be there in a line. Let us also take another factor. It is realistic

    to aim at equal vertical and horizontal resolution. Therefore, the number of alternate

    black and white dots on line can be 575 x 0.69 x 4/3 which is equal to 528.

    It means there are 528 divided by 2 cyclic changes i.e. 264 cycles. These 264 cycles

    are there during 52 micro seconds. Hence the highest frequency is 5 MHz.

    MHz552

    10264f6

    highest =

    =

    Therefore the horizontal resolution of the system is 5 MHz.A similar calculation for

    525 lines system limits the highest frequency to 4 MHz and hence the horizontal

    resolution of same value.

    In view of the above the horizontal bandwidth of signal in 625 lines system is 5 MHz.

    6.3The PAL Colour Television System

    5.3.1 The Colour Television

    It is possible to obtain any desired colour by mixing three primary colours i.e. Red,

    Blue and green in a suitable proportion.

    6.3.1 Additive Colour Mixing

    The figure 10 shows the effect of projecting red, green, blue beams of light so that they

    overlap on screen.

    Y= 0.3 Red + 0.59 Green + 0.11 Blue

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    Fig. 6.1 Additive Colour Mixing

    6.4 The Colour Television

    It is possible to obtain any desired colour by mixing three primary colours i.e., red, blue

    and green in suitable proportion. Thus it is only required to convert optical information

    of these three colours to electrical signals and transmit it on different carriers to be

    decoded by the receiver. This can then be converted back to the optical image at the

    picture tube. The phosphors for all the three colours i.e. R, G and B are easily available

    to the manufacturers of the picture tube. So the pick up from the cameras and output

    for the picture tube should consists of three signals i.e. R, G and B. It is only in

    between the camera and the picture tube of the receiver we need a system to transmit

    this information.

    Fig 6.2 Colour TV

    Colour television has the constraint of compatibility and reverse compatibility with the

    monochrome television system which makes it slightly complicated. Compatibility

    means that when colour TV signal is radiated the monochrome TV sets should also

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    display Black & White pictures. This is achieved by sending Y as monochrome

    information along with the chroma signal. Y is obtained by mixing R,G & B as per the

    well known equation :

    Y = 0.3 R + 0.59 G + 0.11 B

    Reverse compatibility means that when Black & White TV signal is radiated the colour

    TV sets should display the Black & White pictures.

    If we transmit R, G, B, the reverse compatibility cannot be achieved. Let us see how

    If we transmit Y, R & B and derive G then :

    Since Y = 0.3R + 0.59G + 0.11 B

    G = 1.7Y - 0.51 R - 0.19 B

    In such a case what happens with a colour TV set when we transmit black and white

    signal. R and B are zero, but G gun gets 1.7 Y. The net result is black & white pictures

    on a colour TV screen appear as Green pictures. So reverse compatibility is not

    achieved.

    6.5 Colour Difference Signals

    To achieve reverse compatibility, when we transmit Y, R-Y and B-Y instead of Y, R &

    B, we do not take G-Y as this will always be much lower than R-Y and B-Y and hence

    will needs more amplification and will cause more noise into the system. G-Y can be

    derived electronically in the TV receiver.

    In the previous paragraph we have seen

    G = 1.7 Y - 0.51 R - 0.19 B

    So G-Y = -0.51 (R-Y) - 0.19 (B-Y)

    Thus, colour difference signals fulfill the compatibility and reverse compatibility.

    Because in this case the colour difference signals are zero if the original signal is

    monochrome (i.e. R = B = G)

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    So if we take R - Y

    R - Y = R - (0.3 R + 0.59 R + 0.11 R) = 0

    Similarly B - Y = 0

    As such colour difference signals are zero for white or any shade of gray whereas, Y

    carries the entire Luminance information.

    It is to be noted while R, G, B signals always have positive value R-Y, B-Y and G-Y

    signals can either be positive or negative or even zero.

    The R-Y and B - Y chrominance signals may be recovered at the television receiver by

    suitable synchronous demodulation. But sub-carrier is to be generated by a local

    oscillator. This generated sub-carrier in the receiver must have same frequency as that

    of transmitted sub-carrier and also the same phase. This is achieved by transmitting 10

    cycles of sub-carrier frequency on the back porch of H synchronizing pulse. This 10

    cycles sub-carrier signal is known as BURST or colour BURST.

    Fig 6.3 Colour Difference Signal

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    Fig 6..4 Block Digram of PAL Encoder

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    CHAPTER 7

    VIDEO CHAIN IN A TYPICAL DOORDARSHAN STUDIO

    7.1 STUDIO CENTRE

    A Studio centre of Doordarshan has the following objectives:

    1) To originate programmes from studios either for live telecast or for recording on

    a video tape.

    2) To knit various other sources of programs available at the production desk i.e.,

    camera output from studios, feed from other kendras, outdoor, playback from pre

    recorded tape, film based programs slides, video graphics and characters generator

    etc. This knitting or live editing includes generation of special effects and desired

    transitions between various sources.

    3) Processing/distribution of different sources to various destinations in technical

    areas.

    4) Routing of mixed programme for recording/transmission via master switching

    room and Micro Wave to the transmitter or any other desired destinations.

    Activities in a television studio can be divided into three major areas such as :

    1) Action area,

    2) Production control room, and

    3) Central apparatus room,

    7.1.1 Action area

    This place requires large space and ceiling as compared to any other technical area.

    Action in this area includes staging, lighting, performance by artists, and arrangement

    to pick up picture and sound. Hardware required for these activities in a studio (typical

    size 20 x20x8.5 cubic meters) are:

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    2. Timing a production/telecast.

    3. Editing of different sources available at the production desk.

    4. Monitoring of output/off air signal.

    Hardware provided in this area include:

    1. Monitoring facilities for all the input and output sources(audio/video).

    2. Remote control for video mixer, telecine and library store and special effect

    (ADO) etc.

    3. Communication facilities with technical areas and studio floor.

    7.1.3 Vision mixing and switching

    Unlike films, television media allows switching between different sources

    simultaneously at the video switcher in Production control room operated by the Vision

    Mixer on the direction of the program producer. The producer directs the cameramen

    for proper shots on various cameras through intercom and the vision mixer (also called

    VM engineer) switches shots from the selected camera/cameras with split second

    accuracy, in close cooperation with the producer. The shots can be switched from one

    video source to another video source, superimposed, cross faded, faded in or faded out

    electronically with actual switching being done during the vertical intervals between the

    picture frames. Electronics special effects are also used now days as a transition

    between the two sources.

    7.1.4 Vision Mixer (or Video Switcher)

    Though the video switching is done by the VM at the remote panel, the electronics is

    located in CAR. The vision mixer is typically a 10 x 6 or 20 x 10 cross bar switcher

    selecting anyone of the 10 or 20 input sources to 6 or 10 different output lines. The

    input sources include: Camera 1, camera 2, camera 3, VTR1, VTR2, Telecine 1,

    Telecine 2, Test signal etc. The vision mixer provides for the following operational

    facilities for editing of TV programs:-

    (i) Take: Selection of any input source

    or

    Cut: switching clearly from one source to another.

    (ii) DISSOLVE: Fading out of one source of video and fading in another

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    source of video.

    (iii) SUPERPOSITION OF TWO SOURCES: Keyed caption when selected inlay is

    superimposed on the background picture.

    (iv) SPECIAL EFFECTS: A choice of a number of wipe patterns for split screen or wipe

    effects.

    The selected output can be monitored in the corresponding pre-view monitor. All the

    picture sources are available on the monitors. The preview monitors can be used for

    previewing the telecine, VTR; test signals etc. with any desired special effect, prior to

    its actual switching.

    The switcher also provides cue facilities to switch camera tally lights as an indication to

    the cameraman whether his camera is on output of the switcher.

    7.2 Present day PCRs have

    24 input video special effects switchers

    (CD 680 or CD 682-SP)

    Character generators

    Telecine/DLS remote controls

    Adequate monitoring equipment

    7.2.1 Character Generator(CG)

    Character Generator provides titles and credit captions during production in Roman

    script. It provides high resolution characters, different colours for colorizing characters,

    background, edges etc. At present bilingual and trilingual C.G are also being used by

    Doordarshan.

    Character Generator is a microcomputer with Texts along instructions when typed in at

    the keyboard is stored on a floppy or a Hard disk. Many pages of scripts can be stored

    on the disk and recalled when needed, by typing the addresses for the stored pages, to

    appear as one of the video sources.

    7.2.2 Sync Pulse-Generator(SPG)

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    It is essential that all the video sources as input to the switcher are in synchronism i.e.,

    start and end of each line or all the frames of video sources is concurrent. This

    requirement is ensured by the sync pulse generator (SPG). SPG consists of highly

    stable crystal oscillator. Various pulses of standard width and frequency are derived

    from this crystal electronically which form clock for the generation of video signal.

    These pulses are fed to all the video generating equipment to achieve this objective of

    synchronism. Because of its importance, SPG is normally duplicated for change over in

    case of failure.

    It provide the following outputs:

    Line drive

    Field drive

    Mixed blanking

    Mixed sync

    colour subcarrier

    A burst insertion pulse

    PAL phase Indent pulses

    7.2.3Camera Control Unit (CCU)

    The television cameras which include camera head with its optical focusing

    lens, pan and tilt head, video signal pre-amplifier view finder and other associated

    electronic circuitry are mounted on cameras trolleys and operate inside the studios. The

    output of cameras is pre-amplified in the head and then connected to the camera control

    unit (CCU) through long multi-core cable (35 to 40 cores), or triax cable.

    All the camera control voltages are fed from the CCU to the camera head over the

    multi-core camera cable. The view-finder signal is also sent over the camera cable to

    the camera head view-finder for helping the cameraman in proper focusing, adjusting

    and composing the shots.

    The video signal so obtained is amplified, H.F. corrected, equalized for cable delays,

    D.C. clamped, horizontal, and vertical blanking pulses are added to it. The peak white

    level is also clipped to avoid overloading of the following stages and avoiding over

    modulation in the transmitter. The composite sync signals are then added and these

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    video signals are fed to a distribution amplifier, which normally gives multiple outputs

    for monitoring etc.

    7.2.3 Light Control

    The scene to be televised must be well illuminated to produce a clear and noise free

    picture. The lighting should also give the depth, the correct contrast and artistic display

    of various shades without multiple shadows.

    The lighting arrangements in a TV studio have to be very elaborate. A large number of

    lights are used to meet the needs of key, fill, and back lights etc. Lights are

    classified as spot and soft lights. These are suspended from motorized hoists and

    telescopes. The up and down movement is remotely controlled. The switching on and

    off the lights at the required time and their dimming is controlled from the light control

    panel inside a lighting control room using SCR dimmer controls. These remotely

    control various lights are inside the studios.

    7.3 Sound mixing and control

    As a rule, in television, sound accompanies the picture. Several microphones aregenerally required for production of complex television programs besides other audio

    sources also called marred sound from telecine, VTR, and audio tape/disc replays. All

    these audio sources are connected to the sound control console.

    The sounds from different sources are controlled and mixed in accordance with the

    requirement of the program. Split second accuracy is required for providing the correct

    audio source in synchronisation with the picture thus requiring lot of skill from the

    engineer. Even the level of sound sometimes is varied in accordance with the shot

    composition called prospective.

    7.3.1 Audio facilities

    An audio mixing console, with a number of inputs, say about 32 inputs is provided in

    major studio. This includes special facilities such as equalisation, PFL, phase reversal,

    echo send/receive and digital reverberation units at some places Meltron console tape

    recorders and EMI 938 disc reproducers are provided for playing back/creating audio

    effects as independent sources (Unmarried) to the switcher.

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    7.3.2Video Tape recorders

    VTR room is provided at each studio center. It houses a few Broadcast standard

    Videocassette recorders (VCRs). In these recorders, sound and video signals are

    recorded simultaneously on the same tape.

    Most of the TV centers have professional quality B-Format BCN-51 One inch VTRs.

    For broadcast quality playback it is equipped with correction electronics i.e. a processor

    which comprises velocity error compensation, drop-out compensation and time base

    correction. It also comprises a digital variable motion unit enabling still reproduction,

    slow motion and visible search operation.

    New centers are being supplied with Sony U-matic high band VCRs along with

    Sony Betacam SP VCRs, DVC Pro.

    7.3.3Post Production Suites

    Modern videotape editing has revolutionised the production of television programs over

    the years. The latest trend all over the world is to have more of fully equipped post

    production suites than number of studios. Most of the present day shootings are done

    on locations using single camera. The actual production is done in these suites. The

    job for a post production suites is:-

    a. To knit program available on various sources.

    b. While doing editing with multiple sources, it should be possible to have any

    kind of transition.

    c. Adding/Mixing sound tracks.

    d. Voice over facilities.

    e. Creating special effects.The concept of live editing on vision mixer is being replaced by to do it at leisure in

    post production suites.

    A well equipped post production suite will have:-

    1. Five VTRs/VCRs, may be of different format remotely controlled by the editor.

    2. Vision mixing with special effect and wipes etc. with control from a remote

    editor panel.

    3. Ampex Digital Optics (ADO) for special effects.

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    4. Audio mixer with remote control from the editor remote panel.

    5. Multi-track audio recorder with time code facilities and remote operation.

    6. Character generator for titles.

    7. Adequate monitoring facilities.

    8. Supported by Offline editing systems to save time in post production suites.

    9. One man operation.

    7.4 Coverage of Outside events

    Outside broadcasts(or OBs) provide an important part of the television programs.

    Major events like sports, important functions and performances are covered with anO.B. van which contains all the essential production facilities.

    7.4.1 Video Chain :

    The block diagram on facing page connects all these sections and it can be observed

    that the CAR is the nodal area. Now let us follow a CAM-I signal. CAM-I first goes to

    a Camera electronics in CAR via a multi-core cable, the signal is then matched/adjusted

    for quality in CCU and then like any other sources it goes to video switcher via PP(Patch Panel) and respective VDAs(Video Distribution Amplifiers) and optional Hum

    compensator/Cable equilizers.

    Output from the switcher goes to stabilizing amplifier via PP and VDAs.

    Output from the stab. Is further distributed to various destinations.

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    Fig 7.1 video chain

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    7.5 TV LIGHTING

    7.5.1 GENERAL PRINCIPLES

    Lighting for television is very exciting and needs creative talent. There is always a

    tremendous scope for doing experiments to achieve the required effect. Light is a kind

    of electromagnetic radiation with a visible spectrum from red to violet i.e. wave length

    from 700 nm to 380 nm respectively. However to effectively use the hardware and

    software connected with lighting it is important to know more about this energy.

    7.5.2 Light Source

    Any light source has a Luminance intensity (I) which is measured in Candelas. Candela

    is equivalent to an intensity released by standard one candle source of light.

    7.5.3 Basic Three Point Lighting

    a) Key light : This is the principal light source of illumination. It gives shape and

    modeling by casting shadows. It is treated like "sun" in the sky and it should cast only

    one shadow. Normally it is a hard source.

    b) Fill Light : Controls the lighting contrast by filling in shadows. It can also provide

    catch lights in the eyes. Normally it is a soft source.

    c) Back light : Separates the body from the background, gives roundness to the subject

    and reveals texture. Normally it is hard source.

    d) Background Light : Separates the person from the background, reveals background

    interest and shape. Normally it is a hard source.In three point lighting the ratio of 3/2/1

    (Back/Key/Fill) for mono and 3/2/2 for colour provides good portrait lighting.

    7.6 TV CAMERA

    7.6.1 Introduction

    A TV Camera consists of three sections :

    a) A Camera lens and optics : To form optical image on the face

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    plate of a pick up device.

    b) A transducer or pick up device : To convert optical image into an

    electrical signal.

    c) Electronics : To process output of a transducer to

    get a CCVS signal.

    7.6 .2 CCD CAMERAS

    7.6.2.a Introduction

    Any 7.6 TV CAMERA

    convert the light information on it to a charge signal. All we need now is to have an

    arrangement to collect this charge and convert it to voltage. This is the basic principle

    on which CCD cameras are based.

    7.6.3 Latest CCD Cameras

    CCD were launched in 1983 for broadcasting with pixel count from a mere 2,50,000

    which increased to 20,00,000 in 1994 for HDTV application. Noise and aliasing has

    been reduced to negligible level. CCD cameras now offers fully modulated video

    output at light level as low as 6.0 lumens. A typical specification for a studio camera

    now available in market are some thing like 2/3 inch, FIT, lens on chip CCD with

    6,00,000 pixel, 850 lines H resolution, S/N more than 60 dB, sensitivity F-8 (2000 lux)

    etc.

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    Fig 7.2Block Diagram of a typical Camera

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    CHAPTER8

    HIGH POWER TV TRANSMITTER

    8.1 Design

    All the TV transmitters have the same basic design. They consist of an exciter followed

    by power amplifiers which boost the exciter power to the required level.

    8.1.1 Exciter

    The exciter stage determines the quality of a transmitter. It contains pre-corrector units

    both at base band as well as at IF stage, so that after passing through all subsequent

    transmitter stages, an acceptable signal is available. Since the number and type of

    amplifier stages, may differ according to the required output power, the

    characteristics of the pre-correction circuits can be varied over a wide range.

    8.1.2 Vision and Sound Signal Amplification

    In HPTs the vision and sound carriers can be generated, modulated and amplified

    separately and then combined in the diplexer at the transmitter output.

    In LPTs, on the other hand, sound and vision are modulated separately but amplified

    jointly. This is common vision and aural amplification.A special group delay

    equalization circuit is needed in the first case because of errors caused by TV

    diplexer. In the second case the intermodulation products are more prominent and

    special filters for suppressing them is required.As it is difficult to meet the

    intermodulation requirements particularly at higher power ratings, separate

    amplification is used in HPTs though combined amplification requires fewer

    amplifier stages.

    8.1.3 Power Amplifier Stages

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    In BEL mark I & II transmitters three valve stages (BEL 450 CX, BEL 4500 CX and

    BEL 15000 CX) are used in vision transmitter chain and two valves (BEL 450 CX

    and BEL 4500 CX) in aural transmitter chain. In BEL mark III transmitter only

    two valve stages (BEL 4500 CX and BEL 15000 CX) are used in vision transmitter

    chain. Aural transmitter chain is fully solid state in Mark III transmitter.

    BEL 10 kW TV TRANSMITTER

    A block diagram of BEL 10 kW TV Transmitter is shown in Fig. 10. It consists of :

    Monitoring Equipment Rack

    Control Console Input Equipment Rack

    Indoor Co-axial Equipment comprising of :

    U-link Rack with U-link panel A and B, T-Transformer and 10 kW

    Dummy Load.

    Aural Harmonic Filter.

    CIN Diplexer

    Aural Notch Filter and Band Pass Filter.

    Antenna system with junction box, feeder cables etc.

    Fig 8.1 lock Diagram of 10kW TV Transmitter

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    8.1.4 SOLID STATE POWER AMPLIFIERS

    1) Has got two identical sections. Each capable of delivering 10 W.

    2) Gets 28 V power supply through relay in 80 W AMP.

    3) Sample of output is available at front panel for RF monitoring.4) Provides A DC output corresponding to sync peak out put for vision monitoring

    unit.

    5) Thermostat on heat sink is connected in series with thermostat or 80 W AMP

    and provides thermal protection. (Operating temp. 70oC.)

    Fig. 8.2 TX. Block Diagram

    Fig 8.3 Vision Chain of Exciter

    8.2TRANSMITTER CONTROL SYSTEM

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    The transmitter control unit performs the task of transmitter interlocking and control.

    Also it supports operation from control console. The XTR control unit (TCU) has two

    independent system viz.

    1. Main control system. (MCS)

    2. Back-up Control System (BCS)

    8.2.1 System Description of Exciter

    Fig 8.4 Block Diagram of TV Exciter

    8.2.2 Video Chain

    The input video signal is fed to a video processor. In VHF transmitters LPF, Delay

    equalizer and receiver pre-corrector precede the video processor.

    8.2.2.a Low Pass Filter : Limits incoming video signal to 5 MHz.

    8.2.2.b Delay Equalizer : Group delay introduced by LPF is corrected. It also pre-

    distorts the video for compensating group delay errors introduced in the subsequent

    stages and diplexer.

    8.2.2.c Receiver pre-corrector : Pre-distorts the signal providing partial compensation

    of GD which occurs in domestic receivers.Both the delay equaliser and receiver

    precorrector are combined in the delay equaliser module in Mark III version.

    8.3 DP/DG Corrector

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    This is also used in the exciter preceding LPF (mark III) for pre-correcting the

    differential gain and differential phase errors occurring in the transmitter.

    8.3.1 Video Processor

    The block diagram of video processor is given in fig. 3.

    Functions

    Amplification of Video signal

    Clamping at back porch of video signal.

    Clamping gives constant peak power. Zero volt reference line is steady irrespective of

    video signal pattern when clamping takes place otherwise the base line starts an

    excursion about the zero reference depending on the video signal.

    Fig 8.5 Block Diagram of Video Processor

    8.3.2 Vision Modulator

    The block diagram of Vision modulator is given in fig. 4 and schematic diagram is

    shown in fig.

    Functions

    Amplification of Vision IF at 38.9 MHz.

    Linear amplitude modulation of Vision IF by video from the video processor in

    a balanced modulator.

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    8.4 IF Amplifier

    IF is amplified to provide sufficient level to the modulator. It operates as an amplitude

    limiter for maintaining constant output.

    8.4.1 Modulator

    A balanced modulator using two IS-1993 diodes is used in the modulator.

    8.4.2 Band pass amplifier

    Modulated signal is amplified to 10 mW in double tuned amplifier which

    provides a flat response within 0.5 dB in 7 MHz band.

    Fig 8.6 Block Diagram of Vision Modulator

    Fig 8.7Schematic Diagram of Vision Modulator

    8.5 VSBF and Mixer

    The block diagram of VSBF and Mixer is given in fig. 6. It consists of

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    following stages :

    VSB filter

    ALC amplifier

    Mixer

    Helical Filter

    Mixer Amplfier

    Fig 8.8 Block Diagram of VSBF Mixer

    8.5.1 VSB Filter

    Surface Acoustic wave (SAW) filter provide a very steep side band response with high

    attenuation outside designated channel. It has a linear phase characteristic with a low

    amplitude and group delay ripple. (Fig. 7.)

    Fig 8.9 Block Diagram of V.S.B.Filter

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    8.5.2 Local Oscillator

    The block diagram of Local Oscillator is given in fig. 8.It supplies three equal outputs

    of + 8 dBm each at a frequency of fv + fvif. This unit has 3 sub units.

    (1) fc/4 oscillator : Generates frequency which is 1/4 of desired channel frequency.

    Fine freq. control is done by VC1.

    (2) LO Mixer/Power divider : Here the above fc/4 frequency is multiplied by four

    to obtain channel frequency of fc and then mixed with fvif. Power divider is

    also incorporated to provide three isolated outputs of equal level.

    Fig 8.10 Block Diagram of Local Oscillator

    8.5.3 AUDIO CHAIN

    8.5.3.a Aural Modulator

    The aural modulator unit consists of audio amplifier, VCO, mixer and APC.

    The block diagram of Aural modulator is given in fig. 9.

    Fig 8.11 Block Diagram of Aural Modulator

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    8.5.4 Audio Amplifier

    A balanced audio signal at + 10 dBm from studio is converted to unbalanced signal by

    audio transformer T4. The output of this is taken through potentiometer to the input of

    Hybrid Audio Amp BMC 1003. A 50 micro second pre-emphasis is also provided.

    8.5.5 VCO

    This is a varactor tuned oscillator. Its frequency can be varied by coil L4. Transistor

    TR-17 forms the oscillator. VCO output is frequency modulated by the audio signal.

    Output level is 0 dBm

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    CHAPTER 9

    TV TRANSMITTER ANTENNA SYSTEM

    TV Antenna System is that part of the Broadcasting Network which accepts RF Energy

    from transmitter and launches electromagnetic waves in space. The polarization of the

    radiation as adopted by Doordarshan is linear horizontal. The system is installed on a

    supporting tower and consists of antenna panels, power dividers, baluns, branch feeder

    cable, junction boxes and main feeder cables. Dipole antenna elements, in one or the

    other form are common at VHF frequencies where as slot antennae are mostly used at

    UHF frequencies. Omni directional radiation pattern is obtained by arranging the

    dipoles in the form of turnstile and exciting the same in quadrature phase. Desired gainis obtained by stacking the dipoles in vertical plane. As a result of stacking, most of the

    RF energy is directed in the horizontal plane. Radiation in vertical plane is minimized.

    The installed antenna system should fulfil the following requirements :

    a) It should have required gain and provide desired field strength at the point of

    reception.

    b) It should have desired horizontal radiation pattern and directivity for serving the

    planned area of interest. The radiation pattern should be omni directional if the

    location of the transmitting station is at the center of the service area and

    directional one, if the location is otherwise.

    c) It should offer proper impedance to the main feeder cable and thereby to the

    transmitter so that optimum RF energy is transferred into space. Impedance

    mismatch results into reflection of power and formation of standing waves. The

    standard RF impedance at VHF/UHF is 50 ohms.

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    Fig 9.1Turnstile Antenna and its Horizontal Pattern

    9.1 Radiation Pattern and Gain

    The horizontal and vertical radiation pattern are shown in fig. 9.1 and 9.2. The total

    gain depends upon the type of the antenna panel and no. of stacks as given in table-1.

    Fig. 9.2 Typical Horizontal radiation pattern

    9.2 VESTIGIAL SIDE BAND TRANSMISSION

    Another feature of present day TV Transmitters is vestigial side band transmission. If

    normal amplitude modulation technique is used for picture transmission, the minimum

    transmission channel bandwidth should be around 11 MHz taking into account the

    space for sound carrier and a small guard band of around 0.25 MHz. Using such large

    transmission BW will limit the number of channels in the spectrum allotted for TV

    transmission. To accommodate large number of channels in the allotted spectrum,

    reduction in transmission BW was considered necessary. The transmission BW could

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    be reduced to around 5.75 MHz by using single side band (SSB) AM technique,

    because in principle one side band of the double side band (DSB) AM could be

    suppressed, since the two side bands have the same signal content.

    It was not considered feasible to suppress one complete side band in the case of TV

    signal as most of the energy is contained in lower frequencies and these frequencies

    contain the most important information of the picture. If these frequencies are

    removed, it causes objectionable phase distortion at these frequencies which will affect

    picture quality. Thus as a compromise only a part of lower side band is suppressed

    while taking full advantage of the fact that:

    i) Visual disturbance due to phase errors are severe and unacceptable where

    large picture areas are concerned (i.e. at LF) but

    ii) Phase errors become difficult to see on small details (i.e. in HF region) in

    the picture. Thus low modulating frequencies must minimize phase

    distortion where as high frequencies are tolerant of phase distortions as they

    are very difficult to see.

    The radiated signal thus contains full upper side band together with carrier and

    the vestige (remaining part) of the partially suppressed LSB. The lower side band

    contains frequencies up to 0.75 MHz with a slope of 0.5 MHz so that the final cut off is

    at 1.25 MHz.

    9.3 Standards

    The characteristics of the TV signal is sections 1 and 2 refer to CCIR B/G standards.

    Various other standards are given in Table 1.

    Table 1

    Frequency Range Vision/sound carrier spacing channel

    width

    Vision sound carrier spacing 5.5 MHz

    Channel width 7 MHz (B) in VHF OR 8 MHz (G) in UHF

    Sound Modulation FM

    FM deviation (maximum) + 50 kHz

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    CONCLUSION

    The practical training has proved to be knowledge buster for me and I have acquired a

    good practical knowledge of the field which cant be gained nearly by reading books.

    During my training atPARSHAR BHARTI A.I.R and DD, kingsways Delhi

    11ooo9 was really very surprised and delighted to see the system configuration and

    interconnections to such a large extent. I came to know and learn practical about

    various stages and equipment involved right from the production of program to its

    transmission; about which I heard or read only in text books. I also visited workshop

    where, beside my training program, I also learnt various basic things about diodes,

    capacitor, power supplies, multimeter, digital C.R.O. etc. which has a remarkable

    experience. I really feel that my training at A.I.R and DD was very beneficial for

    me.The training has proved me with a good knowledge of working of PARSHAR

    BHARTI A.I.R and DD base for relating the theoretical knowledge with the practical

    one. It was a very exciting, adventurous and exhaustive training which has raised my

    practical skills to a great extent.

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    BIBLIOGRAPHY

    The documents which were a help to me in completion of my report are being obtained

    from the following sources:

    I. Sites :

    1. Tcil-india.com

    2. Google.com

    3. Winkipedia.com

    4. Emory.edu

    5. Tycotelecom.com

    6. Technologyforall.com

    II. White Papers from different sites.

    III. Books :

    1. William Stallings, Wireless Communication & Networks, Pearson

    Education, 2007.

    2. Sanjay Sharma, Analog & Digital Communication System, Laxmi

    Publication, 2009.